Secure Programming for Linux and Unix HOWTO

Table of Contents
1. Introduction
2. Background
    2.1. History of Unix, Linux, and Open Source / Free Software
    2.2. Security Principles
    2.3. Why do Programmers Write Insecure Code?
    2.4. Is Open Source Good for Security?
    2.5. Types of Secure Programs
    2.6. Paranoia is a Virtue
    2.7. Why Did I Write This Document?
    2.8. Sources of Design and Implementation Guidelines
    2.9. Other Sources of Security Information
    2.10. Document Conventions
3. Summary of Linux and Unix Security Features
    3.1. Processes
    3.2. Files
    3.3. System V IPC
    3.4. Sockets and Network Connections
    3.5. Signals
    3.6. Quotas and Limits
    3.7. Dynamically Linked Libraries
    3.8. Audit
    3.9. PAM
    3.10. Specialized Security Extensions for Unix-like Systems
4. Security Requirements
    4.1. Common Criteria Introduction
    4.2. Security Environment and Objectives
    4.3. Security Functionality Requirements
    4.4. Security Assurance Measure Requirements
5. Validate All Input
    5.1. Command line
    5.2. Environment Variables
    5.3. File Descriptors
    5.4. File Names
    5.5. File Contents
    5.6. Web-Based Application Inputs (Especially CGI Scripts)
    5.7. Other Inputs
    5.8. Human Language (Locale) Selection
    5.9. Character Encoding
    5.10. Prevent Cross-site Malicious Content on Input
    5.11. Filter HTML/URIs That May Be Re-presented
    5.12. Forbid HTTP GET To Perform Non-Queries
    5.13. Counter SPAM
    5.14. Limit Valid Input Time and Load Level
6. Avoid Buffer Overflow
    6.1. Dangers in C/C++
    6.2. Library Solutions in C/C++
    6.3. Compilation Solutions in C/C++
    6.4. Other Languages
7. Structure Program Internals and Approach
    7.1. Follow Good Software Engineering Principles for Secure Programs
    7.2. Secure the Interface
    7.3. Separate Data and Control
    7.4. Minimize Privileges
    7.5. Minimize the Functionality of a Component
    7.6. Avoid Creating Setuid/Setgid Scripts
    7.7. Configure Safely and Use Safe Defaults
    7.8. Load Initialization Values Safely
    7.9. Fail Safe
    7.10. Avoid Race Conditions
    7.11. Trust Only Trustworthy Channels
    7.12. Set up a Trusted Path
    7.13. Use Internal Consistency-Checking Code
    7.14. Self-limit Resources
    7.15. Prevent Cross-Site (XSS) Malicious Content
    7.16. Foil Semantic Attacks
    7.17. Be Careful with Data Types
8. Carefully Call Out to Other Resources
    8.1. Call Only Safe Library Routines
    8.2. Limit Call-outs to Valid Values
    8.3. Handle Metacharacters
    8.4. Call Only Interfaces Intended for Programmers
    8.5. Check All System Call Returns
    8.6. Avoid Using vfork(2)
    8.7. Counter Web Bugs When Retrieving Embedded Content
    8.8. Hide Sensitive Information
9. Send Information Back Judiciously
    9.1. Minimize Feedback
    9.2. Don't Include Comments
    9.3. Handle Full/Unresponsive Output
    9.4. Control Data Formatting (Format Strings/Formatation)
    9.5. Control Character Encoding in Output
    9.6. Prevent Include/Configuration File Access
10. Language-Specific Issues
    10.1. C/C++
    10.2. Perl
    10.3. Python
    10.4. Shell Scripting Languages (sh and csh Derivatives)
    10.5. Ada
    10.6. Java
    10.7. Tcl
    10.8. PHP
11. Special Topics
    11.1. Passwords
    11.2. Authenticating on the Web
    11.3. Random Numbers
    11.4. Specially Protect Secrets (Passwords and Keys) in User Memory
    11.5. Cryptographic Algorithms and Protocols
    11.6. Using PAM
    11.7. Tools
    11.8. Windows CE
    11.9. Write Audit Records
    11.10. Physical Emissions
    11.11. Miscellaneous
12. Conclusion
13. Bibliography
A. History
B. Acknowledgements
C. About the Documentation License
D. GNU Free Documentation License
E. Endorsements
F. About the Author

List of Tables
5-1. Legal UTF-8 Sequences

List of Figures
1-1. Abstract View of a Program

Chapter 1. Introduction

                                       A wise man attacks the city of the    
                                       mighty and pulls down the stronghold  
                                       in which they trust.                  
                                                         Proverbs 21:22 (NIV)

This book describes a set of guidelines for writing secure programs on Linux
and Unix systems. For purposes of this book, a ``secure program'' is a
program that sits on a security boundary, taking input from a source that
does not have the same access rights as the program. Such programs include
application programs used as viewers of remote data, web applications
(including CGI scripts), network servers, and setuid/setgid programs. This
book does not address modifying the operating system kernel itself, although
many of the principles discussed here do apply. These guidelines were
developed as a survey of ``lessons learned'' from various sources on how to
create such programs (along with additional observations by the author),
reorganized into a set of larger principles. This book includes specific
guidance for a number of languages, including C, C++, Java, Perl, PHP,
Python, Tcl, and Ada95.

You can find the master copy of this book at [
secure-programs] This book is also
part of the Linux Documentation Project (LDP) at [] http:/
/ It's also mirrored in several other places. Please note that
these mirrors, including the LDP copy and/or the copy in your distribution,
may be older than the master copy. I'd like to hear comments on this book,
but please do not send comments until you've checked to make sure that your
comment is valid for the latest version.

This book does not cover assurance measures, software engineering processes,
and quality assurance approaches, which are important but widely discussed
elsewhere. Such measures include testing, peer review, configuration
management, and formal methods. Documents specifically identifying sets of
development assurance measures for security issues include the Common
Criteria (CC, [CC 1999]) and the Systems Security Engineering Capability
Maturity Model [SSE-CMM 1999]. Inspections and other peer review techniques
are discussed in [Wheeler 1996]. This book does briefly discuss ideas from
the CC, but only as an organizational aid to discuss security requirements.
More general sets of software engineering processes are defined in documents
such as the Software Engineering Institute's Capability Maturity Model for
Software (SW-CMM) [Paulk 1993a, 1993b] and ISO 12207 [ISO 12207]. General
international standards for quality systems are defined in ISO 9000 and ISO
9001 [ISO 9000, 9001].  

This book does not discuss how to configure a system (or network) to be
secure in a given environment. This is clearly necessary for secure use of a
given program, but a great many other documents discuss secure
configurations. An excellent general book on configuring Unix-like systems to
be secure is Garfinkel [1996]. Other books for securing Unix-like systems
include Anonymous [1998]. You can also find information on configuring
Unix-like systems at web sites such as [
security.html] Information on
configuring a Linux system to be secure is available in a wide variety of
documents including Fenzi [1999], Seifried [1999], Wreski [1998], Swan
[2001], and Anonymous [1999]. Geodsoft [2001] describes how to harden
OpenBSD, and many of its suggestions are useful for any Unix-like system.
Information on auditing existing Unix-like systems are discussed in Mookhey
[2002]. For Linux systems (and eventually other Unix-like systems), you may
want to examine the Bastille Hardening System, which attempts to ``harden''
or ``tighten'' the Linux operating system. You can learn more about Bastille
at []; it is
available for free under the General Public License (GPL). Other hardening
systems include [] grsecurity. For Windows 2000, you
might want to look at Cox [2000]. The U.S. National Security Agency (NSA)
maintains a set of security recommendation guides at [http://], including the ``60 Minute
Network Security Guide.'' If you're trying to establish a public key
infrastructure (PKI) using open source tools, you might want to look at the
[] Open Source PKI Book. More about firewalls
and Internet security is found in [Cheswick 1994].

Configuring a computer is only part of Security Management, a larger area
that also covers how to deal with viruses, what kind of organizational
security policy is needed, business continuity plans, and so on. There are
international standards and guidance for security management. ISO 13335 is a
five-part technical report giving guidance on security management [ISO
13335]. ISO/IEC 17799:2000 defines a code of practice [ISO 17799]; its stated
purpose is to give high-level and general ``recommendations for information
security management for use by those who are responsible for initiating,
implementing or maintaining security in their organization.'' The document
specifically identifies itself as "a starting point for developing
organization specific guidance." It also states that not all of the guidance
and controls it contains may be applicable, and that additional controls not
contained may be required. Even more importantly, they are intended to be
broad guidelines covering a number of areas. and not intended to give
definitive details or "how-tos". It's worth noting that the original signing
of ISO/IEC 17799:2000 was controversial; Belgium, Canada, France, Germany,
Italy, Japan and the US voted against its adoption. However, it appears that
these votes were primarily a protest on parliamentary procedure, not on the
content of the document, and certainly people are welcome to use ISO 17799 if
they find it helpful. More information about ISO 17799 can be found in NIST's
17799:2000 FAQ. ISO 17799 is highly related to BS 7799 part 1 and 2; more
information about BS 7799 can be found at [] ISO 17799 is currently under revision. It's
important to note that none of these standards (ISO 13335, ISO 17799, or BS
7799 parts 1 and 2) are intended to be a detailed set of technical guidelines
for software developers; they are all intended to provide broad guidelines in
a number of areas. This is important, because software developers who simply
only follow (for example) ISO 17799 will generally not produce secure
software - developers need much, much, much more detail than ISO 17799

The Commonly Accepted Security Practices & Recommendations (CASPR) project at
[] is trying to distill information
security knowledge into a series of papers available to all (under the GNU
FDL license, so that future document derivatives will continue to be
available to all). Clearly, security management needs to include keeping with
patches as vulnerabilities are found and fixed. Beattie [2002] provides an
interesting analysis on how to determine when to apply patches contrasting
risk of a bad patch to the risk of intrusion (e.g., under certain conditions,
patches are optimally applied 10 or 30 days after they are released).

If you're interested in the current state of vulnerabilities, there are other
resources available to use. The CVE at gives a standard
identifier for each (widespread) vulnerability. The paper [http://] SecurityTracker
Statistics analyzes vulnerabilities to determine what were the most common
vulnerabilities. The Internet Storm Center at shows
the prominence of various Internet attacks around the world.

This book assumes that the reader understands computer security issues in
general, the general security model of Unix-like systems, networking (in
particular TCP/IP based networks), and the C programming language. This book
does include some information about the Linux and Unix programming model for
security. If you need more information on how TCP/IP based networks and
protocols work, including their security protocols, consult general works on
TCP/IP such as [Murhammer 1998].

When I first began writing this document, there were many short articles but
no books on writing secure programs. There are now two other books on writing
secure programs. One is ``Building Secure Software'' by John Viega and Gary
McGraw [Viega 2002]; this is a very good book that discusses a number of
important security issues, but it omits a large number of important security
problems that are instead covered here. Basically, this book selects several
important topics and covers them well, but at the cost of omitting many other
important topics. The Viega book has a little more information for Unix-like
systems than for Windows systems, but much of it is independent of the kind
of system. The other book is ``Writing Secure Code'' by Michael Howard and
David LeBlanc [Howard 2002]. The title of this other book is misleading; the
book is solely about writing secure programs for Windows, and is basically
worthless if you are writing programs for any other system. This shouldn't be
surprising; it's published by Microsoft press, and its copyright is owned by
Microsoft. If you are trying to write secure programs for Microsoft's Windows
systems, it's a good book. Another useful source of secure programming
guidance is the The Open Web Application Security Project (OWASP) Guide to
Building Secure Web Applications and Web Services; it has more on process,
and less specifics than this book, but it has useful material in it.

This book covers all Unix-like systems, including Linux and the various
strains of Unix, and it particularly stresses Linux and provides details
about Linux specifically. There's some material specifically on Windows CE,
and in fact much of this material is not limited to a particular operating
system. If you know relevant information not already included here, please
let me know.

This book is copyright (C) 1999-2002 David A. Wheeler and is covered by the
GNU Free Documentation License (GFDL); see Appendix C and Appendix D for more

Chapter 2 discusses the background of Unix, Linux, and security. Chapter 3
describes the general Unix and Linux security model, giving an overview of
the security attributes and operations of processes, filesystem objects, and
so on. This is followed by the meat of this book, a set of design and
implementation guidelines for developing applications on Linux and Unix
systems. The book ends with conclusions in Chapter 12, followed by a lengthy
bibliography and appendixes.

The design and implementation guidelines are divided into categories which I
believe emphasize the programmer's viewpoint. Programs accept inputs, process
data, call out to other resources, and produce output, as shown in Figure 1-1
; notionally all security guidelines fit into one of these categories. I've
subdivided ``process data'' into structuring program internals and approach,
avoiding buffer overflows (which in some cases can also be considered an
input issue), language-specific information, and special topics. The chapters
are ordered to make the material easier to follow. Thus, the book chapters
giving guidelines discuss validating all input (Chapter 5), avoiding buffer
overflows (Chapter 6), structuring program internals and approach (Chapter 7
), carefully calling out to other resources (Chapter 8), judiciously sending
information back (Chapter 9), language-specific information (Chapter 10), and
finally information on special topics such as how to acquire random numbers (
Chapter 11).

Figure 1-1. Abstract View of a Program


Chapter 2. Background

                                       I issued an order and a search was    
                                       made, and it was found that this city 
                                       has a long history of revolt against  
                                       kings and has been a place of         
                                       rebellion and sedition.               
                                                              Ezra 4:19 (NIV)

2.1. History of Unix, Linux, and Open Source / Free Software

2.1.1. Unix

In 1969-1970, Kenneth Thompson, Dennis Ritchie, and others at AT&T Bell Labs
began developing a small operating system on a little-used PDP-7. The
operating system was soon christened Unix, a pun on an earlier operating
system project called MULTICS. In 1972-1973 the system was rewritten in the
programming language C, an unusual step that was visionary: due to this
decision, Unix was the first widely-used operating system that could switch
from and outlive its original hardware. Other innovations were added to Unix
as well, in part due to synergies between Bell Labs and the academic
community. In 1979, the ``seventh edition'' (V7) version of Unix was
released, the grandfather of all extant Unix systems.

After this point, the history of Unix becomes somewhat convoluted. The
academic community, led by Berkeley, developed a variant called the Berkeley
Software Distribution (BSD), while AT&T continued developing Unix under the
names ``System III'' and later ``System V''. In the late 1980's through early
1990's the ``wars'' between these two major strains raged. After many years
each variant adopted many of the key features of the other. Commercially,
System V won the ``standards wars'' (getting most of its interfaces into the
formal standards), and most hardware vendors switched to AT&T's System V.
However, System V ended up incorporating many BSD innovations, so the
resulting system was more a merger of the two branches. The BSD branch did
not die, but instead became widely used for research, for PC hardware, and
for single-purpose servers (e.g., many web sites use a BSD derivative).

The result was many different versions of Unix, all based on the original
seventh edition. Most versions of Unix were proprietary and maintained by
their respective hardware vendor, for example, Sun Solaris is a variant of
System V. Three versions of the BSD branch of Unix ended up as open source:
FreeBSD (concentrating on ease-of-installation for PC-type hardware), NetBSD
(concentrating on many different CPU architectures), and a variant of NetBSD,
OpenBSD (concentrating on security). More general information about Unix
history can be found at [
and []
unix.html. Much more information about the BSD history can be found in
[McKusick 1999] and [

A slightly old but interesting advocacy piece that presents arguments for
using Unix-like systems (instead of Microsoft's products) is [http://] John Kirch's
paper ``Microsoft Windows NT Server 4.0 versus UNIX''.

2.1.2. Free Software Foundation

In 1984 Richard Stallman's Free Software Foundation (FSF) began the GNU
project, a project to create a free version of the Unix operating system. By
free, Stallman meant software that could be freely used, read, modified, and
redistributed. The FSF successfully built a vast number of useful components,
including a C compiler (gcc), an impressive text editor (emacs), and a host
of fundamental tools. However, in the 1990's the FSF was having trouble
developing the operating system kernel [FSF 1998]; without a kernel their
dream of a completely free operating system would not be realized.

2.1.3. Linux

In 1991 Linus Torvalds began developing an operating system kernel, which he
named ``Linux'' [Torvalds 1999]. This kernel could be combined with the FSF
material and other components (in particular some of the BSD components and
MIT's X-windows software) to produce a freely-modifiable and very useful
operating system. This book will term the kernel itself the ``Linux kernel''
and an entire combination as ``Linux''. Note that many use the term ``GNU/
Linux'' instead for this combination.

In the Linux community, different organizations have combined the available
components differently. Each combination is called a ``distribution'', and
the organizations that develop distributions are called ``distributors''.
Common distributions include Red Hat, Mandrake, SuSE, Caldera, Corel, and
Debian. There are differences between the various distributions, but all
distributions are based on the same foundation: the Linux kernel and the GNU
glibc libraries. Since both are covered by ``copyleft'' style licenses,
changes to these foundations generally must be made available to all, a
unifying force between the Linux distributions at their foundation that does
not exist between the BSD and AT&T-derived Unix systems. This book is not
specific to any Linux distribution; when it discusses Linux it presumes Linux
kernel version 2.2 or greater and the C library glibc 2.1 or greater, valid
assumptions for essentially all current major Linux distributions.

2.1.4. Open Source / Free Software

Increased interest in software that is freely shared has made it increasingly
necessary to define and explain it. A widely used term is ``open source
software'', which is further defined in [OSI 1999]. Eric Raymond [1997, 1998]
wrote several seminal articles examining its various development processes.
Another widely-used term is ``free software'', where the ``free'' is short
for ``freedom'': the usual explanation is ``free speech, not free beer.''
Neither phrase is perfect. The term ``free software'' is often confused with
programs whose executables are given away at no charge, but whose source code
cannot be viewed, modified, or redistributed. Conversely, the term ``open
source'' is sometime (ab)used to mean software whose source code is visible,
but for which there are limitations on use, modification, or redistribution.
This book uses the term ``open source'' for its usual meaning, that is,
software which has its source code freely available for use, viewing,
modification, and redistribution; a more detailed definition is contained in
the [] Open Source Definition. In some
cases, a difference in motive is suggested; those preferring the term ``free
software'' wish to strongly emphasize the need for freedom, while those using
the term may have other motives (e.g., higher reliability) or simply wish to
appear less strident. For information on this definition of free software,
and the motivations behind it, can be found at [] http://

Those interested in reading advocacy pieces for open source software and free
software should see [] and
[] There are other documents which
examine such software, for example, Miller [1995] found that the open source
software were noticeably more reliable than proprietary software (using their
measurement technique, which measured resistance to crashing due to random

2.1.5. Comparing Linux and Unix

This book uses the term ``Unix-like'' to describe systems intentionally like
Unix. In particular, the term ``Unix-like'' includes all major Unix variants
and Linux distributions. Note that many people simply use the term ``Unix''
to describe these systems instead. Originally, the term ``Unix'' meant a
particular product developed by AT&T. Today, the Open Group owns the Unix
trademark, and it defines Unix as ``the worldwide Single UNIX

Linux is not derived from Unix source code, but its interfaces are
intentionally like Unix. Therefore, Unix lessons learned generally apply to
both, including information on security. Most of the information in this book
applies to any Unix-like system. Linux-specific information has been
intentionally added to enable those using Linux to take advantage of Linux's

Unix-like systems share a number of security mechanisms, though there are
subtle differences and not all systems have all mechanisms available. All
include user and group ids (uids and gids) for each process and a filesystem
with read, write, and execute permissions (for user, group, and other). See
Thompson [1974] and Bach [1986] for general information on Unix systems,
including their basic security mechanisms. Chapter 3 summarizes key security
features of Unix and Linux.

2.2. Security Principles

There are many general security principles which you should be familiar with;
one good place for general information on information security is the
Information Assurance Technical Framework (IATF) [NSA 2000]. NIST has
identified high-level ``generally accepted principles and practices''
[Swanson 1996]. You could also look at a general textbook on computer
security, such as [Pfleeger 1997]. NIST Special Publication 800-27 describes
a number of good engineering principles (although, since they're abstract,
they're insufficient for actually building secure programs - hence this
book); you can get a copy at [
sp800-27.pdf. A few security principles are summarized here.

Often computer security objectives (or goals) are described in terms of three
overall objectives:

  * Confidentiality (also known as secrecy), meaning that the computing
    system's assets can be read only by authorized parties.
  * Integrity, meaning that the assets can only be modified or deleted by
    authorized parties in authorized ways.
  * Availability, meaning that the assets are accessible to the authorized
    parties in a timely manner (as determined by the systems requirements).
    The failure to meet this goal is called a denial of service.

Some people define additional major security objectives, while others lump
those additional goals as special cases of these three. For example, some
separately identify non-repudiation as an objective; this is the ability to
``prove'' that a sender sent or receiver received a message (or both), even
if the sender or receiver wishes to deny it later. Privacy is sometimes
addressed separately from confidentiality; some define this as protecting the
confidentiality of a user (e.g., their identity) instead of the data. Most
objectives require identification and authentication, which is sometimes
listed as a separate objective. Often auditing (also called accountability)
is identified as a desirable security objective. Sometimes ``access control''
and ``authenticity'' are listed separately as well. For example, The U.S.
Department of Defense (DoD), in DoD directive 3600.1 defines ``information
assurance'' as ``information operations (IO) that protect and defend
information and information systems by ensuring their availability,
integrity, authentication, confidentiality, and nonrepudiation. This includes
providing for restoration of information systems by incorporating protection,
detection, and reaction capabilities.''

In any case, it is important to identify your program's overall security
objectives, no matter how you group them together, so that you'll know when
you've met them.

Sometimes these objectives are a response to a known set of threats, and
sometimes some of these objectives are required by law. For example, for U.S.
banks and other financial institutions, there's a new privacy law called the
``Gramm-Leach-Bliley'' (GLB) Act. This law mandates disclosure of personal
information shared and means of securing that data, requires disclosure of
personal information that will be shared with third parties, and directs
institutions to give customers a chance to opt out of data sharing. [Jones

There is sometimes conflict between security and some other general system/
software engineering principles. Security can sometimes interfere with ``ease
of use'', for example, installing a secure configuration may take more effort
than a ``trivial'' installation that works but is insecure. Often, this
apparent conflict can be resolved, for example, by re-thinking a problem it's
often possible to make a secure system also easy to use. There's also
sometimes a conflict between security and abstraction (information hiding);
for example, some high-level library routines may be implemented securely or
not, but their specifications won't tell you. In the end, if your application
must be secure, you must do things yourself if you can't be sure otherwise -
yes, the library should be fixed, but it's your users who will be hurt by
your poor choice of library routines.

A good general security principle is ``defense in depth''; you should have
numerous defense mechanisms (``layers'') in place, designed so that an
attacker has to defeat multiple mechanisms to perform a successful attack.

2.3. Why do Programmers Write Insecure Code?

Many programmers don't intend to write insecure code - but do anyway. Here
are a number of purported reasons for this. Most of these were collected and
summarized by Aleph One on Bugtraq (in a posting on December 17, 1998):

  * There is no curriculum that addresses computer security in most schools.
    Even when there is a computer security curriculum, they often don't
    discuss how to write secure programs as a whole. Many such curriculum
    only study certain areas such as cryptography or protocols. These are
    important, but they often fail to discuss common real-world issues such
    as buffer overflows, string formatting, and input checking. I believe
    this is one of the most important problems; even those programmers who go
    through colleges and universities are very unlikely to learn how to write
    secure programs, yet we depend on those very people to write secure
  * Programming books/classes do not teach secure/safe programming
    techniques. Indeed, until recently there were no books on how to write
    secure programs at all (this book is one of those few).
  * No one uses formal verification methods.
  * C is an unsafe language, and the standard C library string functions are
    unsafe. This is particularly important because C is so widely used - the
    ``simple'' ways of using C permit dangerous exploits.
  * Programmers do not think ``multi-user.''
  * Programmers are human, and humans are lazy. Thus, programmers will often
    use the ``easy'' approach instead of a secure approach - and once it
    works, they often fail to fix it later.
  * Most programmers are simply not good programmers.
  * Most programmers are not security people; they simply don't often think
    like an attacker does.
  * Most security people are not programmers. This was a statement made by
    some Bugtraq contributors, but it's not clear that this claim is really
  * Most computer security models are terrible.
  * There is lots of ``broken'' legacy software. Fixing this software (to
    remove security faults or to make it work with more restrictive security
    policies) is difficult.
  * Consumers don't care about security. (Personally, I have hope that
    consumers are beginning to care about security; a computer system that is
    constantly exploited is neither useful nor user-friendly. Also, many
    consumers are unaware that there's even a problem, assume that it can't
    happen to them, or think that that things cannot be made better.)
  * Security costs extra development time.
  * Security costs in terms of additional testing (red teams, etc.).

2.4. Is Open Source Good for Security?

There's been a lot of debate by security practitioners about the impact of
open source approaches on security. One of the key issues is that open source
exposes the source code to examination by everyone, both the attackers and
defenders, and reasonable people disagree about the ultimate impact of this
situation. (Note - you can get the latest version of this essay by going to
the main website for this book, []

2.4.1. View of Various Experts

First, let's exampine what security experts have to say.

Bruce Schneier is a well-known expert on computer security and cryptography.
He argues that smart engineers should ``demand open source code for anything
related to security'' [Schneier 1999], and he also discusses some of the
preconditions which must be met to make open source software secure. Vincent
Rijmen, a developer of the winning Advanced Encryption Standard (AES)
encryption algorithm, believes that the open source nature of Linux provides
a superior vehicle to making security vulnerabilities easier to spot and fix,
``Not only because more people can look at it, but, more importantly, because
the model forces people to write more clear code, and to adhere to standards.
This in turn facilitates security review'' [Rijmen 2000].

Elias Levy (Aleph1) is the former moderator of one of the most popular
security discussion groups - Bugtraq. He discusses some of the problems in
making open source software secure in his article "Is Open Source Really More
Secure than Closed?". His summary is:

    So does all this mean Open Source Software is no better than closed
    source software when it comes to security vulnerabilities? No. Open
    Source Software certainly does have the potential to be more secure than
    its closed source counterpart. But make no mistake, simply being open
    source is no guarantee of security.
Whitfield Diffie is the co-inventor of public-key cryptography (the basis of
all Internet security) and chief security officer and senior staff engineer
at Sun Microsystems. In his 2003 article [
2100-1107-980938.html] Risky business: Keeping security a secret, he argues
that proprietary vendor's claims that their software is more secure because
it's secret is nonsense. He identifies and then counters two main claims made
by proprietary vendors: (1) that release of code benefits attackers more than
anyone else because a lot of hostile eyes can also look at open-source code,
and that (2) a few expert eyes are better than several random ones. He first
notes that while giving programmers access to a piece of software doesn't
guarantee they will study it carefully, there is a group of programmers who
can be expected to care deeply: Those who either use the software personally
or work for an enterprise that depends on it. "In fact, auditing the programs
on which an enterprise depends for its own security is a natural function of
the enterprise's own information-security organization." He then counters the
second argument, noting that "As for the notion that open source's usefulness
to opponents outweighs the advantages to users, that argument flies in the
face of one of the most important principles in security: A secret that
cannot be readily changed should be regarded as a vulnerability." He closes
noting that

    "It's simply unrealistic to depend on secrecy for security in computer
    software. You may be able to keep the exact workings of the program out
    of general circulation, but can you prevent the code from being
    reverse-engineered by serious opponents? Probably not."

John Viega's article [
000526_security.html] "The Myth of Open Source Security" also discusses
issues, and summarizes things this way:

    Open source software projects can be more secure than closed source
    projects. However, the very things that can make open source programs
    secure -- the availability of the source code, and the fact that large
    numbers of users are available to look for and fix security holes -- can
    also lull people into a false sense of security.
[] Michael
H. Warfield's "Musings on open source security" is very positive about the
impact of open source software on security. In contrast, Fred Schneider
doesn't believe that open source helps security, saying ``there is no reason
to believe that the many eyes inspecting (open) source code would be
successful in identifying bugs that allow system security to be compromised''
and claiming that ``bugs in the code are not the dominant means of attack''
[Schneider 2000]. He also claims that open source rules out control of the
construction process, though in practice there is such control - all major
open source programs have one or a few official versions with ``owners'' with
reputations at stake. Peter G. Neumann discusses ``open-box'' software (in
which source code is available, possibly only under certain conditions),
saying ``Will open-box software really improve system security? My answer is
not by itself, although the potential is considerable'' [Neumann 2000].
TruSecure Corporation, under sponsorship by Red Hat (an open source company),
has developed a paper on why they believe open source is more effective for
security [TruSecure 2001]. [
library/l-oss.html?open&I=252,t=gr,p=SeclmpOS] Natalie Walker Whitlock's IBM
DeveloperWorks article discusses the pros and cons as well. Brian Witten,
Carl Landwehr, and Micahel Caloyannides [Witten 2001] published in IEEE
Software an article tentatively concluding that having source code available
should work in the favor of system security; they note:

    ``We can draw four additional conclusions from this discussion. First,
    access to source code lets users improve system security -- if they have
    the capability and resources to do so. Second, limited tests indicate
    that for some cases, open source life cycles produce systems that are
    less vulnerable to nonmalicious faults. Third, a survey of three
    operating systems indicates that one open source operating system
    experienced less exposure in the form of known but unpatched
    vulnerabilities over a 12-month period than was experienced by either of
    two proprietary counterparts. Last, closed and proprietary system
    development models face disincentives toward fielding and supporting more
    secure systems as long as less secure systems are more profitable.
    Notwithstanding these conclusions, arguments in this important matter are
    in their formative stages and in dire need of metrics that can reflect
    security delivered to the customer.''
Scott A. Hissam and Daniel Plakosh's [
/New/IEE_hissam.pdf] ``Trust and Vulnerability in Open Source Software''
discuss the pluses and minuses of open source software. As with other papers,
they note that just because the software is open to review, it should not
automatically follow that such a review has actually been performed. Indeed,
they note that this is a general problem for all software, open or closed -
it is often questionable if many people examine any given piece of software.
One interesting point is that they demonstrate that attackers can learn about
a vulnerability in a closed source program (Windows) from patches made to an
OSS/FS program (Linux). In this example, Linux developers fixed a
vulnerability before attackers tried to attack it, and attackers correctly
surmised that a similar problem might be still be in Windows (and it was).
Unless OSS/FS programs are forbidden, this kind of learning is difficult to
prevent. Therefore, the existance of an OSS/FS program can reveal the
vulnerabilities of both the OSS/FS and proprietary program performing the
same function - but at in this example, the OSS/FS program was fixed first.

2.4.2. Why Closing the Source Doesn't Halt Attacks

It's been argued that a system without source code is more secure because,
since there's less information available for an attacker, it should be harder
for an attacker to find the vulnerabilities. This argument has a number of
weaknesses, however, because although source code is extremely important when
trying to add new capabilities to a program, attackers generally don't need
source code to find a vulnerability.

First, it's important to distinguish between ``destructive'' acts and
``constructive'' acts. In the real world, it is much easier to destroy a car
than to build one. In the software world, it is much easier to find and
exploit a vulnerability than to add new significant new functionality to that
software. Attackers have many advantages against defenders because of this
difference. Software developers must try to have no security-relevant
mistakes anywhere in their code, while attackers only need to find one.
Developers are primarily paid to get their programs to work... attackers
don't need to make the program work, they only need to find a single
weakness. And as I'll describe in a moment, it takes less information to
attack a program than to modify one.

Generally attackers (against both open and closed programs) start by knowing
about the general kinds of security problems programs have. There's no point
in hiding this information; it's already out, and in any case, defenders need
that kind of information to defend themselves. Attackers then use techniques
to try to find those problems; I'll group the techniques into ``dynamic''
techniques (where you run the program) and ``static'' techniques (where you
examine the program's code - be it source code or machine code).

In ``dynamic'' approaches, an attacker runs the program, sending it data
(often problematic data), and sees if the programs' response indicates a
common vulnerability. Open and closed programs have no difference here, since
the attacker isn't looking at code. Attackers may also look at the code, the
``static'' approach. For open source software, they'll probably look at the
source code and search it for patterns. For closed source software, they
might search the machine code (usually presented in assembly language format
to simplify the task) for essentially the same patterns. They might also use
tools called ``decompilers'' that turn the machine code back into source code
and then search the source code for the vulnerable patterns (the same way
they would search for vulnerabilities in open source software). See Flake
[2001] for one discussion of how closed code can still be examined for
security vulnerabilities (e.g., using disassemblers). This point is
important: even if an attacker wanted to use source code to find a
vulnerability, a closed source program has no advantage, because the attacker
can use a disassembler to re-create the source code of the product.

Non-developers might ask ``if decompilers can create source code from machine
code, then why do developers say they need source code instead of just
machine code?'' The problem is that although developers don't need source
code to find security problems, developers do need source code to make
substantial improvements to the program. Although decompilers can turn
machine code back into a ``source code'' of sorts, the resulting source code
is extremely hard to modify. Typically most understandable names are lost, so
instead of variables like ``grand_total'' you get ``x123123'', instead of
methods like ``display_warning'' you get ``f123124'', and the code itself may
have spatterings of assembly in it. Also, _ALL_ comments and design
information are lost. This isn't a serious problem for finding security
problems, because generally you're searching for patterns indicating
vulnerabilities, not for internal variable or method names. Thus, decompilers
can be useful for finding ways to attack programs, but aren't helpful for
updating programs.

Thus, developers will say ``source code is vital'' when they intend to add
functionality), but the fact that the source code for closed source programs
is hidden doesn't protect the program very much.

2.4.3. Why Keeping Vulnerabilities Secret Doesn't Make Them Go Away

Sometimes it's noted that a vulnerability that exists but is unknown can't be
exploited, so the system ``practically secure.'' In theory this is true, but
the problem is that once someone finds the vulnerability, the finder may just
exploit the vulnerability instead of helping to fix it. Having unknown
vulnerabilities doesn't really make the vulnerabilities go away; it simply
means that the vulnerabilities are a time bomb, with no way to know when
they'll be exploited. Fundamentally, the problem of someone exploiting a
vulnerability they discover is a problem for both open and closed source

One related claim sometimes made (though not as directly related to OSS/FS)
is that people should not post warnings about vulnerabilities and discuss
them. This sounds good in theory, but the problem is that attackers already
distribute information about vulnerabilities through a large number of
channels. In short, such approaches would leave defenders vulnerable, while
doing nothing to inhibit attackers. In the past, companies actively tried to
prevent disclosure of vulnerabilities, but experience showed that, in
general, companies didn't fix vulnerabilities until they were widely known to
their users (who could then insist that the vulnerabilities be fixed). This
is all part of the argument for ``full disclosure.'' Gartner Group has a
blunt commentary in a article titled ``Commentary: Hype is the real
issue - Tech News.'' They stated:

    The comments of Microsoft's Scott Culp, manager of the company's security
    response center, echo a common refrain in a long, ongoing battle over
    information. Discussions of morality regarding the distribution of
    information go way back and are very familiar. Several centuries ago, for
    example, the church tried to squelch Copernicus' and Galileo's theory of
    the sun being at the center of the solar system... Culp's attempt to
    blame "information security professionals" for the recent spate of
    vulnerabilities in Microsoft products is at best disingenuous. Perhaps,
    it also represents an attempt to deflect criticism from the company that
    built those products... [The] efforts of all parties contribute to a
    continuous process of improvement. The more widely vulnerabilities become
    known, the more quickly they get fixed.

2.4.4. How OSS/FS Counters Trojan Horses

It's sometimes argued that open source programs, because there's no enforced
control by a single company, permit people to insert Trojan Horses and other
malicious code. Trojan horses can be inserted into open source code, true,
but they can also be inserted into proprietary code. A disgruntled or bribed
employee can insert malicious code, and in many organizations it's much less
likely to be found than in an open source program. After all, no one outside
the organization can review the source code, and few companies review their
code internally (or, even if they do, few can be assured that the reviewed
code is actually what is used). And the notion that a closed-source company
can be sued later has little evidence; nearly all licenses disclaim all
warranties, and courts have generally not held software development companies

Borland's InterBase server is an interesting case in point. Some time between
1992 and 1994, Borland inserted an intentional ``back door'' into their
database server, ``InterBase''. This back door allowed any local or remote
user to manipulate any database object and install arbitrary programs, and in
some cases could lead to controlling the machine as ``root''. This
vulnerability stayed in the product for at least 6 years - no one else could
review the product, and Borland had no incentive to remove the vulnerability.
Then Borland released its source code on July 2000. The "Firebird" project
began working with the source code, and uncovered this serious security
problem with InterBase in December 2000. By January 2001 the CERT announced
the existence of this back door as CERT advisory CA-2001-01. What's
discouraging is that the backdoor can be easily found simply by looking at an
ASCII dump of the program (a common cracker trick). Once this problem was
found by open source developers reviewing the code, it was patched quickly.
You could argue that, by keeping the password unknown, the program stayed
safe, and that opening the source made the program less secure. I think this
is nonsense, since ASCII dumps are trivial to do and well-known as a standard
attack technique, and not all attackers have sudden urges to announce
vulnerabilities - in fact, there's no way to be certain that this
vulnerability has not been exploited many times. It's clear that after the
source was opened, the source code was reviewed over time, and the
vulnerabilities found and fixed. One way to characterize this is to say that
the original code was vulnerable, its vulnerabilities became easier to
exploit when it was first made open source, and then finally these
vulnerabilities were fixed.

2.4.5. Other Advantages

The advantages of having source code open extends not just to software that
is being attacked, but also extends to vulnerability assessment scanners.
Vulnerability assessment scanners intentionally look for vulnerabilities in
configured systems. A recent Network Computing evaluation found that the best
scanner (which, among other things, found the most legitimate
vulnerabilities) was Nessus, an open source scanner [Forristal 2001].

2.4.6. Bottom Line

So, what's the bottom line? I personally believe that when a program began as
closed source and is then first made open source, it often starts less secure
for any users (through exposure of vulnerabilities), and over time (say a few
years) it has the potential to be much more secure than a closed program. If
the program began as open source software, the public scrutiny is more likely
to improve its security before it's ready for use by significant numbers of
users, but there are several caveats to this statement (it's not an ironclad
rule). Just making a program open source doesn't suddenly make a program
secure, and just because a program is open source does not guarantee

  * First, people have to actually review the code. This is one of the key
    points of debate - will people really review code in an open source
    project? All sorts of factors can reduce the amount of review: being a
    niche or rarely-used product (where there are few potential reviewers),
    having few developers, and use of a rarely-used computer language.
    Clearly, a program that has a single developer and no other contributors
    of any kind doesn't have this kind of review. On the other hand, a
    program that has a primary author and many other people who occasionally
    examine the code and contribute suggests that there are others reviewing
    the code (at least to create contributions). In general, if there are
    more reviewers, there's generally a higher likelihood that someone will
    identify a flaw - this is the basis of the ``many eyeballs'' theory. Note
    that, for example, the OpenBSD project continuously examines programs for
    security flaws, so the components in its innermost parts have certainly
    undergone a lengthy review. Since OSS/FS discussions are often held
    publicly, this level of review is something that potential users can
    judge for themselves.
    One factor that can particularly reduce review likelihood is not actually
    being open source. Some vendors like to posture their ``disclosed
    source'' (also called ``source available'') programs as being open
    source, but since the program owner has extensive exclusive rights,
    others will have far less incentive to work ``for free'' for the owner on
    the code. Even open source licenses which have unusually asymmetric
    rights (such as the MPL) have this problem. After all, people are less
    likely to voluntarily participate if someone else will have rights to
    their results that they don't have (as Bruce Perens says, ``who wants to
    be someone else's unpaid employee?''). In particular, since the reviewers
    with the most incentive tend to be people trying to modify the program,
    this disincentive to participate reduces the number of ``eyeballs''.
    Elias Levy made this mistake in his article about open source security;
    his examples of software that had been broken into (e.g., TIS's Gauntlet)
    were not, at the time, open source.
  * Second, at least some of the people developing and reviewing the code
    must know how to write secure programs. Hopefully the existence of this
    book will help. Clearly, it doesn't matter if there are ``many eyeballs''
    if none of the eyeballs know what to look for. Note that it's not
    necessary for everyone to know how to write secure programs, as long as
    those who do know how are examining the code changes.
  * Third, once found, these problems need to be fixed quickly and their
    fixes distributed. Open source systems tend to fix the problems quickly,
    but the distribution is not always smooth. For example, the OpenBSD
    developers do an excellent job of reviewing code for security flaws - but
    they don't always report the identified problems back to the original
    developer. Thus, it's quite possible for there to be a fixed version in
    one system, but for the flaw to remain in another. I believe this problem
    is lessening over time, since no one ``downstream'' likes to repeatedly
    fix the same problem. Of course, ensuring that security patches are
    actually installed on end-user systems is a problem for both open source
    and closed source software.

Another advantage of open source is that, if you find a problem, you can fix
it immediately. This really doesn't have any counterpart in closed source.

In short, the effect on security of open source software is still a major
debate in the security community, though a large number of prominent experts
believe that it has great potential to be more secure.

2.5. Types of Secure Programs

Many different types of programs may need to be secure programs (as the term
is defined in this book). Some common types are:

  * Application programs used as viewers of remote data. Programs used as
    viewers (such as word processors or file format viewers) are often asked
    to view data sent remotely by an untrusted user (this request may be
    automatically invoked by a web browser). Clearly, the untrusted user's
    input should not be allowed to cause the application to run arbitrary
    programs. It's usually unwise to support initialization macros (run when
    the data is displayed); if you must, then you must create a secure
    sandbox (a complex and error-prone task that almost never succeeds, which
    is why you shouldn't support macros in the first place). Be careful of
    issues such as buffer overflow, discussed in Chapter 6, which might allow
    an untrusted user to force the viewer to run an arbitrary program.
  * Application programs used by the administrator (root). Such programs
    shouldn't trust information that can be controlled by non-administrators.
  * Local servers (also called daemons).
  * Network-accessible servers (sometimes called network daemons).
  * Web-based applications (including CGI scripts). These are a special case
    of network-accessible servers, but they're so common they deserve their
    own category. Such programs are invoked indirectly via a web server,
    which filters out some attacks but nevertheless leaves many attacks that
    must be withstood.
  * Applets (i.e., programs downloaded to the client for automatic
    execution). This is something Java is especially famous for, though other
    languages (such as Python) support mobile code as well. There are several
    security viewpoints here; the implementer of the applet infrastructure on
    the client side has to make sure that the only operations allowed are
    ``safe'' ones, and the writer of an applet has to deal with the problem
    of hostile hosts (in other words, you can't normally trust the client).
    There is some research attempting to deal with running applets on hostile
    hosts, but frankly I'm skeptical of the value of these approaches and
    this subject is exotic enough that I don't cover it further here.
  * setuid/setgid programs. These programs are invoked by a local user and,
    when executed, are immediately granted the privileges of the program's
    owner and/or owner's group. In many ways these are the hardest programs
    to secure, because so many of their inputs are under the control of the
    untrusted user and some of those inputs are not obvious.


This book merges the issues of these different types of program into a single
set. The disadvantage of this approach is that some of the issues identified
here don't apply to all types of programs. In particular, setuid/setgid
programs have many surprising inputs and several of the guidelines here only
apply to them. However, things are not so clear-cut, because a particular
program may cut across these boundaries (e.g., a CGI script may be setuid or
setgid, or be configured in a way that has the same effect), and some
programs are divided into several executables each of which can be considered
a different ``type'' of program. The advantage of considering all of these
program types together is that we can consider all issues without trying to
apply an inappropriate category to a program. As will be seen, many of the
principles apply to all programs that need to be secured.

There is a slight bias in this book toward programs written in C, with some
notes on other languages such as C++, Perl, PHP, Python, Ada95, and Java.
This is because C is the most common language for implementing secure
programs on Unix-like systems (other than CGI scripts, which tend to use
languages such as Perl, PHP, or Python). Also, most other languages'
implementations call the C library. This is not to imply that C is somehow
the ``best'' language for this purpose, and most of the principles described
here apply regardless of the programming language used.

2.6. Paranoia is a Virtue

The primary difficulty in writing secure programs is that writing them
requires a different mind-set, in short, a paranoid mind-set. The reason is
that the impact of errors (also called defects or bugs) can be profoundly

Normal non-secure programs have many errors. While these errors are
undesirable, these errors usually involve rare or unlikely situations, and if
a user should stumble upon one they will try to avoid using the tool that way
in the future.

In secure programs, the situation is reversed. Certain users will
intentionally search out and cause rare or unlikely situations, in the hope
that such attacks will give them unwarranted privileges. As a result, when
writing secure programs, paranoia is a virtue.

2.7. Why Did I Write This Document?

One question I've been asked is ``why did you write this book''? Here's my
answer: Over the last several years I've noticed that many developers for
Linux and Unix seem to keep falling into the same security pitfalls, again
and again. Auditors were slowly catching problems, but it would have been
better if the problems weren't put into the code in the first place. I
believe that part of the problem was that there wasn't a single, obvious
place where developers could go and get information on how to avoid known
pitfalls. The information was publicly available, but it was often hard to
find, out-of-date, incomplete, or had other problems. Most such information
didn't particularly discuss Linux at all, even though it was becoming widely
used! That leads up to the answer: I developed this book in the hope that
future software developers won't repeat past mistakes, resulting in more
secure systems. You can see a larger discussion of this at [http://] http://

A related question that could be asked is ``why did you write your own book
instead of just referring to other documents''? There are several answers:

  * Much of this information was scattered about; placing the critical
    information in one organized document makes it easier to use.
  * Some of this information is not written for the programmer, but is
    written for an administrator or user.
  * Much of the available information emphasizes portable constructs
    (constructs that work on all Unix-like systems), and failed to discuss
    Linux at all. It's often best to avoid Linux-unique abilities for
    portability's sake, but sometimes the Linux-unique abilities can really
    aid security. Even if non-Linux portability is desired, you may want to
    support the Linux-unique abilities when running on Linux. And, by
    emphasizing Linux, I can include references to information that is
    helpful to someone targeting Linux that is not necessarily true for


2.8. Sources of Design and Implementation Guidelines

Several documents help describe how to write secure programs (or,
alternatively, how to find security problems in existing programs), and were
the basis for the guidelines highlighted in the rest of this book. 

For general-purpose servers and setuid/setgid programs, there are a number of
valuable documents (though some are difficult to find without having a
reference to them).

Matt Bishop [1996, 1997] has developed several extremely valuable papers and
presentations on the topic, and in fact he has a web page dedicated to the
topic at [] http:// AUSCERT has released a
programming checklist [
secure_programming_checklist] [AUSCERT 1996], based in part on chapter 23 of
Garfinkel and Spafford's book discussing how to write secure SUID and network
programs [] [Garfinkel 1996]. [http://] Galvin [1998a] described
a simple process and checklist for developing secure programs; he later
updated the checklist in [
/swol-08-security.html] Galvin [1998b]. [
security-holes.html] Sitaker [1999] presents a list of issues for the ``Linux
security audit'' team to search for. [
review.html] Shostack [1999] defines another checklist for reviewing
security-sensitive code. The NCSA [
/security/programming] [NCSA] provides a set of terse but useful secure
programming guidelines. Other useful information sources include the Secure
Unix Programming FAQ [] [Al-Herbish 1999], the 
Security-Audit's Frequently Asked Questions [] [Graham
1999], and [] Ranum [1998]. Some
recommendations must be taken with caution, for example, the BSD setuid(7)
man page [] [Unknown] recommends
the use of access(3) without noting the dangerous race conditions that
usually accompany it. Wood [1985] has some useful but dated advice in its
``Security for Programmers'' chapter. [
talks] Bellovin [1994] includes useful guidelines and some specific examples,
such as how to restructure an ftpd implementation to be simpler and more
secure. FreeBSD provides some guidelines [
security.html] FreeBSD [1999] [
programming-guidelines/book1.html] [Quintero 1999] is primarily concerned
with GNOME programming guidelines, but it includes a section on security
considerations. [] [Venema 1996]
provides a detailed discussion (with examples) of some common errors when
programming secure programs (widely-known or predictable passwords, burning
yourself with malicious data, secrets in user-accessible data, and depending
on other programs). [] [Sibert 1996]
describes threats arising from malicious data. Michael Bacarella's article
[] The Peon's Guide To Secure
System Development provides a nice short set of guidelines.

There are many documents giving security guidelines for programs using the
Common Gateway Interface (CGI) to interface with the web. These include
[] Van Biesbrouck [1996],
[] Gundavaram
[unknown], [] [Garfinkle 1997]
[] Kim [1996], [
/paulp/cgi-security/safe-cgi.txt] Phillips [1995], [
Security/Faq/www-security-faq.html] Stein [1999], [
razvan.peteanu] [Peteanu 2000], and [
web-security.html] [Advosys 2000].

There are many documents specific to a language, which are further discussed
in the language-specific sections of this book. For example, the Perl
distribution includes [
perlsec.html] perlsec(1), which describes how to use Perl more securely. The
Secure Internet Programming site at [] http:// is interested in computer security issues in
general, but focuses on mobile code systems such as Java, ActiveX, and
JavaScript; Ed Felten (one of its principles) co-wrote a book on securing
Java ([] [McGraw 1999]) which is discussed in 
Section 10.6. Sun's security code guidelines provide some guidelines
primarily for Java and C; it is available at [

Yoder [1998] contains a collection of patterns to be used when dealing with
application security. It's not really a specific set of guidelines, but a set
of commonly-used patterns for programming that you may find useful. The
Schmoo group maintains a web page linking to information on how to write
secure code at []

There are many documents describing the issue from the other direction (i.e.,
``how to crack a system''). One example is McClure [1999], and there's
countless amounts of material from that vantage point on the Internet. There
are also more general documents on computer architectures on how attacks must
be developed to exploit them, e.g., [LSD 2001]. The Honeynet Project has been
collecting information (including statistics) on how attackers actually
perform their attacks; see their website at [] for more information.

There's also a large body of information on vulnerabilities already
identified in existing programs. This can be a useful set of examples of
``what not to do,'' though it takes effort to extract more general guidelines
from the large body of specific examples. There are mailing lists that
discuss security issues; one of the most well-known is [http://] Bugtraq, which among other things
develops a list of vulnerabilities. The CERT Coordination Center (CERT/CC) is
a major reporting center for Internet security problems which reports on
vulnerabilities. The CERT/CC occasionally produces advisories that provide a
description of a serious security problem and its impact, along with
instructions on how to obtain a patch or details of a workaround; for more
information see [] Note that
originally the CERT was a small computer emergency response team, but
officially ``CERT'' doesn't stand for anything now. The Department of
Energy's Computer Incident Advisory Capability (CIAC) also reports on
vulnerabilities. These different groups may identify the same vulnerabilities
but use different names. To resolve this problem, MITRE supports the Common
Vulnerabilities and Exposures (CVE) list which creates a single unique
identifier (``name'') for all publicly known vulnerabilities and security
exposures identified by others; see [] http:// NIST's ICAT is a searchable catalog of computer
vulnerabilities, categorizing each CVE vulnerability so that they can be
searched and compared later; see [] http://

This book is a summary of what I believe are the most useful and important
guidelines. My goal is a book that a good programmer can just read and then
be fairly well prepared to implement a secure program. No single document can
really meet this goal, but I believe the attempt is worthwhile. My objective
is to strike a balance somewhere between a ``complete list of all possible
guidelines'' (that would be unending and unreadable) and the various
``short'' lists available on-line that are nice and short but omit a large
number of critical issues. When in doubt, I include the guidance; I believe
in that case it's better to make the information available to everyone in
this ``one stop shop'' document. The organization presented here is my own
(every list has its own, different structure), and some of the guidelines
(especially the Linux-unique ones, such as those on capabilities and the
FSUID value) are also my own. Reading all of the referenced documents listed
above as well is highly recommended, though I realize that for many it's

2.9. Other Sources of Security Information

There are a vast number of web sites and mailing lists dedicated to security
issues. Here are some other sources of security information:

  * [] has a wealth of general
    security-related news and information, and hosts a number of
    security-related mailing lists. See their website for information on how
    to subscribe and view their archives. A few of the most relevant mailing
    lists on SecurityFocus are:
      + The ``Bugtraq'' mailing list is, as noted above, a ``full disclosure
        moderated mailing list for the detailed discussion and announcement
        of computer security vulnerabilities: what they are, how to exploit
        them, and how to fix them.''
      + The ``secprog'' mailing list is a moderated mailing list for the
        discussion of secure software development methodologies and
        techniques. I specifically monitor this list, and I coordinate with
        its moderator to ensure that resolutions reached in SECPROG (if I
        agree with them) are incorporated into this document.
      + The ``vuln-dev'' mailing list discusses potential or undeveloped
  * IBM's ``developerWorks: Security'' has a library of interesting articles.
    You can learn more from [] http://
  * For Linux-specific security information, a good source is [http://] If you're interested in
    auditing Linux code, places to see include the Linux Security-Audit
    Project FAQ and [] Linux Kernel Auditing Project are
    dedicated to auditing Linux code for security issues.

Of course, if you're securing specific systems, you should sign up to their
security mailing lists (e.g., Microsoft's, Red Hat's, etc.) so you can be
warned of any security updates.

2.10. Document Conventions

System manual pages are referenced in the format name(number), where number
is the section number of the manual. The pointer value that means ``does not
point anywhere'' is called NULL; C compilers will convert the integer 0 to
the value NULL in most circumstances where a pointer is needed, but note that
nothing in the C standard requires that NULL actually be implemented by a
series of all-zero bits. C and C++ treat the character '\0' (ASCII 0)
specially, and this value is referred to as NIL in this book (this is usually
called ``NUL'', but ``NUL'' and ``NULL'' sound identical). Function and
method names always use the correct case, even if that means that some
sentences must begin with a lower case letter. I use the term ``Unix-like''
to mean Unix, Linux, or other systems whose underlying models are very
similar to Unix; I can't say POSIX, because there are systems such as Windows
2000 that implement portions of POSIX yet have vastly different security

An attacker is called an ``attacker'', ``cracker'', or ``adversary'', and not
a ``hacker''. Some journalists mistakenly use the word ``hacker'' instead of
``attacker''; this book avoids this misuse, because many Linux and Unix
developers refer to themselves as ``hackers'' in the traditional non-evil
sense of the term. To many Linux and Unix developers, the term ``hacker''
continues to mean simply an expert or enthusiast, particularly regarding
computers. It is true that some hackers commit malicious or intrusive
actions, but many other hackers do not, and it's unfair to claim that all
hackers perform malicious activities. Many other glossaries and books note
that not all hackers are attackers. For example, the Industry Advisory
Council's Information Assurance (IA) Special Interest Group (SIG)'s [http://] Information Assurance Glossary defines
hacker as ``A person who delights in having an intimate understanding of the
internal workings of computers and computer networks. The term is misused in
a negative context where `cracker' should be used.'' The Jargon File has a
[] long and complicate
definition for hacker, starting with ``A person who enjoys exploring the
details of programmable systems and how to stretch their capabilities, as
opposed to most users, who prefer to learn only the minimum necessary.''; it
notes although some people use the term to mean ``A malicious meddler who
tries to discover sensitive information by poking around'', it also states
that this definition is deprecated and that the correct term for this sense
is ``cracker''.

This book uses the ``new'' or ``logical'' quoting system, instead of the
traditional American quoting system: quoted information does not include any
trailing punctuation if the punctuation is not part of the material being
quoted. While this may cause a minor loss of typographical beauty, the
traditional American system causes extraneous characters to be placed inside
the quotes. These extraneous characters have no effect on prose but can be
disastrous in code or computer commands. I use standard American (not
British) spelling; I've yet to meet an English speaker on any continent who
has trouble with this.

Chapter 3. Summary of Linux and Unix Security Features

                                       Discretion will protect you, and      
                                       understanding will guard you.         
                                                          Proverbs 2:11 (NIV)

Before discussing guidelines on how to use Linux or Unix security features,
it's useful to know what those features are from a programmer's viewpoint.
This section briefly describes those features that are widely available on
nearly all Unix-like systems. However, note that there is considerable
variation between different versions of Unix-like systems, and not all
systems have the abilities described here. This chapter also notes some
extensions or features specific to Linux; Linux distributions tend to be
fairly similar to each other from the point-of-view of programming for
security, because they all use essentially the same kernel and C library (and
the GPL-based licenses encourage rapid dissemination of any innovations). It
also notes some of the security-relevant differences between different Unix
implementations, but please note that this isn't an exhaustive list. This
chapter doesn't discuss issues such as implementations of mandatory access
control (MAC) which many Unix-like systems do not implement. If you already
know what those features are, please feel free to skip this section.

Many programming guides skim briefly over the security-relevant portions of
Linux or Unix and skip important information. In particular, they often
discuss ``how to use'' something in general terms but gloss over the security
attributes that affect their use. Conversely, there's a great deal of
detailed information in the manual pages about individual functions, but the
manual pages sometimes obscure key security issues with detailed discussions
on how to use each individual function. This section tries to bridge that
gap; it gives an overview of the security mechanisms in Linux that are likely
to be used by a programmer, but concentrating specifically on the security
ramifications. This section has more depth than the typical programming
guides, focusing specifically on security-related matters, and points to
references where you can get more details.

First, the basics. Linux and Unix are fundamentally divided into two parts:
the kernel and ``user space''. Most programs execute in user space (on top of
the kernel). Linux supports the concept of ``kernel modules'', which is
simply the ability to dynamically load code into the kernel, but note that it
still has this fundamental division. Some other systems (such as the HURD)
are ``microkernel'' based systems; they have a small kernel with more limited
functionality, and a set of ``user'' programs that implement the lower-level
functions traditionally implemented by the kernel.

Some Unix-like systems have been extensively modified to support strong
security, in particular to support U.S. Department of Defense requirements
for Mandatory Access Control (level B1 or higher). This version of this book
doesn't cover these systems or issues; I hope to expand to that in a future
version. More detailed information on some of them is available elsewhere,
for example, details on SGI's ``Trusted IRIX/B'' are available in NSA's Final
Evaluation Reports (FERs).

When users log in, their usernames are mapped to integers marking their
``UID'' (for ``user id'') and the ``GID''s (for ``group id'') that they are a
member of. UID 0 is a special privileged user (role) traditionally called
``root''; on most Unix-like systems (including Unix) root can overrule most
security checks and is used to administrate the system. On some Unix systems,
GID 0 is also special and permits unrestricted access to resources at the
group level [Gay 2000, 228]; this isn't true on other systems (such as
Linux), but even in those systems group 0 is essentially all-powerful because
so many special system files are owned by group 0. Processes are the only
``subjects'' in terms of security (that is, only processes are active
objects). Processes can access various data objects, in particular filesystem
objects (FSOs), System V Interprocess Communication (IPC) objects, and
network ports. Processes can also set signals. Other security-relevant topics
include quotas and limits, libraries, auditing, and PAM. The next few
subsections detail this.

3.1. Processes

In Unix-like systems, user-level activities are implemented by running
processes. Most Unix systems support a ``thread'' as a separate concept;
threads share memory inside a process, and the system scheduler actually
schedules threads. Linux does this differently (and in my opinion uses a
better approach): there is no essential difference between a thread and a
process. Instead, in Linux, when a process creates another process it can
choose what resources are shared (e.g., memory can be shared). The Linux
kernel then performs optimizations to get thread-level speeds; see clone(2)
for more information. It's worth noting that the Linux kernel developers tend
to use the word ``task'', not ``thread'' or ``process'', but the external
documentation tends to use the word process (so I'll use the term ``process''
here). When programming a multi-threaded application, it's usually better to
use one of the standard thread libraries that hide these differences. Not
only does this make threading more portable, but some libraries provide an
additional level of indirection, by implementing more than one
application-level thread as a single operating system thread; this can
provide some improved performance on some systems for some applications.

3.1.1. Process Attributes

Here are typical attributes associated with each process in a Unix-like

  * RUID, RGID - real UID and GID of the user on whose behalf the process is
  * EUID, EGID - effective UID and GID used for privilege checks (except for
    the filesystem)
  * SUID, SGID - Saved UID and GID; used to support switching permissions
    ``on and off'' as discussed below. Not all Unix-like systems support
    this, but the vast majority do (including Linux and Solaris); if you want
    to check if a given system implements this option in the POSIX standard,
    you can use sysconf(2) to determine if _POSIX_SAVED_IDS is in effect.
  * supplemental groups - a list of groups (GIDs) in which this user has
    membership. In the original version 7 Unix, this didn't exist - processes
    were only a member of one group at a time, and a special command had to
    be executed to change that group. BSD added support for a list of groups
    in each process, which is more flexible, and this addition is now widely
    implemented (including by Linux and Solaris).
  * umask - a set of bits determining the default access control settings
    when a new filesystem object is created; see umask(2).
  * scheduling parameters - each process has a scheduling policy, and those
    with the default policy SCHED_OTHER have the additional parameters nice,
    priority, and counter. See sched_setscheduler(2) for more information.
  * limits - per-process resource limits (see below).
  * filesystem root - the process' idea of where the root filesystem ("/")
    begins; see chroot(2).


Here are less-common attributes associated with processes:

  * FSUID, FSGID - UID and GID used for filesystem access checks; this is
    usually equal to the EUID and EGID respectively. This is a Linux-unique
  * capabilities - POSIX capability information; there are actually three
    sets of capabilities on a process: the effective, inheritable, and
    permitted capabilities. See below for more information on POSIX
    capabilities. Linux kernel version 2.2 and greater support this; some
    other Unix-like systems do too, but it's not as widespread.


In Linux, if you really need to know exactly what attributes are associated
with each process, the most definitive source is the Linux source code, in
particular /usr/include/linux/sched.h's definition of task_struct.

The portable way to create new processes it use the fork(2) call. BSD
introduced a variant called vfork(2) as an optimization technique. The bottom
line with vfork(2) is simple: don't use it if you can avoid it. See Section
8.6 for more information.

Linux supports the Linux-unique clone(2) call. This call works like fork(2),
but allows specification of which resources should be shared (e.g., memory,
file descriptors, etc.). Various BSD systems implement an rfork() system call
(originally developed in Plan9); it has different semantics but the same
general idea (it also creates a process with tighter control over what is
shared). Portable programs shouldn't use these calls directly, if possible;
as noted earlier, they should instead rely on threading libraries that use
such calls to implement threads.

This book is not a full tutorial on writing programs, so I will skip
widely-available information handling processes. You can see the
documentation for wait(2), exit(2), and so on for more information.

3.1.2. POSIX Capabilities

POSIX capabilities are sets of bits that permit splitting of the privileges
typically held by root into a larger set of more specific privileges. POSIX
capabilities are defined by a draft IEEE standard; they're not unique to
Linux but they're not universally supported by other Unix-like systems
either. Linux kernel 2.0 did not support POSIX capabilities, while version
2.2 added support for POSIX capabilities to processes. When Linux
documentation (including this one) says ``requires root privilege'', in
nearly all cases it really means ``requires a capability'' as documented in
the capability documentation. If you need to know the specific capability
required, look it up in the capability documentation.

In Linux, the eventual intent is to permit capabilities to be attached to
files in the filesystem; as of this writing, however, this is not yet
supported. There is support for transferring capabilities, but this is
disabled by default. Linux version 2.2.11 added a feature that makes
capabilities more directly useful, called the ``capability bounding set''.
The capability bounding set is a list of capabilities that are allowed to be
held by any process on the system (otherwise, only the special init process
can hold it). If a capability does not appear in the bounding set, it may not
be exercised by any process, no matter how privileged. This feature can be
used to, for example, disable kernel module loading. A sample tool that takes
advantage of this is LCAP at [] http://

More information about POSIX capabilities is available at [ftp://]

3.1.3. Process Creation and Manipulation

Processes may be created using fork(2), the non-recommended vfork(2), or the
Linux-unique clone(2); all of these system calls duplicate the existing
process, creating two processes out of it. A process can execute a different
program by calling execve(2), or various front-ends to it (for example, see
exec(3), system(3), and popen(3)).

When a program is executed, and its file has its setuid or setgid bit set,
the process' EUID or EGID (respectively) is usually set to the file's value.
This functionality was the source of an old Unix security weakness when used
to support setuid or setgid scripts, due to a race condition. Between the
time the kernel opens the file to see which interpreter to run, and when the
(now-set-id) interpreter turns around and reopens the file to interpret it,
an attacker might change the file (directly or via symbolic links).

Different Unix-like systems handle the security issue for setuid scripts in
different ways. Some systems, such as Linux, completely ignore the setuid and
setgid bits when executing scripts, which is clearly a safe approach. Most
modern releases of SysVr4 and BSD 4.4 use a different approach to avoid the
kernel race condition. On these systems, when the kernel passes the name of
the set-id script to open to the interpreter, rather than using a pathname
(which would permit the race condition) it instead passes the filename /dev/
fd/3. This is a special file already opened on the script, so that there can
be no race condition for attackers to exploit. Even on these systems I
recommend against using the setuid/setgid shell scripts language for secure
programs, as discussed below.

In some cases a process can affect the various UID and GID values; see setuid
(2), seteuid(2), setreuid(2), and the Linux-unique setfsuid(2). In particular
the saved user id (SUID) attribute is there to permit trusted programs to
temporarily switch UIDs. Unix-like systems supporting the SUID use the
following rules: If the RUID is changed, or the EUID is set to a value not
equal to the RUID, the SUID is set to the new EUID. Unprivileged users can
set their EUID from their SUID, the RUID to the EUID, and the EUID to the

The Linux-unique FSUID process attribute is intended to permit programs like
the NFS server to limit themselves to only the filesystem rights of some
given UID without giving that UID permission to send signals to the process.
Whenever the EUID is changed, the FSUID is changed to the new EUID value; the
FSUID value can be set separately using setfsuid(2), a Linux-unique call.
Note that non-root callers can only set FSUID to the current RUID, EUID,
SEUID, or current FSUID values.

3.2. Files

On all Unix-like systems, the primary repository of information is the file
tree, rooted at ``/''. The file tree is a hierarchical set of directories,
each of which may contain filesystem objects (FSOs).

In Linux, filesystem objects (FSOs) may be ordinary files, directories,
symbolic links, named pipes (also called first-in first-outs or FIFOs),
sockets (see below), character special (device) files, or block special
(device) files (in Linux, this list is given in the find(1) command). Other
Unix-like systems have an identical or similar list of FSO types.

Filesystem objects are collected on filesystems, which can be mounted and
unmounted on directories in the file tree. A filesystem type (e.g., ext2 and
FAT) is a specific set of conventions for arranging data on the disk to
optimize speed, reliability, and so on; many people use the term
``filesystem'' as a synonym for the filesystem type.

3.2.1. Filesystem Object Attributes

Different Unix-like systems support different filesystem types. Filesystems
may have slightly different sets of access control attributes and access
controls can be affected by options selected at mount time. On Linux, the
ext2 filesystems is currently the most popular filesystem, but Linux supports
a vast number of filesystems. Most Unix-like systems tend to support multiple
filesystems too.

Most filesystems on Unix-like systems store at least the following:

  * owning UID and GID - identifies the ``owner'' of the filesystem object.
    Only the owner or root can change the access control attributes unless
    otherwise noted.
  * permission bits - read, write, execute bits for each of user (owner),
    group, and other. For ordinary files, read, write, and execute have their
    typical meanings. In directories, the ``read'' permission is necessary to
    display a directory's contents, while the ``execute'' permission is
    sometimes called ``search'' permission and is necessary to actually enter
    the directory to use its contents. In a directory ``write'' permission on
    a directory permits adding, removing, and renaming files in that
    directory; if you only want to permit adding, set the sticky bit noted
    below. Note that the permission values of symbolic links are never used;
    it's only the values of their containing directories and the linked-to
    file that matter.
  * ``sticky'' bit - when set on a directory, unlinks (removes) and renames
    of files in that directory are limited to the file owner, the directory
    owner, or root privileges. This is a very common Unix extension and is
    specified in the Open Group's Single Unix Specification version 2. Old
    versions of Unix called this the ``save program text'' bit and used this
    to indicate executable files that should stay in memory. Systems that did
    this ensured that only root could set this bit (otherwise users could
    have crashed systems by forcing ``everything'' into memory). In Linux,
    this bit has no effect on ordinary files and ordinary users can modify
    this bit on the files they own: Linux's virtual memory management makes
    this old use irrelevant.
  * setuid, setgid - when set on an executable file, executing the file will
    set the process' effective UID or effective GID to the value of the
    file's owning UID or GID (respectively). All Unix-like systems support
    this. In Linux and System V systems, when setgid is set on a file that
    does not have any execute privileges, this indicates a file that is
    subject to mandatory locking during access (if the filesystem is mounted
    to support mandatory locking); this overload of meaning surprises many
    and is not universal across Unix-like systems. In fact, the Open Group's
    Single Unix Specification version 2 for chmod(3) permits systems to
    ignore requests to turn on setgid for files that aren't executable if
    such a setting has no meaning. In Linux and Solaris, when setgid is set
    on a directory, files created in the directory will have their GID
    automatically reset to that of the directory's GID. The purpose of this
    approach is to support ``project directories'': users can save files into
    such specially-set directories and the group owner automatically changes.
    However, setting the setgid bit on directories is not specified by
    standards such as the Single Unix Specification [Open Group 1997].
  * timestamps - access and modification times are stored for each filesystem
    object. However, the owner is allowed to set these values arbitrarily
    (see touch(1)), so be careful about trusting this information. All
    Unix-like systems support this.


The following attributes are Linux-unique extensions on the ext2 filesystem,
though many other filesystems have similar functionality:

  * immutable bit - no changes to the filesystem object are allowed; only
    root can set or clear this bit. This is only supported by ext2 and is not
    portable across all Unix systems (or even all Linux filesystems).
  * append-only bit - only appending to the filesystem object are allowed;
    only root can set or clear this bit. This is only supported by ext2 and
    is not portable across all Unix systems (or even all Linux filesystems).


Other common extensions include some sort of bit indicating ``cannot delete
this file''.

Many of these values can be influenced at mount time, so that, for example,
certain bits can be treated as though they had a certain value (regardless of
their values on the media). See mount(1) for more information about this.
These bits are useful, but be aware that some of these are intended to
simplify ease-of-use and aren't really sufficient to prevent certain actions.
For example, on Linux, mounting with ``noexec'' will disable execution of
programs on that file system; as noted in the manual, it's intended for
mounting filesystems containing binaries for incompatible systems. On Linux,
this option won't completely prevent someone from running the files; they can
copy the files somewhere else to run them, or even use the command ``/lib/'' to run the file directly.

Some filesystems don't support some of these access control values; again,
see mount(1) for how these filesystems are handled. In particular, many
Unix-like systems support MS-DOS disks, which by default support very few of
these attributes (and there's not standard way to define these attributes).
In that case, Unix-like systems emulate the standard attributes (possibly
implementing them through special on-disk files), and these attributes are
generally influenced by the mount(1) command.

It's important to note that, for adding and removing files, only the
permission bits and owner of the file's directory really matter unless the
Unix-like system supports more complex schemes (such as POSIX ACLs). Unless
the system has other extensions, and stock Linux 2.2 doesn't, a file that has
no permissions in its permission bits can still be removed if its containing
directory permits it. Also, if an ancestor directory permits its children to
be changed by some user or group, then any of that directory's descendants
can be replaced by that user or group.

The draft IEEE POSIX standard on security defines a technique for true ACLs
that support a list of users and groups with their permissions.
Unfortunately, this is not widely supported nor supported exactly the same
way across Unix-like systems. Stock Linux 2.2, for example, has neither ACLs
nor POSIX capability values in the filesystem.

It's worth noting that in Linux, the Linux ext2 filesystem by default
reserves a small amount of space for the root user. This is a partial defense
against denial-of-service attacks; even if a user fills a disk that is shared
with the root user, the root user has a little space left over (e.g., for
critical functions). The default is 5% of the filesystem space; see mke2fs
(8), in particular its ``-m'' option.

3.2.2. Creation Time Initial Values

At creation time, the following rules apply. On most Unix systems, when a new
filesystem object is created via creat(2) or open(2), the FSO UID is set to
the process' EUID and the FSO's GID is set to the process' EGID. Linux works
slightly differently due to its FSUID extensions; the FSO's UID is set to the
process' FSUID, and the FSO GID is set to the process' FSGUID; if the
containing directory's setgid bit is set or the filesystem's ``GRPID'' flag
is set, the FSO GID is actually set to the GID of the containing directory.
Many systems, including Sun Solaris and Linux, also support the setgid
directory extensions. As noted earlier, this special case supports
``project'' directories: to make a ``project'' directory, create a special
group for the project, create a directory for the project owned by that
group, then make the directory setgid: files placed there are automatically
owned by the project. Similarly, if a new subdirectory is created inside a
directory with the setgid bit set (and the filesystem GRPID isn't set), the
new subdirectory will also have its setgid bit set (so that project
subdirectories will ``do the right thing''.); in all other cases the setgid
is clear for a new file. This is the rationale for the ``user-private group''
scheme (used by Red Hat Linux and some others). In this scheme, every user is
a member of a ``private'' group with just themselves as members, so their
defaults can permit the group to read and write any file (since they're the
only member of the group). Thus, when the file's group membership is
transferred this way, read and write privileges are transferred too. FSO
basic access control values (read, write, execute) are computed from
(requested values & ~ umask of process). New files always start with a clear
sticky bit and clear setuid bit.

3.2.3. Changing Access Control Attributes

You can set most of these values with chmod(2), fchmod(2), or chmod(1) but
see also chown(1), and chgrp(1). In Linux, some of the Linux-unique
attributes are manipulated using chattr(1).

Note that in Linux, only root can change the owner of a given file. Some
Unix-like systems allow ordinary users to transfer ownership of their files
to another, but this causes complications and is forbidden by Linux. For
example, if you're trying to limit disk usage, allowing such operations would
allow users to claim that large files actually belonged to some other

3.2.4. Using Access Control Attributes

Under Linux and most Unix-like systems, reading and writing attribute values
are only checked when the file is opened; they are not re-checked on every
read or write. Still, a large number of calls do check these attributes,
since the filesystem is so central to Unix-like systems. Calls that check
these attributes include open(2), creat(2), link(2), unlink(2), rename(2),
mknod(2), symlink(2), and socket(2).

3.2.5. Filesystem Hierarchy

Over the years conventions have been built on ``what files to place where''.
Where possible, please follow conventional use when placing information in
the hierarchy. For example, place global configuration information in /etc.
The Filesystem Hierarchy Standard (FHS) tries to define these conventions in
a logical manner, and is widely used by Linux systems. The FHS is an update
to the previous Linux Filesystem Structure standard (FSSTND), incorporating
lessons learned and approaches from Linux, BSD, and System V systems. See
[] for more
information about the FHS. A summary of these conventions is in hier(5) for
Linux and hier(7) for Solaris. Sometimes different conventions disagree;
where possible, make these situations configurable at compile or installation

I should note that the FHS has been adopted by the []
Linux Standard Base which is developing and promoting a set of standards to
increase compatibility among Linux distributions and to enable software
applications to run on any compliant Linux system.

3.3. System V IPC

Many Unix-like systems, including Linux and System V systems, support System
V interprocess communication (IPC) objects. Indeed System V IPC is required
by the Open Group's Single UNIX Specification, Version 2 [Open Group 1997].
System V IPC objects can be one of three kinds: System V message queues,
semaphore sets, and shared memory segments. Each such object has the
following attributes:

  * read and write permissions for each of creator, creator group, and
  * creator UID and GID - UID and GID of the creator of the object.
  * owning UID and GID - UID and GID of the owner of the object (initially
    equal to the creator UID).


When accessing such objects, the rules are as follows:

  * if the process has root privileges, the access is granted.
  * if the process' EUID is the owner or creator UID of the object, then the
    appropriate creator permission bit is checked to see if access is
  * if the process' EGID is the owner or creator GID of the object, or one of
    the process' groups is the owning or creating GID of the object, then the
    appropriate creator group permission bit is checked for access.
  * otherwise, the appropriate ``other'' permission bit is checked for


Note that root, or a process with the EUID of either the owner or creator,
can set the owning UID and owning GID and/or remove the object. More
information is available in ipc(5).

3.4. Sockets and Network Connections

Sockets are used for communication, particularly over a network. Sockets were
originally developed by the BSD branch of Unix systems, but they are
generally portable to other Unix-like systems: Linux and System V variants
support sockets as well, and socket support is required by the Open Group's
Single Unix Specification [Open Group 1997]. System V systems traditionally
used a different (incompatible) network communication interface, but it's
worth noting that systems like Solaris include support for sockets. Socket(2)
creates an endpoint for communication and returns a descriptor, in a manner
similar to open(2) for files. The parameters for socket specify the protocol
family and type, such as the Internet domain (TCP/IP version 4), Novell's
IPX, or the ``Unix domain''. A server then typically calls bind(2), listen
(2), and accept(2) or select(2). A client typically calls bind(2) (though
that may be omitted) and connect(2). See these routine's respective man pages
for more information. It can be difficult to understand how to use sockets
from their man pages; you might want to consult other papers such as Hall
"Beej" [1999] to learn how these calls are used together.

The ``Unix domain sockets'' don't actually represent a network protocol; they
can only connect to sockets on the same machine. (at the time of this writing
for the standard Linux kernel). When used as a stream, they are fairly
similar to named pipes, but with significant advantages. In particular, Unix
domain socket is connection-oriented; each new connection to the socket
results in a new communication channel, a very different situation than with
named pipes. Because of this property, Unix domain sockets are often used
instead of named pipes to implement IPC for many important services. Just
like you can have unnamed pipes, you can have unnamed Unix domain sockets
using socketpair(2); unnamed Unix domain sockets are useful for IPC in a way
similar to unnamed pipes.

There are several interesting security implications of Unix domain sockets.
First, although Unix domain sockets can appear in the filesystem and can have
stat(2) applied to them, you can't use open(2) to open them (you have to use
the socket(2) and friends interface). Second, Unix domain sockets can be used
to pass file descriptors between processes (not just the file's contents).
This odd capability, not available in any other IPC mechanism, has been used
to hack all sorts of schemes (the descriptors can basically be used as a
limited version of the ``capability'' in the computer science sense of the
term). File descriptors are sent using sendmsg(2), where the msg (message)'s
field msg_control points to an array of control message headers (field
msg_controllen must specify the number of bytes contained in the array). Each
control message is a struct cmsghdr followed by data, and for this purpose
you want the cmsg_type set to SCM_RIGHTS. A file descriptor is retrieved
through recvmsg(2) and then tracked down in the analogous way. Frankly, this
feature is quite baroque, but it's worth knowing about.

Linux 2.2 and later supports an additional feature in Unix domain sockets:
you can acquire the peer's ``credentials'' (the pid, uid, and gid). Here's
some sample code:
 /* fd= file descriptor of Unix domain socket connected                      
    to the client you wish to identify */                                    
 struct ucred cr;                                                            
 int cl=sizeof(cr);                                                          
 if (getsockopt(fd, SOL_SOCKET, SO_PEERCRED, &cr, &cl)==0) {                 
   printf("Peer's pid=%d, uid=%d, gid=%d\n",                                 
 , cr.uid, cr.gid);                                          

Standard Unix convention is that binding to TCP and UDP local port numbers
less than 1024 requires root privilege, while any process can bind to an
unbound port number of 1024 or greater. Linux follows this convention, more
specifically, Linux requires a process to have the capability
CAP_NET_BIND_SERVICE to bind to a port number less than 1024; this capability
is normally only held by processes with an EUID of 0. The adventurous can
check this in Linux by examining its Linux's source; in Linux 2.2.12, it's
file /usr/src/linux/net/ipv4/af_inet.c, function inet_bind().

3.5. Signals

Signals are a simple form of ``interruption'' in the Unix-like OS world, and
are an ancient part of Unix. A process can set a ``signal'' on another
process (say using kill(1) or kill(2)), and that other process would receive
and handle the signal asynchronously. For a process to have permission to
send an arbitrary signal to some other process, the sending process must
either have root privileges, or the real or effective user ID of the sending
process must equal the real or saved set-user-ID of the receiving process.
However, some signals can be sent in other ways. In particular, SIGURG can be
delivered over a network through the TCP/IP out-of-band (OOB) message.

Although signals are an ancient part of Unix, they've had different semantics
in different implementations. Basically, they involve questions such as
``what happens when a signal occurs while handling another signal''? The
older Linux libc 5 used a different set of semantics for some signal
operations than the newer GNU libc libraries. Calling C library functions is
often unsafe within a signal handler, and even some system calls aren't safe;
you need to examine the documentation for each call you make to see if it
promises to be safe to call inside a signal. For more information, see the
glibc FAQ (on some systems a local copy is available at /usr/doc/glibc-*/

For new programs, just use the POSIX signal system (which in turn was based
on BSD work); this set is widely supported and doesn't have some of the
problems that some of the older signal systems did. The POSIX signal system
is based on using the sigset_t datatype, which can be manipulated through a
set of operations: sigemptyset(), sigfillset(), sigaddset(), sigdelset(), and
sigismember(). You can read about these in sigsetops(3). Then use sigaction
(2), sigprocmask(2), sigpending(2), and sigsuspend(2) to set up an manipulate
signal handling (see their man pages for more information).

In general, make any signal handlers very short and simple, and look
carefully for race conditions. Signals, since they are by nature
asynchronous, can easily cause race conditions.

A common convention exists for servers: if you receive SIGHUP, you should
close any log files, reopen and reread configuration files, and then re-open
the log files. This supports reconfiguration without halting the server and
log rotation without data loss. If you are writing a server where this
convention makes sense, please support it.

Michal Zalewski [2001] has written an excellent tutorial on how signal
handlers are exploited, and has recommendations for how to eliminate signal
race problems. I encourage looking at his summary for more information; here
are my recommendations, which are similar to Michal's work:

  * Where possible, have your signal handlers unconditionally set a specific
    flag and do nothing else.
  * If you must have more complex signal handlers, use only calls
    specifically designated as being safe for use in signal handlers. In
    particular, don't use malloc() or free() in C (which on most systems
    aren't protected against signals), nor the many functions that depend on
    them (such as the printf() family and syslog()). You could try to
    ``wrap'' calls to insecure library calls with a check to a global flag
    (to avoid re-entry), but I wouldn't recommend it.
  * Block signal delivery during all non-atomic operations in the program,
    and block signal delivery inside signal handlers.

3.6. Quotas and Limits

Many Unix-like systems have mechanisms to support filesystem quotas and
process resource limits. This certainly includes Linux. These mechanisms are
particularly useful for preventing denial of service attacks; by limiting the
resources available to each user, you can make it hard for a single user to
use up all the system resources. Be careful with terminology here, because
both filesystem quotas and process resource limits have ``hard'' and ``soft''
limits but the terms mean slightly different things.

You can define storage (filesystem) quota limits on each mountpoint for the
number of blocks of storage and/or the number of unique files (inodes) that
can be used, and you can set such limits for a given user or a given group. A
``hard'' quota limit is a never-to-exceed limit, while a ``soft'' quota can
be temporarily exceeded. See quota(1), quotactl(2), and quotaon(8).

The rlimit mechanism supports a large number of process quotas, such as file
size, number of child processes, number of open files, and so on. There is a
``soft'' limit (also called the current limit) and a ``hard limit'' (also
called the upper limit). The soft limit cannot be exceeded at any time, but
through calls it can be raised up to the value of the hard limit. See
getrlimit(2), setrlimit(2), and getrusage(2), sysconf(3), and ulimit(1). Note
that there are several ways to set these limits, including the PAM module

3.7. Dynamically Linked Libraries

Practically all programs depend on libraries to execute. In most modern
Unix-like systems, including Linux, programs are by default compiled to use 
dynamically linked libraries (DLLs). That way, you can update a library and
all the programs using that library will use the new (hopefully improved)
version if they can.

Dynamically linked libraries are typically placed in one a few special
directories. The usual directories include /lib, /usr/lib, /lib/security for
PAM modules, /usr/X11R6/lib for X-windows, and /usr/local/lib. You should use
these standard conventions in your programs, in particular, except during
debugging you shouldn't use value computed from the current directory as a
source for dynamically linked libraries (an attacker may be able to add their
own choice ``library'' values).

There are special conventions for naming libraries and having symbolic links
for them, with the result that you can update libraries and still support
programs that want to use old, non-backward-compatible versions of those
libraries. There are also ways to override specific libraries or even just
specific functions in a library when executing a particular program. This is
a real advantage of Unix-like systems over Windows-like systems; I believe
Unix-like systems have a much better system for handling library updates, one
reason that Unix and Linux systems are reputed to be more stable than
Windows-based systems.

On GNU glibc-based systems, including all Linux systems, the list of
directories automatically searched during program start-up is stored in the
file /etc/ Many Red Hat-derived distributions don't normally
include /usr/local/lib in the file /etc/ I consider this a bug,
and adding /usr/local/lib to /etc/ is a common ``fix'' required to
run many programs on Red Hat-derived systems. If you want to just override a
few functions in a library, but keep the rest of the library, you can enter
the names of overriding libraries (.o files) in /etc/; these
``preloading'' libraries will take precedence over the standard set. This
preloading file is typically used for emergency patches; a distribution
usually won't include such a file when delivered. Searching all of these
directories at program start-up would be too time-consuming, so a caching
arrangement is actually used. The program ldconfig(8) by default reads in the
file /etc/, sets up the appropriate symbolic links in the dynamic
link directories (so they'll follow the standard conventions), and then
writes a cache to /etc/ that's then used by other programs. So,
ldconfig has to be run whenever a DLL is added, when a DLL is removed, or
when the set of DLL directories changes; running ldconfig is often one of the
steps performed by package managers when installing a library. On start-up,
then, a program uses the dynamic loader to read the file /etc/ and
then load the libraries it needs.

Various environment variables can control this process, and in fact there are
environment variables that permit you to override this process (so, for
example, you can temporarily substitute a different library for this
particular execution). In Linux, the environment variable LD_LIBRARY_PATH is
a colon-separated set of directories where libraries are searched for first,
before the standard set of directories; this is useful when debugging a new
library or using a nonstandard library for special purposes, but be sure you
trust those who can control those directories. The variable LD_PRELOAD lists
object files with functions that override the standard set, just as /etc/ does. The variable LD_DEBUG, displays debugging information; if
set to ``all'', voluminous information about the dynamic linking process is
displayed while it's occurring.

Permitting user control over dynamically linked libraries would be disastrous
for setuid/setgid programs if special measures weren't taken. Therefore, in
the GNU glibc implementation, if the program is setuid or setgid these
variables (and other similar variables) are ignored or greatly limited in
what they can do. The GNU glibc library determines if a program is setuid or
setgid by checking the program's credentials; if the UID and EUID differ, or
the GID and the EGID differ, the library presumes the program is setuid/
setgid (or descended from one) and therefore greatly limits its abilities to
control linking. If you load the GNU glibc libraries, you can see this; see
especially the files elf/rtld.c and sysdeps/generic/dl-sysdep.c. This means
that if you cause the UID and GID to equal the EUID and EGID, and then call a
program, these variables will have full effect. Other Unix-like systems
handle the situation differently but for the same reason: a setuid/setgid
program should not be unduly affected by the environment variables set. Note
that graphical user interface toolkits generally do permit user control over
dynamically linked libraries, because executables that directly invoke
graphical user inteface toolkits should never, ever, be setuid (or have other
special privileges) at all. For more about how to develop secure GUI
applications, see Section 7.4.4.

For Linux systems, you can get more information from my document, the Program
Library HOWTO.

3.8. Audit

Different Unix-like systems handle auditing differently. In Linux, the most
common ``audit'' mechanism is syslogd(8), usually working in conjunction with
klogd(8). You might also want to look at wtmp(5), utmp(5), lastlog(8), and
acct(2). Some server programs (such as the Apache web server) also have their
own audit trail mechanisms. According to the FHS, audit logs should be stored
in /var/log or its subdirectories.

3.9. PAM

Sun Solaris and nearly all Linux systems use the Pluggable Authentication
Modules (PAM) system for authentication. PAM permits run-time configuration
of authentication methods (e.g., use of passwords, smart cards, etc.). See 
Section 11.6 for more information on using PAM.

3.10. Specialized Security Extensions for Unix-like Systems

A vast amount of research and development has gone into extending Unix-like
systems to support security needs of various communities. For example,
several Unix-like systems have been extended to support the U.S. military's
desire for multilevel security. If you're developing software, you should try
to design your software so that it can work within these extensions.

FreeBSD has a new system call, [
jail.html] jail(2). The jail system call supports sub-partitioning an
environment into many virtual machines (in a sense, a ``super-chroot''); its
most popular use has been to provide virtual machine services for Internet
Service Provider environments. Inside a jail, all processes (even those owned
by root) have the the scope of their requests limited to the jail. When a
FreeBSD system is booted up after a fresh install, no processes will be in
jail. When a process is placed in a jail, it, and any descendants of that
process created will be in that jail. Once in a jail, access to the file
name-space is restricted in the style of chroot(2) (with typical chroot
escape routes blocked), the ability to bind network resources is limited to a
specific IP address, the ability to manipulate system resources and perform
privileged operations is sharply curtailed, and the ability to interact with
other processes is limited to only processes inside the same jail. Note that
each jail is bound to a single IP address; processes within the jail may not
make use of any other IP address for outgoing or incoming connections.

Some extensions available in Linux, such as POSIX capabilities and special
mount-time options, have already been discussed. Here are a few of these
efforts for Linux systems for creating restricted execution environments;
there are many different approaches. The U.S. National Security Agency (NSA)
has developed [] Security-Enhanced Linux (Flask),
which supports defining a security policy in a specialized language and then
enforces that policy. The [] Medusa DS9 extends Linux
by supporting, at the kernel level, a user-space authorization server. [http:
//] LIDS protects files and processes, allowing administrators to
``lock down'' their system. The ``Rule Set Based Access Control'' system,
[] RSBAC is based on the Generalized Framework for Access
Control (GFAC) by Abrams and LaPadula and provides a flexible system of
access control based on several kernel modules. []
Subterfugue is a framework for ``observing and playing with the reality of
software''; it can intercept system calls and change their parameters and/or
change their return values to implement sandboxes, tracers, and so on; it
runs under Linux 2.4 with no changes (it doesn't require any kernel
modifications). [] Janus is a security
tool for sandboxing untrusted applications within a restricted execution
environment. Some have even used []
User-mode Linux, which implements ``Linux on Linux'', as a sandbox
implementation. Because there are so many different approaches to
implementing more sophisticated security models, Linus Torvalds has requested
that a generic approach be developed so different security policies can be
inserted; for more information about this, see [

There are many other extensions for security on various Unix-like systems,
but these are really outside the scope of this document.

Chapter 4. Security Requirements

                                       You will know that your tent is       
                                       secure; you will take stock of your   
                                       property and find nothing missing.    
                                                               Job 5:24 (NIV)

Before you can determine if a program is secure, you need to determine
exactly what its security requirements are. Thankfully, there's an
international standard for identifying and defining security requirements
that is useful for many such circumstances: the Common Criteria [CC 1999],
standardized as ISO/IEC 15408:1999. The CC is the culmination of decades of
work to identify information technology security requirements. There are
other schemes for defining security requirements and evaluating products to
see if products meet the requirements, such as NIST FIPS-140 for
cryptographic equipment, but these other schemes are generally focused on a
specialized area and won't be considered further here.

This chapter briefly describes the Common Criteria (CC) and how to use its
concepts to help you informally identify security requirements and talk with
others about security requirements using standard terminology. The language
of the CC is more precise, but it's also more formal and harder to
understand; hopefully the text in this section will help you "get the jist".

Note that, in some circumstances, software cannot be used unless it has
undergone a CC evaluation by an accredited laboratory. This includes certain
kinds of uses in the U.S. Department of Defense (as specified by NSTISSP
Number 11, which requires that before some products can be used they must be
evaluated or enter evaluation), and in the future such a requirement may also
include some kinds of uses for software in the U.S. federal government. This
section doesn't provide enough information if you plan to actually go through
a CC evaluation by an accredited laboratory. If you plan to go through a
formal evaluation, you need to read the real CC, examine various websites to
really understand the basics of the CC, and eventually contract a lab
accredited to do a CC evaluation.

4.1. Common Criteria Introduction

First, some general information about the CC will help understand how to
apply its concepts. The CC's official name is "The Common Criteria for
Information Technology Security Evaluation", though it's normally just called
the Common Criteria. The CC document has three parts: the introduction (that
describes the CC overall), security functional requirements (that lists
various kinds of security functions that products might want to include), and
security assurance requirements (that lists various methods of assuring that
a product is secure). There is also a related document, the "Common
Evaluation Methodology" (CEM), that guides evaluators how to apply the CC
when doing formal evaluations (in particular, it amplifies what the CC means
in certain cases).

Although the CC is International Standard ISO/IEC 15408:1999, it is
outrageously expensive to order the CC from ISO. Hopefully someday ISO will
follow the lead of other standards organizations such as the IETF and the
W3C, which freely redistribute standards. Not surprisingly, IETF and W3C
standards are followed more often than many ISO standards, in part because
ISO's fees for standards simply make them inaccessible to most developers. (I
don't mind authors being paid for their work, but ISO doesn't fund most of
the standards development work - indeed, many of the developers of ISO
documents are volunteers - so ISO's indefensible fees only line their own
pockets and don't actually aid the authors or users at all.) Thankfully, the
CC developers anticipated this problem and have made sure that the CC's
technical content is freely available to all; you can download the CC's
technical content from [] http:// Even those doing formal evaluation
processes usually use these editions of the CC, and not the ISO versions;
there's simply no good reason to pay ISO for them.

Although it can be used in other ways, the CC is typically used to create two
kinds of documents, a ``Protection Profile'' (PP) or a ``Security Target''
(ST). A ``protection profile'' (PP) is a document created by group of users
(for example, a consumer group or large organization) that identifies the
desired security properties of a product. Basically, a PP is a list of user
security requirements, described in a very specific way defined by the CC. If
you're building a product similar to other existing products, it's quite
possible that there are one or more PPs that define what some users believe
are necessary for that kind of product (e.g., an operating system or
firewall). A ``security target'' (ST) is a document that identifies what a
product actually does, or a subset of it, that is security-relevant. An ST
doesn't need to meet the requirements of any particular PP, but an ST could
meet the requirements of one or more PPs.

Both PPs and STs can go through a formal evaluation. An evaluation of a PP
simply ensures that the PP meets various documentation rules and sanity
checks. An ST evaluation involves not just examining the ST document, but
more importantly it involves evaluating an actual system (called the ``target
of evaluation'', or TOE). The purpose of an ST evaluation is to ensure that,
to the level of the assurance requirements specified by the ST, the actual
product (the TOE) meets the ST's security functional requirements. Customers
can then compare evaluated STs to PPs describing what they want. Through this
comparison, consumers can determine if the products meet their requirements -
and if not, where the limitations are.

To create a PP or ST, you go through a process of identifying the security
environment, namely, your assumptions, threats, and relevant organizational
security policies (if any). From the security environment, you derive the
security objectives for the product or product type. Finally, the security
requirements are selected so that they meet the objectives. There are two
kinds of security requirements: functional requirements (what a product has
to be able to do), and assurance requirements (measures to inspire confidence
that the objectives have been met). Actually creating a PP or ST is often not
a simple straight line as outlined here, but the final result needs to show a
clear relationship so that no critical point is easily overlooked. Even if
you don't plan to write an ST or PP, the ideas in the CC can still be
helpful; the process of identifying the security environment, objectives, and
requirements is still helpful in identifying what's really important.

The vast majority of the CC's text is used to define standardized functional
requirements and assurance requirements. In essence, the majority of the CC
is a ``chinese menu'' of possible security requirements that someone might
want. PP authors pick from the various options to describe what they want,
and ST authors pick from the options to describe what they provide.

Since many people might have difficulty identifying a reasonable set of
assurance requirements, so pre-created sets of assurance requirements called
``evaluation assurance levels'' (EALs) have been defined, ranging from 1 to
7. EAL 2 is simply a standard shorthand for the set of assurance requirements
defined for EAL 2. Products can add additional assurance measures, for
example, they might choose EAL 2 plus some additional assurance measures (if
the combination isn't enough to achieve a higher EAL level, such a
combination would be called "EAL 2 plus"). There are mutual recognition
agreements signed between many of the world's nations that will accept an
evaluation done by an accredited laboratory in the other countries as long as
all of the assurance measures taken were at the EAL 4 level or less.

If you want to actually write an ST or PP, there's an open source software
program that can help you, called the ``CC Toolbox''. It can make sure that
dependencies between requirements are met, suggest common requirements, and
help you quickly develop a document, but it obviously can't do your thinking
for you. The specification of exactly what information must be in a PP or ST
are in CC part 1, annexes B and C respectively.

If you do decide to have your product (or PP) evaluated by an accredited
laboratory, be prepared to spend money, spend time, and work throughout the
process. In particular, evaluations require paying an accredited lab to do
the evaluation, and higher levels of assurance become rapidly more expensive.
Simply believing your product is secure isn't good enough; evaluators will
require evidence to justify any claims made. Thus, evaluations require
documentation, and usually the available documentation has to be improved or
developed to meet CC requirements (especially at the higher assurance
levels). Every claim has to be justified to some level of confidence, so the
more claims made, the stronger the claims, and the more complicated the
design, the more expensive an evaluation is. Obviously, when flaws are found,
they will usually need to be fixed. Note that a laboratory is paid to
evaluate a product and determine the truth. If the product doesn't meet its
claims, then you basically have two choices: fix the product, or change
(reduce) the claims.

It's important to discuss with customers what's desired before beginning a
formal ST evaluation; an ST that includes functional or assurance
requirements not truly needed by customers will be unnecessarily expensive to
evaluate, and an ST that omits necessary requirements may not be acceptable
to the customers (because that necessary piece won't have been evaluated).
PPs identify such requirements, but make sure that the PP accurately reflects
the customer's real requirements (perhaps the customer only wants a part of
the functionality or assurance in the PP, or has a different environment in
mind, or wants something else instead for the situations where your product
will be used). Note that an ST need not include every security feature in a
product; an ST only states what will be (or has been) evaluated. A product
that has a higher EAL rating is not necessarily more secure than a similar
product with a lower rating or no rating; the environment might be different,
the evaluation may have saved money and time by not evaluating the other
product at a higher level, or perhaps the evaluation missed something
important. Evaluations are not proofs; they simply impose a defined minimum
bar to gain confidence in the requirements or product.

4.2. Security Environment and Objectives

The first step in defining a PP or ST is identify the ``security
environment''. This means that you have to consider the physical environment
(can attackers access the computer hardware?), the assets requiring
protection (files, databases, authorization credentials, and so on), and the
purpose of the TOE (what kind of product is it? what is the intended use?).

In developing a PP or ST, you'd end up with a statement of assumptions (who
is trusted? is the network or platform benign?), threats (that the system or
its environment must counter), and organizational security policies (that the
system or its environment must meet). A threat is characterized in terms of a
threat agent (who might perform the attack?), a presumed attack method, any
vulnerabilities that are the basis for the attack, and what asset is under

You'd then define a set of security objectives for the system and
environment, and show that those objectives counter the threats and satisfy
the policies. Even if you aren't creating a PP or ST, thinking about your
assumptions, threats, and possible policies can help you avoid foolish
decisions. For example, if the computer network you're using can be sniffed
(e.g., the Internet), then unencrypted passwords are a foolish idea in most

For the CC, you'd then identify the functional and assurance requirements
that would be met by the TOE, and which ones would be met by the environment,
to meet those security objectives. These requirements would be selected from
the ``chinese menu'' of the CC's possible requirements, and the next sections
will briefly describe the major classes of requirements. In the CC,
requirements are grouped into classes, which are subdivided into families,
which are further subdivided into components; the details of all this are in
the CC itself if you need to know about this. A good diagram showing how this
works is in the CC part 1, figure 4.5, which I cannot reproduce here.

Again, if you're not intending for your product to undergo a CC evaluation,
it's still good to briefly determine this kind of information and informally
write include that information in your documentation (e.g., the man page or
whatever your documentation is).

4.3. Security Functionality Requirements

This section briefly describes the CC security functionality requirements (by
CC class), primarily to give you an idea of the kinds of security
requirements you might want in your software. If you want more detail about
the CC's requirements, see CC part 2. Here are the major classes of CC
security requirements, along with the 3-letter CC abbreviation for that

  * Security Audit (FAU). Perhaps you'll need to recognize, record, store,
    and analyze security-relevant activities. You'll need to identify what
    you want to make auditable, since often you can't leave all possible
    auditing capabilities enabled. Also, consider what to do when there's no
    room left for auditing - if you stop the system, an attacker may
    intentionally do things to be logged and thus stop the system.
  * Communication/Non-repudiation (FCO). This class is poorly named in the
    CC; officially it's called communication, but the real meaning is
    non-repudiation. Is it important that an originator cannot deny having
    sent a message, or that a recipient cannot deny having received it? There
    are limits to how well technology itself can support non-repudiation
    (e.g., a user might be able to give their private key away ahead of time
    if they wanted to be able to repudiate something later), but nevertheless
    for some applications supporting non-repudiation capabilities is very
  * Cryptographic Support (FCS). If you're using cryptography, what
    operations use cryptography, what algorithms and key sizes are you using,
    and how are you managing their keys (including distribution and
  * User Data Protection (FDP). This class specifies requirement for
    protecting user data, and is a big class in the CC with many families
    inside it. The basic idea is that you should specify a policy for data
    (access control or information flow rules), develop various means to
    implement the policy, possibly support off-line storage, import, and
    export, and provide integrity when transferring user data between TOEs.
    One often-forgotten issue is residual information protection - is it
    acceptable if an attacker can later recover ``deleted'' data?
  * Identification and authentication (FIA). Generally you don't just want a
    user to report who they are (identification) - you need to verify their
    identity, a process called authentication. Passwords are the most common
    mechanism for authentication. It's often useful to limit the number of
    authentication attempts (if you can) and limit the feedback during
    authentication (e.g., displaying asterisks instead of the actual
    password). Certainly, limit what a user can do before authenticating; in
    many cases, don't let the user do anything without authenticating. There
    may be many issues controlling when a session can start, but in the CC
    world this is handled by the "TOE access" (FTA) class described below
  * Security Management (FMT). Many systems will require some sort of
    management (e.g., to control who can do what), generally by those who are
    given a more trusted role (e.g., administrator). Be sure you think
    through what those special operations are, and ensure that only those
    with the trusted roles can invoke them. You want to limit trust; ideally,
    even more trusted roles should be limited in what they can do.
  * Privacy (FPR). Do you need to support anonymity, pseudonymity,
    unlinkability, or unobservability? If so, are there conditions where you
    want or don't want these (e.g., should an administrator be able to
    determine the real identity of someone hiding behind a pseudonym?). Note
    that these can seriously conflict with non-repudiation, if you want those
    too. If you're worried about sophisticated threats, these functions can
    be hard to provide.
  * Protection of the TOE Security Functions/Self-protection (FPT). Clearly,
    if the TOE can be subverted, any security functions it provides aren't
    worthwhile, and in many cases a TOE has to provide at least some
    self-protection. Perhaps you should "test the underlying abstract
    machine" - i.e., test that the underlying components meet your
    assumptions, or have the product run self-tests (say during start-up,
    periodically, or on request). You should probably "fail secure", at least
    under certain conditions; determine what those conditions are. Consider
    phyical protection of the TOE. You may want some sort of secure recovery
    function after a failure. It's often useful to have replay detection
    (detect when an attacker is trying to replay older actions) and counter
    it. Usually a TOE must make sure that any access checks are always
    invoked and actually succeed before performing a restricted action.
  * Resource Utilization (FRU). Perhaps you need to provide fault tolerance,
    a priority of service scheme, or support resource allocation (such as a
    quota system).
  * TOE Access (FTA). There may be many issues controlling sessions. Perhaps
    there should be a limit on the number of concurrent sessions (if you're
    running a web service, would it make sense for the same user to be logged
    in simultaneously, or from two different machines?). Perhaps you should
    lock or terminate a session automatically (e.g., after a timeout), or let
    users initiate a session lock. You might want to include a standard
    warning banner. One surprisingly useful piece of information is
    displaying, on login, information about the last session (e.g., the date/
    time and location of the last login) and the date/time of the last
    unsuccessful attempt - this gives users information that can help them
    detect interlopers. Perhaps sessions can only be established based on
    other criteria (e.g., perhaps you can only use the program during
    business hours).
  * Trusted path/channels (FTP). A common trick used by attackers is to make
    the screen appear to be something it isn't, e.g., run an ordinary program
    that looks like a login screen or a forged web site. Thus, perhaps there
    needs to be a "trusted path" - a way that users can ensure that they are
    talking to the "real" program.


4.4. Security Assurance Measure Requirements

As noted above, the CC has a set of possible assurance requirements that can
be selected, and several predefined sets of assurance requirements (EAL
levels 1 through 7). Again, if you're actually going to go through a CC
evaluation, you should examine the CC documents; I'll skip describing the
measures involving reviewing official CC documents (evaluating PPs and STs).
Here are some assurance measures that can increase the confidence others have
in your software:

  * Configuration management (ACM). At least, have unique a version
    identifier for each TOE release, so that users will know what they have.
    You gain more assurance if you have good automated tools to control your
    software, and have separate version identifiers for each piece (typical
    CM tools like CVS can do this, although CVS doesn't record changes as
    atomic changes which is a weakness of it). The more that's under
    configuration management, the better; don't just control your code, but
    also control documentation, track all problem reports (especially
    security-related ones), and all development tools.
  * Delivery and operation (ADO). Your delivery mechanism should ideally let
    users detect unauthorized modifications to prevent someone else
    masquerading as the developer, and even better, prevent modification in
    the first place. You should provide documentation on how to securely
    install, generate, and start-up the TOE, possibly generating a log
    describing how the TOE was generated.
  * Development (ADV). These CC requirements deal with documentation
    describing the TOE implementation, and that they need to be consistent
    between each other (e.g., the information in the ST, functional
    specification, high-level design, low-level design, and code, as well as
    any models of the security policy).
  * Guidance documents (AGD). Users and administrators of your product will
    probably need some sort of guidance to help them use it correctly. It
    doesn't need to be on paper; on-line help and "wizards" can help too. The
    guidance should include warnings about actions that may be a problem in a
    secure environemnt, and describe how to use the system securely.
  * Life-cycle support (ALC). This includes development security (securing
    the systems being used for development, including physical security), a
    flaw remediation process (to track and correct all security flaws), and
    selecting development tools wisely.
  * Tests (ATE). Simply testing can help, but remember that you need to test
    the security functions and not just general functions. You should check
    if something is set to permit, it's permitted, and if it's forbidden, it
    is no longer permitted. Of course, there may be clever ways to subvert
    this, which is what vulnerability assessment is all about (described
  * Vulnerability Assessment (AVA). Doing a vulnerability analysis is useful,
    where someone pretends to be an attacker and tries to find
    vulnerabilities in the product using the available information, including
    documentation (look for "don't do X" statements and see if an attacker
    could exploit them) and publicly known past vulnerabilities of this or
    similar products. This book describes various ways of countering known
    vulnerabilities of previous products to problems such as replay attacks
    (where known-good information is stored and retransmitted), buffer
    overflow attacks, race conditions, and other issues that the rest of this
    book describes. The user and administrator guidance documents should be
    examined to ensure that misleading, unreasonable, or conflicting guidance
    is removed, and that secrity procedures for all modes of operation have
    been addressed. Specialized systems may need to worry about covert
    channels; read the CC if you wish to learn more about covert channels.
  * Maintenance of assurance (AMA). If you're not going through a CC
    evaluation, you don't need a formal AMA process, but all software
    undergoes change. What is your process to give all your users strong
    confidence that future changes to your software will not create new
    vulnerabilities? For example, you could establish a process where
    multiple people review any proposed changes.

Chapter 5. Validate All Input

                                       Wisdom will save you from the ways of 
                                       wicked men, from men whose words are  
                                                          Proverbs 2:12 (NIV)

Some inputs are from untrustable users, so those inputs must be validated
(filtered) before being used. You should determine what is legal and reject
anything that does not match that definition. Do not do the reverse (identify
what is illegal and write code to reject those cases), because you are likely
to forget to handle an important case of illegal input.

There is a good reason for identifying ``illegal'' values, though, and that's
as a set of tests (usually just executed in your head) to be sure that your
validation code is thorough. When I set up an input filter, I mentally attack
the filter to see if there are illegal values that could get through.
Depending on the input, here are a few examples of common ``illegal'' values
that your input filters may need to prevent: the empty string, ".", "..", "..
/", anything starting with "/" or ".", anything with "/" or "&" inside it,
any control characters (especially NIL and newline), and/or any characters
with the ``high bit'' set (especially values decimal 254 and 255, and
character 133 is the Unicode Next-of-line character used by OS/390). Again,
your code should not be checking for ``bad'' values; you should do this check
mentally to be sure that your pattern ruthlessly limits input values to legal
values. If your pattern isn't sufficiently narrow, you need to carefully
re-examine the pattern to see if there are other problems.

Limit the maximum character length (and minimum length if appropriate), and
be sure to not lose control when such lengths are exceeded (see Chapter 6 for
more about buffer overflows).

Here are a few common data types, and things you should validate before using
them from an untrusted user:

  * For strings, identify the legal characters or legal patterns (e.g., as a
    regular expression) and reject anything not matching that form. There are
    special problems when strings contain control characters (especially
    linefeed or NIL) or metacharacters (especially shell metacharacters); it
    is often best to ``escape'' such metacharacters immediately when the
    input is received so that such characters are not accidentally sent. CERT
    goes further and recommends escaping all characters that aren't in a list
    of characters not needing escaping [CERT 1998, CMU 1998]. See Section 8.3
    for more information on metacharacters. Note that [
    2001/NOTE-newline-20010314] line ending encodings vary on different
    computers: Unix-based systems use character 0x0a (linefeed), CP/M and DOS
    based systems (including Windows) use 0x0d 0x0a (carriage-return
    linefeed, and some programs incorrectly reverse the order), the Apple
    MacOS uses 0x0d (carriage return), and IBM OS/390 uses 0x85 (0x85) (next
    line, sometimes called newline).
  * Limit all numbers to the minimum (often zero) and maximum allowed values.
  * A full email address checker is actually quite complicated, because there
    are legacy formats that greatly complicate validation if you need to
    support all of them; see mailaddr(7) and IETF RFC 822 [RFC 822] for more
    information if such checking is necessary. Friedl [1997] developed a
    regular expression to check if an email address is valid (according to
    the specification); his ``short'' regular expression is 4,724 characters,
    and his ``optimized'' expression (in appendix B) is 6,598 characters
    long. And even that regular expression isn't perfect; it can't recognize
    local email addresses, and it can't handle nested parentheses in comments
    (as the specification permits). Often you can simplify and only permit
    the ``common'' Internet address formats.
  * Filenames should be checked; see Section 5.4 for more information on
  * URIs (including URLs) should be checked for validity. If you are directly
    acting on a URI (i.e., you're implementing a web server or
    web-server-like program and the URL is a request for your data), make
    sure the URI is valid, and be especially careful of URIs that try to
    ``escape'' the document root (the area of the filesystem that the server
    is responding to). The most common ways to escape the document root are
    via ``..'' or a symbolic link, so most servers check any ``..''
    directories themselves and ignore symbolic links unless specially
    directed. Also remember to decode any encoding first (via URL encoding or
    UTF-8 encoding), or an encoded ``..'' could slip through. URIs aren't
    supposed to even include UTF-8 encoding, so the safest thing is to reject
    any URIs that include characters with high bits set.
    If you are implementing a system that uses the URI/URL as data, you're
    not home-free at all; you need to ensure that malicious users can't
    insert URIs that will harm other users. See Section 5.11.4 for more
    information about this.
  * When accepting cookie values, make sure to check the the domain value for
    any cookie you're using is the expected one. Otherwise, a (possibly
    cracked) related site might be able to insert spoofed cookies. Here's an
    example from IETF RFC 2965 of how failing to do this check could cause a
      + User agent makes request to, gets back cookie
        session_id="1234" and sets the default domain
      + User agent makes request to, gets back cookie
        session-id="1111", with Domain="".
      + User agent makes request to again, and passes:
                 Cookie: $Version="1"; session_id="1234",                       
                         $Version="1"; session_id="1111"; $Domain=""
        The server at should detect that the second cookie
        was not one it originated by noticing that the Domain attribute is
        not for itself and ignore it.

Unless you account for them, the legal character patterns must not include
characters or character sequences that have special meaning to either the
program internals or the eventual output:

  * A character sequence may have special meaning to the program's internal
    storage format. For example, if you store data (internally or externally)
    in delimited strings, make sure that the delimiters are not permitted
    data values. A number of programs store data in comma (,) or colon (:)
    delimited text files; inserting the delimiters in the input can be a
    problem unless the program accounts for it (i.e., by preventing it or
    encoding it in some way). Other characters often causing these problems
    include single and double quotes (used for surrounding strings) and the
    less-than sign "<" (used in SGML, XML, and HTML to indicate a tag's
    beginning; this is important if you store data in these formats). Most
    data formats have an escape sequence to handle these cases; use it, or
    filter such data on input.
  * A character sequence may have special meaning if sent back out to a user.
    A common example of this is permitting HTML tags in data input that will
    later be posted to other readers (e.g., in a guestbook or ``reader
    comment'' area). However, the problem is much more general. See Section
    7.15 for a general discussion on the topic, and see Section 5.11 for a
    specific discussion about filtering HTML.

These tests should usually be centralized in one place so that the validity
tests can be easily examined for correctness later.

Make sure that your validity test is actually correct; this is particularly a
problem when checking input that will be used by another program (such as a
filename, email address, or URL). Often these tests have subtle errors,
producing the so-called ``deputy problem'' (where the checking program makes
different assumptions than the program that actually uses the data). If
there's a relevant standard, look at it, but also search to see if the
program has extensions that you need to know about.

While parsing user input, it's a good idea to temporarily drop all
privileges, or even create separate processes (with the parser having
permanently dropped privileges, and the other process performing security
checks against the parser requests). This is especially true if the parsing
task is complex (e.g., if you use a lex-like or yacc-like tool), or if the
programming language doesn't protect against buffer overflows (e.g., C and
C++). See Section 7.4 for more information on minimizing privileges.

When using data for security decisions (e.g., ``let this user in''), be sure
to use trustworthy channels. For example, on a public Internet, don't just
use the machine IP address or port number as the sole way to authenticate
users, because in most environments this information can be set by the
(potentially malicious) user. See Section 7.11 for more information.

The following subsections discuss different kinds of inputs to a program;
note that input includes process state such as environment variables, umask
values, and so on. Not all inputs are under the control of an untrusted user,
so you need only worry about those inputs that are.

5.1. Command line

Many programs take input from the command line. A setuid/setgid program's
command line data is provided by an untrusted user, so a setuid/setgid
program must defend itself from potentially hostile command line values.
Attackers can send just about any kind of data through a command line
(through calls such as the execve(3) call). Therefore, setuid/setgid programs
must completely validate the command line inputs and must not trust the name
of the program reported by command line argument zero (an attacker can set it
to any value including NULL).

5.2. Environment Variables

By default, environment variables are inherited from a process' parent.
However, when a program executes another program, the calling program can set
the environment variables to arbitrary values. This is dangerous to setuid/
setgid programs, because their invoker can completely control the environment
variables they're given. Since they are usually inherited, this also applies
transitively; a secure program might call some other program and, without
special measures, would pass potentially dangerous environment variables
values on to the program it calls. The following subsections discuss
environment variables and what to do with them.

5.2.1. Some Environment Variables are Dangerous

Some environment variables are dangerous because many libraries and programs
are controlled by environment variables in ways that are obscure, subtle, or
undocumented. For example, the IFS variable is used by the sh and bash shell
to determine which characters separate command line arguments. Since the
shell is invoked by several low-level calls (like system(3) and popen(3) in
C, or the back-tick operator in Perl), setting IFS to unusual values can
subvert apparently-safe calls. This behavior is documented in bash and sh,
but it's obscure; many long-time users only know about IFS because of its use
in breaking security, not because it's actually used very often for its
intended purpose. What is worse is that not all environment variables are
documented, and even if they are, those other programs may change and add
dangerous environment variables. Thus, the only real solution (described
below) is to select the ones you need and throw away the rest.

5.2.2. Environment Variable Storage Format is Dangerous

Normally, programs should use the standard access routines to access
environment variables. For example, in C, you should get values using getenv
(3), set them using the POSIX standard routine putenv(3) or the BSD extension
setenv(3) and eliminate environment variables using unsetenv(3). I should
note here that setenv(3) is implemented in Linux, too.

However, crackers need not be so nice; crackers can directly control the
environment variable data area passed to a program using execve(2). This
permits some nasty attacks, which can only be understood by understanding how
environment variables really work. In Linux, you can see environ(5) for a
summary how about environment variables really work. In short, environment
variables are internally stored as a pointer to an array of pointers to
characters; this array is stored in order and terminated by a NULL pointer
(so you'll know when the array ends). The pointers to characters, in turn,
each point to a NIL-terminated string value of the form ``NAME=value''. This
has several implications, for example, environment variable names can't
include the equal sign, and neither the name nor value can have embedded NIL
characters. However, a more dangerous implication of this format is that it
allows multiple entries with the same variable name, but with different
values (e.g., more than one value for SHELL). While typical command shells
prohibit doing this, a locally-executing cracker can create such a situation
using execve(2).

The problem with this storage format (and the way it's set) is that a program
might check one of these values (to see if it's valid) but actually use a
different one. In Linux, the GNU glibc libraries try to shield programs from
this; glibc 2.1's implementation of getenv will always get the first matching
entry, setenv and putenv will always set the first matching entry, and
unsetenv will actually unset all of the matching entries (congratulations to
the GNU glibc implementers for implementing unsetenv this way!). However,
some programs go directly to the environ variable and iterate across all
environment variables; in this case, they might use the last matching entry
instead of the first one. As a result, if checks were made against the first
matching entry instead, but the actual value used is the last matching entry,
a cracker can use this fact to circumvent the protection routines.

5.2.3. The Solution - Extract and Erase

For secure setuid/setgid programs, the short list of environment variables
needed as input (if any) should be carefully extracted. Then the entire
environment should be erased, followed by resetting a small set of necessary
environment variables to safe values. There really isn't a better way if you
make any calls to subordinate programs; there's no practical method of
listing ``all the dangerous values''. Even if you reviewed the source code of
every program you call directly or indirectly, someone may add new
undocumented environment variables after you write your code, and one of them
may be exploitable.

The simple way to erase the environment in C/C++ is by setting the global
variable environ to NULL. The global variable environ is defined in <unistd.h
>; C/C++ users will want to #include this header file. You will need to
manipulate this value before spawning threads, but that's rarely a problem,
since you want to do these manipulations very early in the program's
execution (usually before threads are spawned).

The global variable environ's definition is defined in various standards;
it's not clear that the official standards condone directly changing its
value, but I'm unaware of any Unix-like system that has trouble with doing
this. I normally just modify the ``environ'' directly; manipulating such
low-level components is possibly non-portable, but it assures you that you
get a clean (and safe) environment. In the rare case where you need later
access to the entire set of variables, you could save the ``environ''
variable's value somewhere, but this is rarely necessary; nearly all programs
need only a few values, and the rest can be dropped.

Another way to clear the environment is to use the undocumented clearenv()
function. The function clearenv() has an odd history; it was supposed to be
defined in POSIX.1, but somehow never made it into that standard. However,
clearenv() is defined in POSIX.9 (the Fortran 77 bindings to POSIX), so there
is a quasi-official status for it. In Linux, clearenv() is defined in <
stdlib.h>, but before using #include to include it you must make sure that
__USE_MISC is #defined. A somewhat more ``official'' approach is to cause
__USE_MISC to be defined is to first #define either _SVID_SOURCE or
_BSD_SOURCE, and then #include <features.h> - these are the official feature
test macros.

One environment value you'll almost certainly re-add is PATH, the list of
directories to search for programs; PATH should not include the current
directory and usually be something simple like ``/bin:/usr/bin''. Typically
you'll also set IFS (to its default of `` \t\n'', where space is the first
character) and TZ (timezone). Linux won't die if you don't supply either IFS
or TZ, but some System V based systems have problems if you don't supply a TZ
value, and it's rumored that some shells need the IFS value set. In Linux,
see environ(5) for a list of common environment variables that you might want
to set.

If you really need user-supplied values, check the values first (to ensure
that the values match a pattern for legal values and that they are within
some reasonable maximum length). Ideally there would be some standard trusted
file in /etc with the information for ``standard safe environment variable
values'', but at this time there's no standard file defined for this purpose.
For something similar, you might want to examine the PAM module pam_env on
those systems which have that module. If you allow users to set an arbitrary
environment variable, then you'll let them subvert restricted shells (more on
that below).

If you're using a shell as your programming language, you can use the ``/usr/
bin/env'' program with the ``-'' option (which erases all environment
variables of the program being run). Basically, you call /usr/bin/env, give
it the ``-'' option, follow that with the set of variables and their values
you wish to set (as name=value), and then follow that with the name of the
program to run and its arguments. You usually want to call the program using
the full pathname (/usr/bin/env) and not just as ``env'', in case a user has
created a dangerous PATH value. Note that GNU's env also accepts the options
"-i" and "--ignore-environment" as synonyms (they also erase the environment
of the program being started), but these aren't portable to other versions of

If you're programming a setuid/setgid program in a language that doesn't
allow you to reset the environment directly, one approach is to create a
``wrapper'' program. The wrapper sets the environment program to safe values,
and then calls the other program. Beware: make sure the wrapper will actually
invoke the intended program; if it's an interpreted program, make sure
there's no race condition possible that would allow the interpreter to load a
different program than the one that was granted the special setuid/setgid

5.2.4. Don't Let Users Set Their Own Environment Variables

If you allow users to set their own environment variables, then users will be
able to escape out of restricted accounts (these are accounts that are
supposed to only let the users run certain programs and not work as a
general-purpose machine). This includes letting users write or modify certain
files in their home directory (e.g., like .login), supporting conventions
that load in environment variables from files under the user's control (e.g.,
openssh's .ssh/environment file), or supporting protocols that transfer
environment variables (e.g., the Telnet Environment Option; see CERT Advisory
CA-1995-14 for more). Restricted accounts should never be allowed to modify
or add any file directly contained in their home directory, and instead
should be given only a specific subdirectory that they are allowed to modify
(if they can modify any).

ari posted a detailed discussion of this problem on Bugtraq on June 24, 2002:

    Given the similarities with certain other security issues, i'm surprised
    this hasn't been discussed earlier. If it has, people simply haven't paid
    it enough attention.
    This problem is not necessarily ssh-specific, though most telnet daemons
    that support environment passing should already be configured to remove
    dangerous variables due to a similar (and more serious) issue back in '95
    (ref: [1]). I will give ssh-based examples here.
    Scenario one: Let's say admin bob has a host that he wants to give people
    ftp access to. Bob doesn't want anyone to have the ability to actually
    _log into_ his system, so instead of giving users normal shells, or even
    no shells, bob gives them all (say) /usr/sbin/nologin, a program he wrote
    himself in C to essentially log the attempt to syslog and exit,
    effectively ending the user's session. As far as most people are
    concerned, the user can't do much with this aside from, say, setting up
    an encrypted tunnel.
    The thing is, bob's system uses dynamic libraries (as most do), and /usr/
    sbin/nologin is dynamically linked (as most such programs are). If a user
    can set his environment variables (e.g. by uploading a '.ssh/environment'
    file) and put some arbitrary file on the system (e.g. ''),
    he can bypass any functionality of /usr/sbin/nologin completely via
    LD_PRELOAD (or another member of the LD_* environment family).
    The user can now gain a shell on the system (with his own privileges, of
    course, barring any 'UseLogin' issues (ref: [2])), and administrator bob,
    if he were aware of what just occurred, would be extremely unhappy.
    Granted, there are all kinds of interesting ways to (more or less) do
    away with this problem. Bob could just grit his teeth and give the ftp
    users a nonexistent shell, or he could statically compile nologin,
    assuming his operating system comes with static libraries. Bob could
    also, humorously, make his nologin program setuid and let the standard C
    library take care of the situation. Then, of course, there are also the
    ssh-specific access controls such as AllowGroup and AllowUsers. These may
    appease the situation in this scenario, but it does not correct the
    ... Now, what happens if bob, instead of using /usr/sbin/nologin, wants
    to use (for example) some BBS-type interface that he wrote up or
    downloaded? It can be a script written in perl or tcl or python, or it
    could be a compiled program; doesn't matter. Additionally, bob need not
    be running an ftp server on this host; instead, perhaps bob uses nfs or
    veritas to mount user home directories from a fileserver on his network;
    this exact setup is (unfortunately) employed by many bastion hosts,
    password management hosts and mail servers---to name a few. Perhaps bob
    runs an ISP, and replaces the user's shell when he doesn't pay. With all
    of these possible (and common) scenarios, bob's going to have a somewhat
    more difficult time getting around the problem.
    ... Exploitation of the problem is simple. The circumvention code would
    be compiled into a dynamic library and LD_PRELOAD=/path/to/ should
    be placed into ~user/.ssh/environment (a similar environment option may
    be appended to public keys in the authohrized_keys file). If no
    dynamically loadable programs are executed, this will have no effect.
    ISPs and universities (along with similarly affected organizations)
    should compile their rejection (or otherwise restricted) binaries
    statically (assuming your operating system comes with static
    Ideally, sshd (and all remote access programs that allow user-definable
    environments) should strip any environment settings that libc ignores for
    setuid programs.
5.3. File Descriptors

A program is passed a set of ``open file descriptors'', that is, pre-opened
files. A setuid/setgid program must deal with the fact that the user gets to
select what files are open and to what (within their permission limits). A
setuid/setgid program must not assume that opening a new file will always
open into a fixed file descriptor id, or that the open will succeed at all.
It must also not assume that standard input (stdin), standard output
(stdout), and standard error (stderr) refer to a terminal or are even open.

The rationale behind this is easy; since an attacker can open or close a file
descriptor before starting the program, the attacker could create an
unexpected situation. If the attacker closes the standard output, when the
program opens the next file it will be opened as though it were standard
output, and then it will send all standard output to that file as well. Some
C libraries will automatically open stdin, stdout, and stderr if they aren't
already open (to /dev/null), but this isn't true on all Unix-like systems.
Also, these libraries can't be completely depended on; for example, on some
systems it's possible to create a race condition that causes this automatic
opening to fail (and still run the program).

5.4. File Names

The names of files can, in certain circumstances, cause serious problems.
This is especially a problem for secure programs that run on computers with
local untrusted users, but this isn't limited to that circumstance. Remote
users may be able to trick a program into creating undesirable filenames
(programs should prevent this, but not all do), or remote users may have
partially penetrated a system and try using this trick to penetrate the rest
of the system.

Usually you will want to not include ``..'' (higher directory) as a legal
value from an untrusted user, though that depends on the circumstances. You
might also want to list only the characters you will permit, and forbidding
any filenames that don't match the list. It's best to prohibit any change in
directory, e.g., by not including ``/'' in the set of legal characters, if
you're taking data from an external user and transforming it into a filename.

Often you shouldn't support ``globbing'', that is, expanding filenames using
``*'', ``?'', ``['' (matching ``]''), and possibly ``{'' (matching ``}'').
For example, the command ``ls *.png'' does a glob on ``*.png'' to list all
PNG files. The C fopen(3) command (for example) doesn't do globbing, but the
command shells perform globbing by default, and in C you can request globbing
using (for example) glob(3). If you don't need globbing, just use the calls
that don't do it where possible (e.g., fopen(3)) and/or disable them (e.g.,
escape the globbing characters in a shell). Be especially careful if you want
to permit globbing. Globbing can be useful, but complex globs can take a
great deal of computing time. For example, on some ftp servers, performing a
few of these requests can easily cause a denial-of-service of the entire
ftp> ls */../*/../*/../*/../*/../*/../*/../*/../*/../*/../*/../*/../*        
Trying to allow globbing, yet limit globbing patterns, is probably futile.
Instead, make sure that any such programs run as a separate process and use
process limits to limit the amount of CPU and other resources they can
consume. See Section 7.4.8 for more information on this approach, and see 
Section 3.6 for more information on how to set these limits.

Unix-like systems generally forbid including the NIL character in a filename
(since this marks the end of the name) and the '/' character (since this is
the directory separator). However, they often permit anything else, which is
a problem; it is easy to write programs that can be subverted by
cleverly-created filenames.

Filenames that can especially cause problems include:

  * Filenames with leading dashes (-). If passed to other programs, this may
    cause the other programs to misinterpret the name as option settings.
    Ideally, Unix-like systems shouldn't allow these filenames; they aren't
    needed and create many unnecessary security problems. Unfortunately,
    currently developers have to deal with them. Thus, whenever calling
    another program with a filename, insert ``--'' before the filename
    parameters (to stop option processing, if the program supports this
    common request) or modify the filename (e.g., insert ``./'' in front of
    the filename to keep the dash from being the lead character).
  * Filenames with control characters. This especially includes newlines and
    carriage returns (which are often confused as argument separators inside
    shell scripts, or can split log entries into multiple entries) and the
    ESCAPE character (which can interfere with terminal emulators, causing
    them to perform undesired actions outside the user's control). Ideally,
    Unix-like systems shouldn't allow these filenames either; they aren't
    needed and create many unnecessary security problems.
  * Filenames with spaces; these can sometimes confuse a shell into being
    multiple arguments, with the other arguments causing problems. Since
    other operating systems allow spaces in filenames (including Windows and
    MacOS), for interoperability's sake this will probably always be
    permitted. Please be careful in dealing with them, e.g., in the shell use
    double-quotes around all filename parameters whenever calling another
    program. You might want to forbid leading and trailing spaces at least;
    these aren't as visible as when they occur in other places, and can
    confuse human users.
  * Invalid character encoding. For example, a program may believe that the
    filename is UTF-8 encoded, but it may have an invalidly long UTF-8
    encoding. See Section 5.9.2 for more information. I'd like to see
    agreement on the character encoding used for filenames (e.g., UTF-8), and
    then have the operating system enforce the encoding (so that only legal
    encodings are allowed), but that hasn't happened at this time.
  * Another other character special to internal data formats, such as ``<'',
    ``;'', quote characters, backslash, and so on.

5.5. File Contents

If a program takes directions from a file, it must not trust that file
specially unless only a trusted user can control its contents. Usually this
means that an untrusted user must not be able to modify the file, its
directory, or any of its ancestor directories. Otherwise, the file must be
treated as suspect.

If the directions in the file are supposed to be from an untrusted user, then
make sure that the inputs from the file are protected as describe throughout
this book. In particular, check that values match the set of legal values,
and that buffers are not overflowed.

5.6. Web-Based Application Inputs (Especially CGI Scripts)

Web-based applications (such as CGI scripts) run on some trusted server and
must get their input data somehow through the web. Since the input data
generally come from untrusted users, this input data must be validated.
Indeed, this information may have actually come from an untrusted third
party; see Section 7.15 for more information. For example, CGI scripts are
passed this information through a standard set of environment variables and
through standard input. The rest of this text will specifically discuss CGI,
because it's the most common technique for implementing dynamic web content,
but the general issues are the same for most other dynamic web content

One additional complication is that many CGI inputs are provided in so-called
``URL-encoded'' format, that is, some values are written in the format %HH
where HH is the hexadecimal code for that byte. You or your CGI library must
handle these inputs correctly by URL-decoding the input and then checking if
the resulting byte value is acceptable. You must correctly handle all values,
including problematic values such as  (NIL) and %0A (newline). Don't
decode inputs more than once, or input such as ``%2500'' will be mishandled
(the %25 would be translated to ``%'', and the resulting ``'' would be
erroneously translated to the NIL character).

CGI scripts are commonly attacked by including special characters in their
inputs; see the comments above.

Another form of data available to web-based applications are ``cookies.''
Again, users can provide arbitrary cookie values, so they cannot be trusted
unless special precautions are taken. Also, cookies can be used to track
users, potentially invading user privacy. As a result, many users disable
cookies, so if possible your web application should be designed so that it
does not require the use of cookies (but see my later discussion for when you
must authenticate individual users). I encourage you to avoid or limit the
use of persistent cookies (cookies that last beyond a current session),
because they are easily abused. Indeed, U.S. agencies are currently forbidden
to use persistent cookies except in special circumstances, because of the
concern about invading user privacy; see the OMB guidance in memorandum
M-00-13 (June 22, 2000). Note that to use cookies, some browsers may insist
that you have a privacy profile (named p3p.xml on the root directory of the

Some HTML forms include client-side input checking to prevent some illegal
values; these are typically implemented using Javascript/ECMAscript or Java.
This checking can be helpful for the user, since it can happen
``immediately'' without requiring any network access. However, this kind of
input checking is useless for security, because attackers can send such
``illegal'' values directly to the web server without going through the
checks. It's not even hard to subvert this; you don't have to write a program
to send arbitrary data to a web application. In general, servers must perform
all their own input checking (of form data, cookies, and so on) because they
cannot trust clients to do this securely. In short, clients are generally not
``trustworthy channels''. See Section 7.11 for more information on
trustworthy channels.

A brief discussion on input validation for those using Microsoft's Active
Server Pages (ASP) is available from Jerry Connolly at [http://]

5.7. Other Inputs

Programs must ensure that all inputs are controlled; this is particularly
difficult for setuid/setgid programs because they have so many such inputs.
Other inputs programs must consider include the current directory, signals,
memory maps (mmaps), System V IPC, pending timers, resource limits, the
scheduling priority, and the umask (which determines the default permissions
of newly-created files). Consider explicitly changing directories (using
chdir(2)) to an appropriately fully named directory at program startup.

5.8. Human Language (Locale) Selection

As more people have computers and the Internet available to them, there has
been increasing pressure for programs to support multiple human languages and
cultures. This combination of language and other cultural factors is usually
called a ``locale''. The process of modifying a program so it can support
multiple locales is called ``internationalization'' (i18n), and the process
of providing the information for a particular locale to a program is called
``localization'' (l10n).

Overall, internationalization is a good thing, but this process provides
another opportunity for a security exploit. Since a potentially untrusted
user provides information on the desired locale, locale selection becomes
another input that, if not properly protected, can be exploited.

5.8.1. How Locales are Selected

In locally-run programs (including setuid/setgid programs), locale
information is provided by an environment variable. Thus, like all other
environment variables, these values must be extracted and checked against
valid patterns before use.

For web applications, this information can be obtained from the web browser
(via the Accept-Language request header). However, since not all web browsers
properly pass this information (and not all users configure their browsers
properly), this is used less often than you might think. Often, the language
requested in a web browser is simply passed in as a form value. Again, these
values must be checked for validity before use, as with any other form value.

In either case, locale information is really just a special case of input
discussed in the previous sections. However, because this input is so rarely
considered, I'm discussing it separately. In particular, when combined with
format strings (discussed later), user-controlled strings can permit
attackers to force other programs to run arbitrary instructions, corrupt
data, and do other unfortunate actions.

5.8.2. Locale Support Mechanisms

There are two major library interfaces for supporting locale-selected
messages on Unix-like systems, one called ``catgets'' and the other called
``gettext''. In the catgets approach, every string is assigned a unique
number, which is used as an index into a table of messages. In contrast, in
the gettext approach, a string (usually in English) is used to look up a
table that translates the original string. catgets(3) is an accepted standard
(via the X/Open Portability Guide, Volume 3 and Single Unix Specification),
so it's possible your program uses it. The ``gettext'' interface is not an
official standard, (though it was originally a UniForum proposal), but I
believe it's the more widely used interface (it's used by Sun and essentially
all GNU programs).

In theory, catgets should be slightly faster, but this is at best marginal on
today's machines, and the bookkeeping effort to keep unique identifiers valid
in catgets() makes the gettext() interface much easier to use. I'd suggest
using gettext(), just because it's easier to use. However, don't take my word
for it; see GNU's documentation on gettext (info:gettext#catgets) for a
longer and more descriptive comparison.

The catgets(3) call (and its associated catopen(3) call) in particular is
vulnerable to security problems, because the environment variable NLSPATH can
be used to control the filenames used to acquire internationalized messages.
The GNU C library ignores NLSPATH for setuid/setgid programs, which helps,
but that doesn't protect programs running on other implementations, nor other
programs (like CGI scripts) which don't ``appear'' to require such

The widely-used ``gettext'' interface is at least not vulnerable to a
malicious NLSPATH setting to my knowledge. However, it appears likely to me
that malicious settings of LC_ALL or LC_MESSAGES could cause problems. Also,
if you use gettext's bindtextdomain() routine in its file cat-compat.c, that
does depend on NLSPATH.

5.8.3. Legal Values

For the moment, if you must permit untrusted users to set information on
their desired locales, make sure the provided internationalization
information meets a narrow filter that only permits legitimate locale names.
For user programs (especially setuid/setgid programs), these values will come
in via NLSPATH, LANGUAGE, LANG, the old LINGUAS, LC_ALL, and the other LC_*
values (especially LC_MESSAGES, but also including LC_COLLATE, LC_CTYPE,
LC_MONETARY, LC_NUMERIC, and LC_TIME). For web applications, this
user-requested set of language information would be done via the
Accept-Language request header or a form value (the application should
indicate the actual language setting of the data being returned via the
Content-Language heading). You can check this value as part of your
environment variable filtering if your users can set your environment
variables (i.e., setuid/setgid programs) or as part of your input filtering
(e.g., for CGI scripts). The GNU C library "glibc" doesn't accept some values
of LANG for setuid/setgid programs (in particular anything with "/"), but
errors have been found in that filtering (e.g., Red Hat released an update to
fix this error in glibc on September 1, 2000). This kind of filtering isn't
required by any standard, so you're safer doing this filtering yourself. I
have not found any guidance on filtering language settings, so here are my
suggestions based on my own research into the issue.

First, a few words about the legal values of these settings. Language
settings are generally set using the standard tags defined in IETF RFC 1766
(which uses two-letter country codes as its basic tag, followed by an
optional subtag separated by a dash; I've found that environment variable
settings use the underscore instead). However, some find this insufficiently
flexible, so three-letter country codes may soon be used as well. Also, there
are two major not-quite compatible extended formats, the X/Open Format and
the CEN Format (European Community Standard); you'd like to permit both.
Typical values include ``C'' (the C locale), ``EN'' (English''), and
``FR_fr'' (French using the territory of France's conventions). Also, so many
people use nonstandard names that programs have had to develop ``alias''
systems to cope with nonstandard names (for GNU gettext, see /usr/share/
locale/locale.alias, and for X11, see /usr/lib/X11/locale/locale.alias; you
might need "aliases" instead of "alias"); they should usually be permitted as
well. Libraries like gettext() have to accept all these variants and find an
appropriate value, where possible. One source of further information is FSF
[1999]; another source is the web site. A filter should not
permit characters that aren't needed, in particular ``/'' (which might permit
escaping out of the trusted directories) and ``..'' (which might permit going
up one directory). Other dangerous characters in NLSPATH include ``%'' (which
indicates substitution) and ``:'' (which is the directory separator); the
documentation I have for other machines suggests that some implementations
may use them for other values, so it's safest to prohibit them.

5.8.4. Bottom Line

In short, I suggest simply erasing or re-setting the NLSPATH, unless you have
a trusted user supplying the value. For the Accept-Language heading in HTTP
(if you use it), form values specifying the locale, and the environment
variables LANGUAGE, LANG, the old LINGUAS, LC_ALL, and the other LC_* values
listed above, filter the locales from untrusted users to permit null (empty)
values or to only permit values that match in total this regular expression
(note that I've recently added "="):
I haven't found any legitimate locale which doesn't match this pattern, but
this pattern does appear to protect against locale attacks. Of course,
there's no guarantee that there are messages available in the requested
locale, but in such a case these routines will fall back to the default
messages (usually in English), which at least is not a security problem.

If you wish to be really picky, and only patterns that match li18nux's locale
pattern, you can use this pattern instead:
In both cases, these patterns use POSIX's extended (``modern'') regular
expression notation (see regex(3) and regex(7) on Unix-like systems).

Of course, languages cannot be supported without a standard way to represent
their written symbols, which brings us to the issue of character encoding.

5.9. Character Encoding

5.9.1. Introduction to Character Encoding

For many years Americans have exchanged text using the ASCII character set;
since essentially all U.S. systems support ASCII, this permits easy exchange
of English text. Unfortunately, ASCII is completely inadequate in handling
the characters of nearly all other languages. For many years different
countries have adopted different techniques for exchanging text in different
languages, making it difficult to exchange data in an increasingly
interconnected world.

More recently, ISO has developed ISO 10646, the ``Universal Mulitple-Octet
Coded Character Set (UCS). UCS is a coded character set which defines a
single 31-bit value for each of all of the world's characters. The first
65536 characters of the UCS (which thus fit into 16 bits) are termed the
``Basic Multilingual Plane'' (BMP), and the BMP is intended to cover nearly
all of today's spoken languages. The Unicode forum develops the Unicode
standard, which concentrates on the UCS and adds some additional conventions
to aid interoperability. Historically, Unicode and ISO 10646 were developed
by competing groups, but thankfully they realized that they needed to work
together and they now coordinate with each other.

If you're writing new software that handles internationalized characters, you
should be using ISO 10646/Unicode as your basis for handling international
characters. However, you may need to process older documents in various older
(language-specific) character sets, in which case, you need to ensure that an
untrusted user cannot control the setting of another document's character set
(since this would significantly affect the document's interpretation).

5.9.2. Introduction to UTF-8

Most software is not designed to handle 16 bit or 32 bit characters, yet to
create a universal character set more than 8 bits was required. Therefore, a
special format called ``UTF-8'' was developed to encode these potentially
international characters in a format more easily handled by existing programs
and libraries. UTF-8 is defined, among other places, in IETF RFC 2279, so
it's a well-defined standard that can be freely read and used. UTF-8 is a
variable-width encoding; characters numbered 0 to 0x7f (127) encode to
themselves as a single byte, while characters with larger values are encoded
into 2 to 6 bytes of information (depending on their value). The encoding has
been specially designed to have the following nice properties (this
information is from the RFC and Linux utf-8 man page):

  * The classical US ASCII characters (0 to 0x7f) encode as themselves, so
    files and strings which contain only 7-bit ASCII characters have the same
    encoding under both ASCII and UTF-8. This is fabulous for backward
    compatibility with the many existing U.S. programs and data files.
  * All UCS characters beyond 0x7f are encoded as a multibyte sequence
    consisting only of bytes in the range 0x80 to 0xfd. This means that no
    ASCII byte can appear as part of another character. Many other encodings
    permit characters such as an embedded NIL, causing programs to fail.
  * It's easy to convert between UTF-8 and a 2-byte or 4-byte fixed-width
    representations of characters (these are called UCS-2 and UCS-4
  * The lexicographic sorting order of UCS-4 strings is preserved, and the
    Boyer-Moore fast search algorithm can be used directly with UTF-8 data.
  * All possible 2^31 UCS codes can be encoded using UTF-8.
  * The first byte of a multibyte sequence which represents a single
    non-ASCII UCS character is always in the range 0xc0 to 0xfd and indicates
    how long this multibyte sequence is. All further bytes in a multibyte
    sequence are in the range 0x80 to 0xbf. This allows easy
    resynchronization; if a byte is missing, it's easy to skip forward to the
    ``next'' character, and it's always easy to skip forward and back to the
    ``next'' or ``preceding'' character.

In short, the UTF-8 transformation format is becoming a dominant method for
exchanging international text information because it can support all of the
world's languages, yet it is backward compatible with U.S. ASCII files as
well as having other nice properties. For many purposes I recommend its use,
particularly when storing data in a ``text'' file.

5.9.3. UTF-8 Security Issues

The reason to mention UTF-8 is that some byte sequences are not legal UTF-8,
and this might be an exploitable security hole. UTF-8 encoders are supposed
to use the ``shortest possible'' encoding, but naive decoders may accept
encodings that are longer than necessary. Indeed, earlier standards permitted
decoders to accept ``non-shortest form'' encodings. The problem here is that
this means that potentially dangerous input could be represented multiple
ways, and thus might defeat the security routines checking for dangerous
inputs. The RFC describes the problem this way:

    Implementers of UTF-8 need to consider the security aspects of how they
    handle illegal UTF-8 sequences. It is conceivable that in some
    circumstances an attacker would be able to exploit an incautious UTF-8
    parser by sending it an octet sequence that is not permitted by the UTF-8
    A particularly subtle form of this attack could be carried out against a
    parser which performs security-critical validity checks against the UTF-8
    encoded form of its input, but interprets certain illegal octet sequences
    as characters. For example, a parser might prohibit the NUL character
    when encoded as the single-octet sequence 00, but allow the illegal
    two-octet sequence C0 80 (illegal because it's longer than necessary) and
    interpret it as a NUL character (00). Another example might be a parser
    which prohibits the octet sequence 2F 2E 2E 2F ("/../"), yet permits the
    illegal octet sequence 2F C0 AE 2E 2F.

A longer discussion about this is available at Markus Kuhn's UTF-8 and
Unicode FAQ for Unix/Linux at []

5.9.4. UTF-8 Legal Values

Thus, when accepting UTF-8 input, you need to check if the input is valid
UTF-8. Here is a list of all legal UTF-8 sequences; any character sequence
not matching this table is not a legal UTF-8 sequence. In the following
table, the first column shows the various character values being encoded into
UTF-8. The second column shows how those characters are encoded as binary
values; an ``x'' indicates where the data is placed (either a 0 or 1), though
some values should not be allowed because they're not the shortest possible
encoding. The last row shows the valid values each byte can have (in
hexadecimal). Thus, a program should check that every character meets one of
the patterns in the right-hand column. A ``-'' indicates a range of legal
values (inclusive). Of course, just because a sequence is a legal UTF-8
sequence doesn't mean that you should accept it (you still need to do all
your other checking), but generally you should check any UTF-8 data for UTF-8
legality before performing other checks.

Table 5-1. Legal UTF-8 Sequences
|UCS Code (Hex)          |Binary UTF-8 Format      |Legal UTF-8 Values (Hex)|
|00-7F                   |0xxxxxxx                 |00-7F                   |
|80-7FF                  |110xxxxx 10xxxxxx        |C2-DF 80-BF             |
|800-FFF                 |1110xxxx 10xxxxxx        |E0 A0*-BF 80-BF         |
|                        |10xxxxxx                 |                        |
|1000-FFFF               |1110xxxx 10xxxxxx        |E1-EF 80-BF 80-BF       |
|                        |10xxxxxx                 |                        |
|10000-3FFFF             |11110xxx 10xxxxxx        |F0 90*-BF 80-BF 80-BF   |
|                        |10xxxxxx 10xxxxxx        |                        |
|40000-FFFFFF            |11110xxx 10xxxxxx        |F1-F3 80-BF 80-BF 80-BF |
|                        |10xxxxxx 10xxxxxx        |                        |
|40000-FFFFFF            |11110xxx 10xxxxxx        |F1-F3 80-BF 80-BF 80-BF |
|                        |10xxxxxx 10xxxxxx        |                        |
|100000-10FFFFF          |11110xxx 10xxxxxx        |F4 80-8F* 80-BF 80-BF   |
|                        |10xxxxxx 10xxxxxx        |                        |
|200000-3FFFFFF          |111110xx 10xxxxxx        |too large; see below    |
|                        |10xxxxxx 10xxxxxx        |                        |
|                        |10xxxxxx                 |                        |
|04000000-7FFFFFFF       |1111110x 10xxxxxx        |too large; see below    |
|                        |10xxxxxx 10xxxxxx        |                        |
|                        |10xxxxxx 10xxxxxx        |                        |

As I noted earlier, there are two standards for character sets, ISO 10646 and
Unicode, who have agreed to synchronize their character assignments. The
definition of UTF-8 in ISO/IEC 10646-1:2000 and the IETF RFC also currently
support five and six byte sequences to encode characters outside the range
supported by Uniforum's Unicode, but such values can't be used to support
Unicode characters and it's expected that a future version of ISO 10646 will
have the same limits. Thus, for most purposes the five and six byte UTF-8
encodings aren't legal, and you should normally reject them (unless you have
a special purpose for them).

This is set of valid values is tricky to determine, and in fact earlier
versions of this document got some entries wrong (in some cases it permitted
overlong characters). Language developers should include a function in their
libraries to check for valid UTF-8 values, just because it's so hard to get

I should note that in some cases, you might want to cut slack (or use
internally) the hexadecimal sequence C0 80. This is an overlong sequence
that, if permitted, can represent ASCII NUL (NIL). Since C and C++ have
trouble including a NIL character in an ordinary string, some people have
taken to using this sequence when they want to represent NIL as part of the
data stream; Java even enshrines the practice. Feel free to use C0 80
internally while processing data, but technically you really should translate
this back to 00 before saving the data. Depending on your needs, you might
decide to be ``sloppy'' and accept C0 80 as input in a UTF-8 data stream. If
it doesn't harm security, it's probably a good practice to accept this
sequence since accepting it aids interoperability.

Handling this can be tricky. You might want to examine the C routines
developed by Unicode to handle conversions, available at [ftp://]
Public/PROGRAMS/CVTUTF/ConvertUTF.c. It's unclear to me if these routines are
open source software (the licenses don't clearly say whether or not they can
be modified), so beware of that.

5.9.5. UTF-8 Related Issues

This section has discussed UTF-8, because it's the most popular multibyte
encoding of UCS, simplifying a lot of international text handling issues.
However, it's certainly not the only encoding; there are other encodings,
such as UTF-16 and UTF-7, which have the same kinds of issues and must be
validated for the same reasons.

Another issue is that some phrases can be expressed in more than one way in
ISO 10646/Unicode. For example, some accented characters can be represented
as a single character (with the accent) and also as a set of characters
(e.g., the base character plus a separate composing accent). These two forms
may appear identical. There's also a zero-width space that could be inserted,
with the result that apparently-similar items are considered different.
Beware of situations where such hidden text could interfere with the program.
This is an issue that in general is hard to solve; most programs don't have
such tight control over the clients that they know completely how a
particular sequence will be displayed (since this depends on the client's
font, display characteristics, locale, and so on).

5.10. Prevent Cross-site Malicious Content on Input

Some programs accept data from one untrusted user and pass that data on to a
second user; the second user's application may then process that data in a
way harmful to the second user. This is a particularly common problem for web
applications, we'll call this problem ``cross-site malicious content.'' In
short, you cannot accept input (including any form data) without checking,
filtering, or encoding it. For more information, see Section 7.15.

Fundamentally, this means that all web application input must be filtered (so
characters that can cause this problem are removed), encoded (so the
characters that can cause this problem are encoded in a way to prevent the
problem), or validated (to ensure that only ``safe'' data gets through).
Filtering and validation should often be done at the input, but encoding can
be done either at input or output time. If you're just passing the data
through without analysis, it's probably better to encode the data on input
(so it won't be forgotten), but if you're processing the data, there are
arguments for encoding on output instead.

5.11. Filter HTML/URIs That May Be Re-presented

One special case where cross-site malicious content must be prevented are web
applications which are designed to accept HTML or XHTML from one user, and
then send it on to other users (see Section 7.15 for more information on
cross-site malicious content). The following subsections discuss filtering
this specific kind of input, since handling it is such a common requirement.

5.11.1. Remove or Forbid Some HTML Data

It's safest to remove all possible (X)HTML tags so they cannot affect
anything, and this is relatively easy to do. As noted above, you should
already be identifying the list of legal characters, and rejecting or
removing those characters that aren't in the list. In this filter, simply
don't include the following characters in the list of legal characters: ``<
'', ``>'', and ``&'' (and if they're used in attributes, the double-quote
character ``"''). If browsers only operated according the HTML
specifications, the ``>"'' wouldn't need to be removed, but in practice it
must be removed. This is because some browsers assume that the author of the
page really meant to put in an opening "<" and ``helpfully'' insert one -
attackers can exploit this behavior and use the ">" to create an undesired "<

Usually the character set for transmitting HTML is ISO-8859-1 (even when
sending international text), so the filter should also omit most control
characters (linefeed and tab are usually okay) and characters with their
high-order bit set.

One problem with this approach is that it can really surprise users,
especially those entering international text if all international text is
quietly removed. If the invalid characters are quietly removed without
warning, that data will be irrevocably lost and cannot be reconstructed
later. One alternative is forbidding such characters and sending error
messages back to users who attempt to use them. This at least warns users,
but doesn't give them the functionality they were looking for. Other
alternatives are encoding this data or validating this data, which are
discussed next.

5.11.2. Encoding HTML Data

An alternative that is nearly as safe is to transform the critical characters
so they won't have their usual meaning in HTML. This can be done by
translating all "<" into "&lt;", ">" into "&gt;", and "&" into "&amp;".
Arbitrary international characters can be encoded in Latin-1 using the format
"&#value;" - do not forget the ending semicolon. Encoding the international
characters means you must know what the input encoding was, of course.

One possible danger here is that if these encodings are accidentally
interpreted twice, they will become a vulnerability. However, this approach
at least permits later users to see the "intent" of the input.

5.11.3. Validating HTML Data

Some applications, to work at all, must accept HTML from third parties and
send them on to their users. Beware - you are treading dangerous ground at
this point; be sure that you really want to do this. Even the idea of
accepting HTML from arbitrary places is controversial among some security
practitioners, because it is extremely difficult to get it right.

However, if your application must accept HTML, and you believe that it's
worth the risk, at least identify a list of ``safe'' HTML commands and only
permit those commands.

Here is a minimal set of safe HTML tags that might be useful for applications
(such as guestbooks) that support short comments: <p> (paragraph), <b>
(bold), <i> (italics), <em> (emphasis), <strong> (strong emphasis), <pre>
(preformatted text), <br> (forced line break - note it doesn't require a
closing tag), as well as all their ending tags.

Not only do you need to ensure that only a small set of ``safe'' HTML
commands are accepted, you also need to ensure that they are properly nested
and closed (i.e., that the HTML commands are ``balanced''). In XML, this is
termed ``well-formed'' data. A few exceptions could be made if you're
accepting standard HTML (e.g., supporting an implied </p> where not provided
before a <p> would be fine), but trying to accept HTML in its full generality
(which can infer balancing closing tags in many cases) is not needed for most
applications. Indeed, if you're trying to stick to XHTML (instead of HTML),
then well-formedness is a requirement. Also, HTML tags are case-insensitive;
tags can be upper case, lower case, or a mixture. However, if you intend to
accept XHTML then you need to require all tags to be in lower case (XML is
case-sensitive; XHTML uses XML and requires the tags to be in lower case).

Here are a few random tips about doing this. Usually you should design
whatever surrounds the HTML text and the set of permitted tags so that the
contributed text cannot be misinterpreted as text from the ``main'' site (to
prevent forgeries). Don't accept any attributes unless you've checked the
attribute type and its value; there are many attributes that support things
such as Javascript that can cause trouble for your users. You'll notice that
in the above list I didn't include any attributes at all, which is certainly
the safest course. You should probably give a warning message if an unsafe
tag is used, but if that's not practical, encoding the critical characters
(e.g., "<" becomes "&lt;") prevents data loss while simultaneously keeping
the users safe.

Be careful when expanding this set, and in general be restrictive of what you
accept. If your patterns are too generous, the browser may interpret the
sequences differently than you expect, resulting in a potential exploit. For
example, FozZy posted on Bugtraq (1 April 2002) some sequences that permitted
exploitation in various web-based mail systems, which may give you an idea of
the kinds of problems you need to defend against. Here's some exploit text
that, at one time, could subvert user accounts in Microsoft Hotmail:
   <!-- --> -->                                                              
Here's some similar exploit text for Yahoo! Mail:
  <<script>        (Note: this was found by BugSan)                          
Here's some exploit text for Vizzavi:
  <b onmousover="...">go here</b>                                            
  <img [line_break] src="javascript:alert(document.location)">               
Andrew Clover posted to Bugtraq (on May 11, 2002) a list of various text that
invokes Javascript yet manages to bypass many filters. Here are his examples
(which he says he cut and pasted from elsewhere); some only apply to specific
browsers (IE means Internet Explorer, N4 means Netscape version 4).
  <a href="javas&#99;ript&#35;[code]">                                       
  <div onmouseover="[code]">                                                 
  <img src="javascript:[code]">                                              
  <img dynsrc="javascript:[code]"> [IE]                                      
  <input type="image" dynsrc="javascript:[code]"> [IE]                       
  <bgsound src="javascript:[code]"> [IE]                                     
  &{[code]}; [N4]                                                            
  <img src=&{[code]};> [N4]                                                  
  <link rel="stylesheet" href="javascript:[code]">                           
  <iframe src="vbscript:[code]"> [IE]                                        
  <img src="mocha:[code]"> [N4]                                              
  <img src="livescript:[code]"> [N4]                                         
  <a href="about:<s&#99;ript>[code]</script>">                               
  <meta http-equiv="refresh" content="0;url=javascript:[code]">              
  <body onload="[code]">                                                     
  <div style="background-image: url(javascript:[code]);">                    
  <div style="behaviour: url([link to code]);"> [IE]                         
  <div style="binding: url([link to code]);"> [Mozilla]                      
  <div style="width: expression([code]);"> [IE]                              
  <style type="text/javascript">[code]</style> [N4]                          
  <object classid="clsid:..." codebase="javascript:[code]"> [IE]             
  <!-- -- --><script>[code]</script><!-- -- -->                              
  <img src="blah"onmouseover="[code]">                                       
  <img src="blah>" onmouseover="[code]">                                     
  <xml src="javascript:[code]">                                              
  <xml id="X"><a><b>&lt;script>[code]&lt;/script>;</b></a></xml>             
    <div datafld="b" dataformatas="html" datasrc="#X"></div>                 
  [\xC0][\xBC]script>[code][\xC0][\xBC]/script> [UTF-8; IE, Opera]           
  <![CDATA[<!--]] ><script>[code]//--></script>                              
This is not a complete list, of course, but it at least is a sample of the
kinds of attacks that you must prevent by strictly limiting the tags and
attributes you can allow from untrusted users.

Konstantin Riabitsev has posted []
some PHP code to filter HTML (GPL); I've not examined it closely, but you
might want to take a look.

5.11.4. Validating Hypertext Links (URIs/URLs)

Careful readers will notice that I did not include the hypertext link tag <a>
as a safe tag in HTML. Clearly, you could add <a href="safe URI"> (hypertext
link) to the safe list (not permitting any other attributes unless you've
checked their contents). If your application requires it, then do so.
However, permitting third parties to create links is much less safe, because
defining a ``safe URI''[1] turns out to be very difficult. Many browsers
accept all sorts of URIs which may be dangerous to the user. This section
discusses how to validate URIs from third parties for re-presenting to
others, including URIs incorporated into HTML.

First, let's look briefly at URI syntax (as defined by various
specifications). URIs can be either ``absolute'' or ``relative''. The syntax
of an absolute URI looks like this:
A URI starts with a scheme name (such as ``http''), the characters ``://'',
the authority (such as ``''), a path (which looks like a
directory or file name), a question mark followed by a query, and a hash (``#
'') followed by a fragment identifier. The square brackets surround optional
portions - e.g., many URIs don't actually include the query or fragment. Some
schemes may not permit some of the data (e.g., paths, queries, or fragments),
and many schemes have additional requirements unique to them. Many schemes
permit the ``authority'' field to identify optional usernames, passwords, and
ports, using this syntax for the ``authority'' section:
The ``host'' can either be a name (``'') or an IPv4 numeric
address ( A ``relative'' URI references one object relative to the
``current'' one, and its syntax looks a lot like a filename:
There are a limited number of characters permitted in most of the URI, so to
get around this problem, other 8-bit characters may be ``URL encoded'' as %hh
(where hh is the hexadecimal value of the 8-bit character). For more detailed
information on valid URIs, see IETF RFC 2396 and its related specifications.

Now that we've looked at the syntax of URIs, let's examine the risks of each

  * Scheme: Many schemes are downright dangerous. Permitting someone to
    insert a ``javascript'' scheme into your material would allow them to
    trivially mount denial-of-service attacks (e.g., by repeatedly creating
    windows so the user's machine freezes or becomes unusable). More
    seriously, they might be able to exploit a known vulnerability in the
    javascript implementation. Some schemes can be a nuisance, such as
    ``mailto:'' when a mailing is not expected, and some schemes may not be
    sufficiently secure on the client machine. Thus, it's necessary to limit
    the set of allowed schemes to just a few safe schemes.
  * Authority: Ideally, you should limit user links to ``safe'' sites, but
    this is difficult to do in practice. However, you can certainly do
    something about usernames, passwords, and port numbers: you should forbid
    them. Systems expecting usernames (especially with passwords!) are
    probably guarding more important material; rarely is this needed in
    publicly-posted URIs, and someone could try to use this functionality to
    convince users to expose information they have access to and/or use it to
    modify the information. Such URIs permit semantic attacks; see Section
    7.16 for more information. Usernames without passwords are no less
    dangerous, since browsers typically cache the passwords. You should not
    usually permit specification of ports, because different ports expect
    different protocols and the resulting ``protocol confusion'' can produce
    an exploit. For example, on some systems it's possible to use the
    ``gopher'' scheme and specify the SMTP (email) port to cause a user to
    send email of the attacker's choosing. You might permit a few special
    cases (e.g., http ports 8008 and 8080), but on the whole it's not worth
    it. The host when specified by name actually has a fairly limited
    character set (using the DNS standards). Technically, the standard
    doesn't permit the underscore (``_'') character, but Microsoft ignored
    this part of the standard and even requires the use of the underscore in
    some circumstances, so you probably should allow it. Also, there's been a
    great deal of work on supporting international characters in DNS names,
    which is not further discussed here.
  * Path: Permitting a path is usually okay, but unfortunately some
    applications use part of the path as query data, creating an opening
    we'll discuss next. Also, paths are allowed to contain phrases like
    ``..'', which can expose private data in a poorly-written web server;
    this is less a problem than it once was and really should be fixed by the
    web server. Since it's only the phrase ``..'' that's special, it's
    reasonable to look at paths (and possibly query data) and forbid ``../''
    as a content. However, if your validator permits URL escapes, this can be
    difficult; now you need to prevent versions where some of these
    characters are escaped, and may also have to deal with various
    ``illegal'' character encodings of these characters as well.
  * Query: Query formats (beginning with "?") can be a security risk because
    some query formats actually cause actions to occur on the serving end.
    They shouldn't, and your applications shouldn't, as discussed in Section
    5.12 for more information. However, we have to acknowledge the reality as
    a serious problem. In addition, many web sites are actually
    ``redirectors'' - they take a parameter specifying where the user should
    be redirected, and send back a command redirecting the user to the new
    location. If an attacker references such sites and provides a more
    dangerous URI as the redirection value, and the browser blithely obeys
    the redirection, this could be a problem. Again, the user's browser
    should be more careful, but not all user browsers are sufficiently
    cautious. Also, many web applications have vulnerabilities that can be
    exploited with certain query values, but in general this is hard to
    prevent. The official URI specifications don't sanction the ``+'' (plus)
    character, but in practice the ``+'' character often represents the space
  * Fragment: Fragments basically locate a portion of a document; I'm unaware
    of an attack based on fragments as long as the syntax is legal, but the
    legality of its syntax does need checking. Otherwise, an attacker might
    be able to insert a character such as the double-quote (") and
    prematurely end the URI (foiling any checking).
  * URL escapes: URL escapes are useful because they can represent arbitrary
    8-bit characters; they can also be very dangerous for the same reasons.
    In particular, URL escapes can represent control characters, which many
    poorly-written web applications are vulnerable to. In fact, with or
    without URL escapes, many web applications are vulnerable to certain
    characters (such as backslash, ampersand, etc.), but again this is
    difficult to generalize.
  * Relative URIs: Relative URIs should be reasonably safe (if you manage the
    web site well), although in some applications there's no good reason to
    allow them either.

Of course, there is a trade-off with simplicity as well. Simple patterns are
easier to understand, but they aren't very refined (so they tend to be too
permissive or too restrictive, even more than a refined pattern). Complex
patterns can be more exact, but they are more likely to have errors, require
more performance to use, and can be hard to implement in some circumstances.

Here's my suggestion for a ``simple mostly safe'' URI pattern which is very
simple and can be implemented ``by hand'' or through a regular expression;
permit the following pattern:

This pattern doesn't permit many potentially dangerous capabilities such as
queries, fragments, ports, or relative URIs, and it only permits a few
schemes. It prevents the use of the ``%'' character, which is used in URL
escapes and can be used to specify characters that the server may not be
prepared to handle. Since it doesn't permit either ``:'' or URL escapes, it
doesn't permit specifying port numbers, and even using it to redirect to a
more dangerous URI would be difficult (due to the lack of the escape
character). It also prevents the use of a number of other characters; again,
many poorly-designed web applications can't handle a number of ``unexpected''

Even this ``mostly safe'' URI permits a number of questionable URIs, such as
subdirectories (via ``/'') and attempts to move up directories (via `..'');
illegal queries of this kind should be caught by the server. It permits some
illegal host identifiers (e.g., ``20.20''), though I know of no case where
this would be a security weakness. Some web applications treat subdirectories
as query data (or worse, as command data); this is hard to prevent in general
since finding ``all poorly designed web applications'' is hopeless. You could
prevent the use of all paths, but this would make it impossible to reference
most Internet information. The pattern also allows references to local server
information (through patterns such as "http:///", "http://localhost/", and
"") and access to servers on an internal network; here you'll
have to depend on the servers correctly interpreting the resulting HTTP GET
request as solely a request for information and not a request for an action,
as recommended in Section 5.12. Since query forms aren't permitted by this
pattern, in many environments this should be sufficient.

Unfortunately, the ``mostly safe'' pattern also prevents a number of quite
legitimate and useful URIs. For example, many web sites use the ``?''
character to identify specific documents (e.g., articles on a news site). The
``#'' character is useful for specifying specific sections of a document, and
permitting relative URIs can be handy in a discussion. Various permitted
characters and URL escapes aren't included in the ``mostly safe'' pattern.
For example, without permitting URL escapes, it's difficult to access many
non-English pages. If you truly need such functionality, then you can use
less safe patterns, realizing that you're exposing your users to higher risk
while giving your users greater functionality.

One pattern that permits queries, but at least limits the protocols and ports
used is the following, which I'll call the ``simple somewhat safe pattern'':
This pattern actually isn't very smart, since it permits illegal escapes,
multiple queries, queries in ftp, and so on. It does have the advantage of
being relatively simple.

Creating a ``somewhat safe'' pattern that really limits URIs to legal values
is quite difficult. Here's my current attempt to do so, which I call the
``sophisticated somewhat safe pattern'', expressed in a form where whitespace
is ignored and comments are introduced with "#":
  # Handle http, https, and relative URIs:                                      
  ((/([A-Za-z0-9\-\_\.\!\~\*\'\(\)]|(%[2-9A-Fa-f][0-9a-fA-F]))+)*/?) # path     
   (\?(                                                              # query:   
       (([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+  # isindex 
   (\#([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+)? # fragment 
 # Handle ftp:                                                                  
  ((/([A-Za-z0-9\-\_\.\!\~\*\'\(\)]|(%[2-9A-Fa-f][0-9a-fA-F]))+)*/?) # path     
  (\#([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+)? # fragment  

Even the sophisticated pattern shown above doesn't forbid all illegal URIs.
For example, again, "20.20" isn't a legal domain name, but it's allowed by
the pattern; however, to my knowledge this shouldn't cause any security
problems. The sophisticated pattern forbids URL escapes that represent
control characters (e.g.,  through $1F) - the smallest permitted escape
value is %20 (ASCII space). Forbidding control characters prevents some
trouble, but it's also limiting; change "2-9" to "0-9" everywhere if you need
to support sending all control characters to arbitrary web applications. This
pattern does permit all other URL escape values in paths, which is useful for
international characters but could cause trouble for a few systems which
can't handle it. The pattern at least prevents spaces, linefeeds,
double-quotes, and other dangerous characters from being in the URI, which
prevents other kinds of attacks when incorporating the URI into a generated
document. Note that the pattern permits ``+'' in many places, since in
practice the plus is often used to replace the space character in queries and

Unfortunately, as noted above, there are attacks which can work through any
technique that permit query data, and there don't seem to be really good
defenses for them once you permit queries. So, you could strip out the
ability to use query data from the pattern above, but permit the other forms,
producing a ``sophisticated mostly safe'' pattern:
  # Handle http, https, and relative URIs:                                      
  ((/([A-Za-z0-9\-\_\.\!\~\*\'\(\)]|(%[2-9A-Fa-f][0-9a-fA-F]))+)*/?) # path     
   (\#([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+)? # fragment 
 # Handle ftp:                                                                  
  ((/([A-Za-z0-9\-\_\.\!\~\*\'\(\)]|(%[2-9A-Fa-f][0-9a-fA-F]))+)*/?) # path     
  (\#([A-Za-z0-9\-\_\.\!\~\*\'\(\)\+]|(%[2-9A-Fa-f][0-9a-fA-F]))+)? # fragment  

As far as I can tell, as long as these patterns are only used to check
hypertext anchors selected by the user (the "<a>" tag) this approach also
prevents the insertion of ``web bugs''. Web bugs are simply text that allow
someone other than the originating web server of the main page to track
information such as who read the content and when they read it - see Section
8.7 for more information. This isn't true if you use the <img> (image) tag
with the same checking rules - the image tag is loaded immediately,
permitting someone to add a ``web bug''. Once again, this presumes that
you're not permitting any attributes; many attributes can be quite dangerous
and pierce the security you're trying to provide.

Please note that all of these patterns require the entire URI match the
pattern. An unfortunate fact of these patterns is that they limit the
allowable patterns in a way that forbids many useful ones (e.g., they prevent
the use of new URI schemes). Also, none of them can prevent the very real
problem that some web sites perform more than queries when presented with a
query - and some of these web sites are internal to an organization. As a
result, no URI can really be safe until there are no web sites that accept
GET queries as an action (see Section 5.12). For more information about legal
URLs/URIs, see IETF RFC 2396; domain name syntax is further discussed in IETF
RFC 1034.

5.11.5. Other HTML tags

You might even consider supporting more HTML tags. Obvious next choices are
the list-oriented tags, such as <ol> (ordered list), <ul> (unordered list),
and <li> (list item). However, after a certain point you're really permitting
full publishing (in which case you need to trust the provider or perform more
serious checking than will be described here). Even more importantly, every
new functionality you add creates an opportunity for error (and exploit).

One example would be permitting the <img> (image) tag with the same URI
pattern. It turns out this is substantially less safe, because this permits
third parties to insert ``web bugs'' into the document, identifying who read
the document and when. See Section 8.7 for more information on web bugs.

5.11.6. Related Issues

Web applications should also explicitly specify the character set (usually
ISO-8859-1), and not permit other characters, if data from untrusted users is
being used. See Section 9.5 for more information.

Since filtering this kind of input is easy to get wrong, other alternatives
have been discussed as well. One option is to ask users to use a different
language, much simpler than HTML, that you've designed - and you give that
language very limited functionality. Another approach is parsing the HTML
into some internal ``safe'' format, and then translating that safe format
back to HTML.

Filtering can be done during input, output, or both. The CERT recommends
filtering data during the output process, just before it is rendered as part
of the dynamic page. This is because, if it is done correctly, this approach
ensures that all dynamic content is filtered. The CERT believes that
filtering on the input side is less effective because dynamic content can be
entered into a web sites database(s) via methods other than HTTP, and in this
case, the web server may never see the data as part of the input process.
Unless the filtering is implemented in all places where dynamic data is
entered, the data elements may still be remain tainted.

However, I don't agree with CERT on this point for all cases. The problem is
that it's just as easy to forget to filter all the output as the input, and
allowing ``tainted'' input into your system is a disaster waiting to happen
anyway. A secure program has to filter its inputs anyway, so it's sometimes
better to include all of these checks as part of the input filtering (so that
maintainers can see what the rules really are). And finally, in some secure
programs there are many different program locations that may output a value,
but only a very few ways and locations where a data can be input into it; in
such cases filtering on input may be a better idea.

5.12. Forbid HTTP GET To Perform Non-Queries

Web-based applications using HTTP should prevent the use of the HTTP ``GET''
or ``HEAD'' method for anything other than queries. HTTP includes a number of
different methods; the two most popular methods used are GET and POST. Both
GET and POST can be used to transmit data from a form, but the GET method
transmits data in the URL, while the POST method transmits data separately.

The security problem of using GET to perform non-queries (such as changing
data, transferring money, or signing up for a service) is that an attacker
can create a hypertext link with a URL that includes malicious form data. If
the attacker convinces a victim to click on the link (in the case of a
hypertext link), or even just view a page (in the case of transcluded
information such as images from HTML's img tag), the victim will perform a
GET. When the GET is performed, all of the form data created by the attacker
will be sent by the victim to the link specified. This is a cross-site
malicious content attack, as discussed further in Section 7.15.

If the only action that a malicious cross-site content attack can perform is
to make the user view unexpected data, this isn't as serious a problem. This
can still be a problem, of course, since there are some attacks that can be
made using this capability. For example, there's a potential loss of privacy
due to the user requesting something unexpected, possible real-world effects
from appearing to request illegal or incriminating material, or by making the
user request the information in certain ways the information may be exposed
to an attacker in ways it normally wouldn't be exposed. However, even more
serious effects can be caused if the malicious attacker can cause not just
data viewing, but changes in data, through a cross-site link.

Typical HTTP interfaces (such as most CGI libraries) normally hide the
differences between GET and POST, since for getting data it's useful to treat
the methods ``the same way.'' However, for actions that actually cause
something other than a data query, check to see if the request is something
other than POST; if it is, simply display a filled-in form with the data
given and ask the user to confirm that they really mean the request. This
will prevent cross-site malicious content attacks, while still giving users
the convenience of confirming the action with a single click.

Indeed, this behavior is strongly recommended by the HTTP specification.
According to the HTTP 1.1 specification (IETF RFC 2616 section 9.1.1), ``the
GET and HEAD methods SHOULD NOT have the significance of taking an action
other than retrieval. These methods ought to be considered "safe". This
allows user agents to represent other methods, such as POST, PUT and DELETE,
in a special way, so that the user is made aware of the fact that a possibly
unsafe action is being requested.''

In the interest of fairness, I should note that this doesn't completely solve
the problem, because on some browsers (in some configurations) scripted posts
can do the same thing. For example, imagine a web browser with ECMAscript
(Javascript) enabled receiving the following HTML snippet - on some browsers,
simply displaying this HTML snippet will automatically force the user to send
a POST request to a website chosen by the attacker, with form data defined by
the attacker:
  <form action=http://remote/script.cgi method=post name=b>                  
    <input type=hidden name=action value="do something">                     
    <input type=submit>                                                      
My thanks to David deVitry pointing this out. However, although this advice
doesn't solve all problems, it's still worth doing. In part, this is because
the remaining problem can be solved by smarter web browsers (e.g., by always
confirming the data before allowing ECMAscript to send a web form) or by web
browser configuration (e.g., disabling ECMAscript). Also, this attack doesn't
work in many cross-site scripting exploits, because many websites don't allow
users to post ``script'' commands but do allow arbitrary URL links. Thus,
limiting the actions a GET command can perform to queries significantly
improves web application security.

5.13. Counter SPAM

Any program that can send email elsewhere, by request from the network, can
be used to transport spam. Spam is the usual name for unsolicited bulk email
(UBE) or mass unsolicited email. It's also sometimes called unsolicited
commercial email (UCE), though that name is misleading - not all spam is
commercial. For a discussion of why spam is such a serious problem and more
general discussion about it, see my essay at [
stopspam.html], as well as
[], [], [http://] CAUCE, and [http://] IETF RFC 2635. Spam receivers and
intermediaries bear most of the cost of spam, while the spammer spends very
little to send it. Therefore many people regard spam as a theft of service,
not just some harmless activity, and that number increases as the amount of
spam increases.

If your program can be used to generate email sent to others (such as a mail
transfer agent, generator of data sent by email, or a mailing list manager),
be sure to write your program to prevent its unauthorized use as a mail
relay. A program should usually only allow legitimate authorized users to
send email to others (e.g., those inside that company's mail server or those
legitimately subscribed to the service). More information about this is in
[] IETF RFC 2505 Also, if you manage a
mailing list, make sure that it can enforce the rule that only subscribers
can post to the list, and create a ``log in'' feature that will make it
somewhat harder for spammers to subscribe, spam, and unsubscribe easily.

One way to more directly counter SPAM is to incorporate support for the MAPS
(Mail Abuse Prevention System LLC) RBL (Realtime Blackhole List), which
maintains in real-time a list of IP addresses where SPAM is known to
originate. For more information, see [] http:// Many current Mail Transfer Agents (MTAs) already support
the RBL; see their websites for how to configure them. The usual way to use
the RBL is to simply refuse to accept any requests from IP addresses in the
blackhole list; this is harsh, but it solves the problem. Another similar
service is the Open Relay Database (ORDB) at [] http://, which identifies dynamically those sites that permit open email
relays (open email relays are misconfigured email servers that allow spammers
to send email through them). Another location for more information is [http:/
/] SPEWS. I believe there are other similar services as well.

I suggest that many systems and programs, by default, enable spam blocking if
they can send email on to others whose identity is under control of a remote
user - and that includes MTAs. At the least, consider this. There are real
problems with this suggestion, of course - you might (rarely) inhibit
communication with a legitimate user. On the other hand, if you don't block
spam, then it's likely that everyone else will blackhole your system (and
thus ignore your emails). It's not a simple issue, because no matter what you
do, some people will not allow you to send them email. And of course, how
well do you trust the organization keeping up the real-time blackhole list -
will they add truly innocent sites to the blackhole list, and will they
remove sites from the blackhole list once all is okay? Thus, it becomes a
trade-off - is it more important to talk to spammers (and a few innocents as
well), or is it more important to talk to those many other systems with spam
blocks (losing those innocents who share equipment with spammers)? Obviously,
this must be configurable. This is somewhat controversial advice, so consider
your options for your circumstance.

5.14. Limit Valid Input Time and Load Level

Place time-outs and load level limits, especially on incoming network data.
Otherwise, an attacker might be able to easily cause a denial of service by
constantly requesting the service.

Chapter 6. Avoid Buffer Overflow

                                       An enemy will overrun the land; he    
                                       will pull down your strongholds and   
                                       plunder your fortresses.              
                                                              Amos 3:11 (NIV)

An extremely common security flaw is vulnerability to a ``buffer overflow''.
Buffer overflows are also called ``buffer overruns'', and there are many
kinds of buffer overflow attacks (including ``stack smashing'' and ``heap
smashing'' attacks). Technically, a buffer overflow is a problem with the
program's internal implementation, but it's such a common and serious problem
that I've placed this information in its own chapter. To give you an idea of
how important this subject is, at the CERT, 9 of 13 advisories in 1998 and at
least half of the 1999 advisories involved buffer overflows. An informal 1999
survey on Bugtraq found that approximately 2/3 of the respondents felt that
buffer overflows were the leading cause of system security vulnerability (the
remaining respondents identified ``mis-configuration'' as the leading cause)
[Cowan 1999]. This is an old, well-known problem, yet it continues to
resurface [McGraw 2000].

A buffer overflow occurs when you write a set of values (usually a string of
characters) into a fixed length buffer and write at least one value outside
that buffer's boundaries (usually past its end). A buffer overflow can occur
when reading input from the user into a buffer, but it can also occur during
other kinds of processing in a program.

If a secure program permits a buffer overflow, the overflow can often be
exploited by an adversary. If the buffer is a local C variable, the overflow
can be used to force the function to run code of an attackers' choosing. This
specific variation is often called a ``stack smashing'' attack. A buffer in
the heap isn't much better; attackers may be able to use such overflows to
control other variables in the program. More details can be found from Aleph1
[1996], Mudge [1995], LSD [2001], or the Nathan P. Smith's "Stack Smashing
Security Vulnerabilities" website at [] A discussion of the problem and some
ways to counter them is given by Crispin Cowan et al, 2000, at [http://]
discex00.pdf. A discussion of the problem and some ways to counter them in
Linux is given by Pierre-Alain Fayolle and Vincent Glaume at [http://]

Most high-level programming languages are essentially immune to this problem,
either because they automatically resize arrays (e.g., Perl), or because they
normally detect and prevent buffer overflows (e.g., Ada95). However, the C
language provides no protection against such problems, and C++ can be easily
used in ways to cause this problem too. Assembly language also provides no
protection, and some languages that normally include such protection (e.g.,
Ada and Pascal) can have this protection disabled (for performance reasons).
Even if most of your program is written in another language, many library
routines are written in C or C++, as well as ``glue'' code to call them, so
other languages often don't provide as complete a protection from buffer
overflows as you'd like.

6.1. Dangers in C/C++

C users must avoid using dangerous functions that do not check bounds unless
they've ensured that the bounds will never get exceeded. Functions to avoid
in most cases (or ensure protection) include the functions strcpy(3), strcat
(3), sprintf(3) (with cousin vsprintf(3)), and gets(3). These should be
replaced with functions such as strncpy(3), strncat(3), snprintf(3), and
fgets(3) respectively, but see the discussion below. The function strlen(3)
should be avoided unless you can ensure that there will be a terminating NIL
character to find. The scanf() family (scanf(3), fscanf(3), sscanf(3), vscanf
(3), vsscanf(3), and vfscanf(3)) is often dangerous to use; do not use it to
send data to a string without controlling the maximum length (the format %s
is a particularly common problem). Other dangerous functions that may permit
buffer overruns (depending on their use) include realpath(3), getopt(3),
getpass(3), streadd(3), strecpy(3), and strtrns(3). You must be careful with
getwd(3); the buffer sent to getwd(3) must be at least PATH_MAX bytes long.
The select(2) helper macros FD_SET(), FD_CLR(), and FD_ISSET() do not check
that the index fd is within bounds; make sure that fd >= 0 and fd <=
FD_SETSIZE (this particular one has been exploited in pppd).

Unfortunately, snprintf()'s variants have additional problems. Officially,
snprintf() is not a standard C function in the ISO 1990 (ANSI 1989) standard,
though sprintf() is, so not all systems include snprintf(). Even worse, some
systems' snprintf() do not actually protect against buffer overflows; they
just call sprintf directly. Old versions of Linux's libc4 depended on a
``libbsd'' that did this horrible thing, and I'm told that some old HP
systems did the same. Linux's current version of snprintf is known to work
correctly, that is, it does actually respect the boundary requested. The
return value of snprintf() varies as well; the Single Unix Specification
(SUS) version 2 and the C99 standard differ on what is returned by snprintf
(). Finally, it appears that at least some versions of snprintf don't
guarantee that its string will end in NIL; if the string is too long, it
won't include NIL at all. Note that the glib library (the basis of GTK, and
not the same as the GNU C library glibc) has a g_snprintf(), which has a
consistent return semantic, always NIL-terminates, and most importantly
always respects the buffer length.

Of course, the problem is more than just calling string functions poorly.
Here are a few additional examples of types of buffer overflow problems,
graciously suggested by Timo Sirainen, involving manipulation of numbers to
cause buffer overflows.

First, there's the problem of signedness. If you read data that affects the
buffer size, such as the "number of characters to be read," be sure to check
if the number is less than zero or one. Otherwise, the negative number may be
cast to an unsigned number, and the resulting large positive number may then
permit a buffer overflow problem. Note that sometimes an attacker can provide
a large positive number and have the same thing happen; in some cases, the
large value will be interpreted as a negative number (slipping by the check
for large numbers if there's no check for a less-than-one value), and then be
interpreted later into a large positive value.
 /* 1) signedness - DO NOT DO THIS. */                                       
 char *buf;                                                                  
 int i, len;                                                                 
 read(fd, &len, sizeof(len));                                                
 /* OOPS!  We forgot to check for < 0 */                                     
 if (len > 8000) { error("too large length"); return; }                      
 buf = malloc(len);                                                          
 read(fd, buf, len); /* len casted to unsigned and overflows */              

Here's a second example identified by Timo Sirainen, involving integer size
truncation. Sometimes the different sizes of integers can be exploited to
cause a buffer overflow. Basically, make sure that you don't truncate any
integer results used to compute buffer sizes. Here's Timo's example for
64-bit architectures:
 /* An example of an ERROR for some 64-bit architectures,                    
    if "unsigned int" is 32 bits and "size_t" is 64 bits: */                 
 void *mymalloc(unsigned int size) { return malloc(size); }                  
 char *buf;                                                                  
 size_t len;                                                                 
 read(fd, &len, sizeof(len));                                                
 /* we forgot to check the maximum length */                                 
 /* 64-bit size_t gets truncated to 32-bit unsigned int */                   
 buf = mymalloc(len);                                                        
 read(fd, buf, len);                                                         

Here's a third example from Timo Sirainen, involving integer overflow. This
is particularly nasty when combined with malloc(); an attacker may be able to
create a situation where the computed buffer size is less than the data to be
placed in it. Here is Timo's sample:
 /* 3) integer overflow */                                                   
 char *buf;                                                                  
 size_t len;                                                                 
 read(fd, &len, sizeof(len));                                                
 /* we forgot to check the maximum length */                                 
 buf = malloc(len+1); /* +1 can overflow to malloc(0) */                     
 read(fd, buf, len);                                                         
 buf[len] = '\0';                                                            

6.2. Library Solutions in C/C++

One partial solution in C/C++ is to use library functions that do not have
buffer overflow problems. The first subsection describes the ``standard C
library'' solution, which can work but has its disadvantages. The next
subsection describes the general security issues of both fixed length and
dynamically reallocated approaches to buffers. The following subsections
describe various alternative libraries, such as strlcpy and libmib. Note that
these don't solve all problems; you still have to code extremely carefully in
C/C++ to avoid all buffer overflow situations.

6.2.1. Standard C Library Solution

The ``standard'' solution to prevent buffer overflow in C (which is also used
in some C++ programs) is to use the standard C library calls that defend
against these problems. This approach depends heavily on the standard library
functions strncpy(3) and strncat(3). If you choose this approach, beware:
these calls have somewhat surprising semantics and are hard to use correctly.
The function strncpy(3) does not NIL-terminate the destination string if the
source string length is at least equal to the destination's, so be sure to
set the last character of the destination string to NIL after calling strncpy
(3). If you're going to reuse the same buffer many times, an efficient
approach is to tell strncpy() that the buffer is one character shorter than
it actually is and set the last character to NIL once before use. Both
strncpy(3) and strncat(3) require that you pass the amount of space left
available, a computation that is easy to get wrong (and getting it wrong
could permit a buffer overflow attack). Neither provide a simple mechanism to
determine if an overflow has occurred. Finally, strncpy(3) has a significant
performance penalty compared to the strcpy(3) it supposedly replaces, because
strncpy(3) NIL-fills the remainder of the destination. I've gotten emails
expressing surprise over this last point, but this is clearly stated in
Kernighan and Ritchie second edition [Kernighan 1988, page 249], and this
behavior is clearly documented in the man pages for Linux, FreeBSD, and
Solaris. This means that just changing from strcpy to strncpy can cause a
severe reduction in performance, for no good reason in most cases.

Warning!! The function strncpy(s1, s2, n) can also be used as a way of
copying only part of s2, where n is less than strlen(s2). When used this way,
strncpy() basically provides no protection against buffer overflow by itself
- you have to take separate actions to ensure that n is smaller than the
buffer of s1. Also, when used this way, strncpy() does not usually add a
trailing NIL after copying n characters. This makes it harder to determine if
a program using strncpy() is secure.

You can also use sprintf() while preventing buffer overflows, but you need to
be careful when doing so; it's so easy to misapply that it's hard to
recommend. The sprintf control string can contain various conversion
specifiers (e.g., "%s"), and the control specifiers can have optional field
width (e.g., "%10s") and precision (e.g., "%.10s") specifications. These look
quite similar (the only difference is a period) but they are very different.
The field width only specifies a minimum length and is completely worthless
for preventing buffer overflows. In contrast, the precision specification
specifies the maximum length that that particular string may have in its
output when used as a string conversion specifier - and thus it can be used
to protect against buffer overflows. Note that the precision specification
only specifies the total maximum length when dealing with a string; it has a
different meaning for other conversion operations. If the size is given as a
precision of "*", then you can pass the maximum size as a parameter (e.g.,
the result of a sizeof() operation). This is most easily shown by an example
- here's the wrong and right way to use sprintf() to protect against buffer
 char buf[BUFFER_SIZE];                                                      
 sprintf(buf, "%*s",  sizeof(buf)-1, "long-string");  /* WRONG */            
 sprintf(buf, "%.*s", sizeof(buf)-1, "long-string");  /* RIGHT */            
In theory, sprintf() should be very helpful because you can use it to specify
complex formats. Sadly, it's easy to get things wrong with sprintf(). If the
format is complex, you need to make sure that the destination is large enough
for the largest possible size of the entire format, but the precision field
only controls the size of one parameter. The "largest possible" value is
often hard to determine when a complicated output is being created. If a
program doesn't allocate quite enough space for the longest possible
combination, a buffer overflow vulnerability may open up. Also, sprintf()
appends a NUL to the destination after the entire operation is complete -
this extra character is easy to forget and creates an opportunity for
off-by-one errors. So, while this works, it can be painful to use in some

Also, a quick note about the code above - note that the sizeof() operation
used the size of an array. If the code were changed so that ``buf'' was a
pointer to some allocated memory, then all ``sizeof()'' operations would have
to be changed (or sizeof would just measure the size of a pointer, which
isn't enough space for most values).

The scanf() family is sadly a little murky as well. An obvious question is
whether or not the maximum width value can be used in %s to prevent these
attacks. There are multiple official specifications for scanf(); some clearly
state that the width parameter is the absolutely largest number of
characters, while others aren't as clear. The biggest problem is
implementations; modern implementations that I know of do support maximum
widths, but I cannot say with certainty that all libraries properly implement
maximum widths. The safest approach is to do things yourself in such cases.
However, few will fault you if you simply use scanf and include the widths in
the format strings (but don't forget to count \0, or you'll get the wrong
length). If you do use scanf, it's best to include a test in your
installation scripts to ensure that the library properly limits length. 

6.2.2. Static and Dynamically Allocated Buffers

Functions such as strncpy are useful for dealing with statically allocated
buffers. This is a programming approach where a buffer is allocated for the
``longest useful size'' and then it stays a fixed size from then on. The
alternative is to dynamically reallocate buffer sizes as you need them. It
turns out that both approaches have security implications.

There is a general security problem when using fixed-length buffers: the fact
that the buffer is a fixed length may be exploitable. This is a problem with
strncpy(3) and strncat(3), snprintf(3), strlcpy(3), strlcat(3), and other
such functions. The basic idea is that the attacker sets up a really long
string so that, when the string is truncated, the final result will be what
the attacker wanted (instead of what the developer intended). Perhaps the
string is catenated from several smaller pieces; the attacker might make the
first piece as long as the entire buffer, so all later attempts to
concatenate strings do nothing. Here are some specific examples:

  * Imagine code that calls gethostbyname(3) and, if successful, immediately
    copies hostent->h_name to a fixed-size buffer using strncpy or snprintf.
    Using strncpy or snprintf protects against an overflow of an excessively
    long fully-qualified domain name (FQDN), so you might think you're done.
    However, this could result in chopping off the end of the FQDN. This may
    be very undesirable, depending on what happens next.
  * Imagine code that uses strncpy, strncat, snprintf, etc., to copy the full
    path of a filesystem object to some buffer. Further imagine that the
    original value was provided by an untrusted user, and that the copying is
    part of a process to pass a resulting computation to a function. Sounds
    safe, right? Now imagine that an attacker pads a path with a large number
    of '/'s at the beginning. This could result in future operations being
    performed on the file ``/''. If the program appends values in the belief
    that the result will be safe, the program may be exploitable. Or, the
    attacker could devise a long filename near the buffer length, so that
    attempts to append to the filename would silently fail to occur (or only
    partially occur in ways that may be exploitable).


When using statically-allocated buffers, you really need to consider the
length of the source and destination arguments. Sanity checking the input and
the resulting intermediate computation might deal with this, too.

Another alternative is to dynamically reallocate all strings instead of using
fixed-size buffers. This general approach is recommended by the GNU
programming guidelines, since it permits programs to handle arbitrarily-sized
inputs (until they run out of memory). Of course, the major problem with
dynamically allocated strings is that you may run out of memory. The memory
may even be exhausted at some other point in the program than the portion
where you're worried about buffer overflows; any memory allocation can fail.
Also, since dynamic reallocation may cause memory to be inefficiently
allocated, it is entirely possible to run out of memory even though
technically there is enough virtual memory available to the program to
continue. In addition, before running out of memory the program will probably
use a great deal of virtual memory; this can easily result in ``thrashing'',
a situation in which the computer spends all its time just shuttling
information between the disk and memory (instead of doing useful work). This
can have the effect of a denial of service attack. Some rational limits on
input size can help here. In general, the program must be designed to fail
safely when memory is exhausted if you use dynamically allocated strings.

6.2.3. strlcpy and strlcat

An alternative, being employed by OpenBSD, is the strlcpy(3) and strlcat(3)
functions by Miller and de Raadt [Miller 1999]. This is a minimalist,
statically-sized buffer approach that provides C string copying and
concatenation with a different (and less error-prone) interface. Source and
documentation of these functions are available under a newer BSD-style open
source license at [

First, here are their prototypes:
|size_t strlcpy (char *dst, const char *src, size_t size);                  |
|size_t strlcat (char *dst, const char *src, size_t size);                  |
Both strlcpy and strlcat take the full size of the destination buffer as a
parameter (not the maximum number of characters to be copied) and guarantee
to NIL-terminate the result (as long as size is larger than 0). Remember that
you should include a byte for NIL in the size.

The strlcpy function copies up to size-1 characters from the NUL-terminated
string src to dst, NIL-terminating the result. The strlcat function appends
the NIL-terminated string src to the end of dst. It will append at most size
- strlen(dst) - 1 bytes, NIL-terminating the result.

One minor disadvantage of strlcpy(3) and strlcat(3) is that they are not, by
default, installed in most Unix-like systems. In OpenBSD, they are part of <
string.h>. This is not that difficult a problem; since they are small
functions, you can even include them in your own program's source (at least
as an option), and create a small separate package to load them. You can even
use autoconf to handle this case automatically. If more programs use these
functions, it won't be long before these are standard parts of Linux
distributions and other Unix-like systems. Also, these functions have been
recently added to the ``glib'' library (I submitted the patch to do this), so
using recent versions of glib makes them available. In glib these functions
are named g_strlcpy and g_strlcat (not strlcpy or strlcat) to be consistent
with the glib library naming conventions.

Also, strlcat(3) has slightly varying semantics when the provided size is 0
or if there are no NIL characters in the destination string dst (inside the
given number of characters). In OpenBSD, if the size is 0, then the
destination string's length is considered 0. Also, if size is nonzero, but
there are no NIL characters in the destination string (in the size number of
characters), then the length of the destination is considered equal to the
size. These rules make handling strings without embedded NILs consistent.
Unfortunately, at least Solaris doesn't (at this time) obey these rules,
because they weren't specified in the original documentation. I've talked to
Todd Miller, and he and I agree that the OpenBSD semantics are the correct
ones (and that Solaris is incorrect). The reasoning is simple: under no
condition should strlcat or strlcpy ever examine characters in the
destination outside of the range of size; such access might cause core dumps
(from accessing out-of-range memory) and even hardware interactions (through
memory-mapped I/O). Thus, given:
|  a = strlcat ("Y", "123", 0);                                             |
The correct answer is 3 (0+3=3), but Solaris will claim the answer is 4
because it incorrectly looks at characters beyond the "size" length in the
destination. For now, I suggest avoiding cases where the size is 0 or the
destination has no NIL characters. Future versions of glib will hide this
difference and always use the OpenBSD semantics.

6.2.4. libmib

One toolset for C that dynamically reallocates strings automatically is the
``libmib allocated string functions'' by Forrest J. Cavalier III, available
at []
libmib/astring. There are two variations of libmib; ``libmib-open'' appears
to be clearly open source under its own X11-like license that permits
modification and redistribution, but redistributions must choose a different
name, however, the developer states that it ``may not be fully tested.'' To
continuously get libmib-mature, you must pay for a subscription. The
documentation is not open source, but it is freely available.

6.2.5. C++ std::string class

C++ developers can use the std::string class, which is built into the
language. This is a dynamic approach, as the storage grows as necessary.
However, it's important to note that if that class's data is turned into a
``char *'' (e.g., by using data() or c_str()), the possibilities of buffer
overflow resurface, so you need to be careful when when using such methods.
Note that c_str() always returns a NIL-terminated string, but data() may or
may not (it's implementation dependent, and most implementations do not
include the NIL terminator). Avoid using data(), and if you must use it,
don't be dependent on its format.

Many C++ developers use other string libraries as well, such as those that
come with other large libraries or even home-grown string libraries. With
those libraries, be especially careful - many alternative C++ string classes
include routines to automatically convert the class to a ``char *'' type. As
a result, they can silently introduce buffer overflow vulnerabilities.

6.2.6. Libsafe

Arash Baratloo, Timothy Tsai, and Navjot Singh (of Lucent Technologies) have
developed Libsafe, a wrapper of several library functions known to be
vulnerable to stack smashing attacks. This wrapper (which they call a kind of
``middleware'') is a simple dynamically loaded library that contains modified
versions of C library functions such as strcpy(3). These modified versions
implement the original functionality, but in a manner that ensures that any
buffer overflows are contained within the current stack frame. Their initial
performance analysis suggests that this library's overhead is very small.
Libsafe papers and source code are available at [http://]
/project/libsafe. The Libsafe source code is available under the completely
open source LGPL license.

Libsafe's approach appears somewhat useful. Libsafe should certainly be
considered for inclusion by Linux distributors, and its approach is worth
considering by others as well. For example, I know that the Mandrake
distribution of Linux (version 7.1) includes it. However, as a software
developer, Libsafe is a useful mechanism to support defense-in-depth but it
does not really prevent buffer overflows. Here are several reasons why you
shouldn't depend just on Libsafe during code development:

  * Libsafe only protects a small set of known functions with obvious buffer
    overflow issues. At the time of this writing, this list is significantly
    shorter than the list of functions in this book known to have this
    problem. It also won't protect against code you write yourself (e.g., in
    a while loop) that causes buffer overflows.
  * Even if libsafe is installed in a distribution, the way it is installed
    impacts its use. The documentation recommends setting LD_PRELOAD to cause
    libsafe's protections to be enabled, but the problem is that users can
    unset this environment variable... causing the protection to be disabled
    for programs they execute!
  * Libsafe only protects against buffer overflows of the stack onto the
    return address; you can still overrun the heap or other variables in that
    procedure's frame.
  * Unless you can be assured that all deployed platforms will use libsafe
    (or something like it), you'll have to protect your program as though it
    wasn't there.
  * LibSafe seems to assume that saved frame pointers are at the beginning of
    each stack frame. This isn't always true. Compilers (such as gcc) can
    optimize away things, and in particular the option "-fomit-frame-pointer"
    removes the information that libsafe seems to need. Thus, libsafe may
    fail to work for some programs.

The libsafe developers themselves acknowledge that software developers
shouldn't just depend on libsafe. In their words:

    It is generally accepted that the best solution to buffer overflow
    attacks is to fix the defective programs. However, fixing defective
    programs requires knowing that a particular program is defective. The
    true benefit of using libsafe and other alternative security measures is
    protection against future attacks on programs that are not yet known to
    be vulnerable.
6.2.7. Other Libraries

The glib (not glibc) library is a widely-available open source library that
provides a number of useful functions for C programmers. GTK+ and GNOME both
use glib, for example. As I noted earlier, in glib version 1.3.2, g_strlcpy()
and g_strlcat() have been added through a patch which I submitted. This
should make it easier to portably use those functions once these later
versions of glib become widely available. At this time I do not have an
analysis showing definitively that the glib library functions protect against
buffer overflows. However, many of the glib functions automatically allocate
memory, and those functions automatically fail with no reasonable way to
intercept the failure (e.g., to try something else instead). As a result, in
many cases most glib functions cannot be used in most secure programs. The
GNOME guidelines recommend using functions such as g_strdup_printf(), which
is fine as long as it's okay if your program immediately crashes if an
out-of-memory condition occurs. However, if you can't accept this, then using
such routines isn't appropriate.

6.3. Compilation Solutions in C/C++

A completely different approach is to use compilation methods that perform
bounds-checking (see [Sitaker 1999] for a list). In my opinion, such tools
are very useful in having multiple layers of defense, but it's not wise to
use this technique as your sole defense. There are at least two reasons for
this. First of all, such tools generally only provide a partial defense
against buffer overflows (and the ``complete'' defenses are generally 12-30
times slower); C and C++ were simply not designed to protect against buffer
overflows. Second of all, for open source programs you cannot be certain what
tools will be used to compile the program; using the default ``normal''
compiler for a given system might suddenly open security flaws.

One of the more useful tools is ``StackGuard'', a modification of the
standard GNU C compiler gcc. StackGuard works by inserting a ``guard'' value
(called a ``canary'') in front of the return address; if a buffer overflow
overwrites the return address, the canary's value (hopefully) changes and the
system detects this before using it. This is quite valuable, but note that
this does not protect against buffer overflows overwriting other values
(which they may still be able to use to attack a system). There is work to
extend StackGuard to be able to add canaries to other data items, called
``PointGuard''. PointGuard will automatically protect certain values (e.g.,
function pointers and longjump buffers). However, protecting other variable
types using PointGuard requires specific programmer intervention (the
programmer has to identify which data values must be protected with
canaries). This can be valuable, but it's easy to accidentally omit
protection for a data value you didn't think needed protection - but needs it
anyway. More information on StackGuard, PointGuard, and other alternatives is
in Cowan [1999].

IBM has developed a stack protection system called ProPolice based on the
ideas of StackGuard. IBM doesn't include the ProPolice in its current website
- it's just called a "GCC extension for protecting applications from
stack-smashing attacks." Like StackGuard, ProPolice is a GCC (Gnu Compiler
Collection) extension for protecting applications from stack-smashing
attacks. Applications written in C are protected by automatically inserting
protection code into an application at compilation time. ProPolice is
slightly different than StackGuard, however, by adding three features: (1)
reordering local variables to place buffers after pointers (to avoid the
corruption of pointers that could be used to further corrupt arbitrary memory
locations), (2) copying pointers in function arguments to an area preceding
local variable buffers (to prevent the corruption of pointers that could be
used to further corrupt arbitrary memory locations), and (3) omitting
instrumentation code from some functions (it basically assumes that only
character arrays are dangerous; while this isn't strictly true, it's mostly
true, and as a result ProPolice has better performance while retaining most
of its protective capabilities). The IBM website includes information for how
to build Red Hat Linux and FreeBSD with this protection; OpenBSD has already
added ProPolice to their base system. I think this is extremely promising,
and I hope to see this capability included in future versions of gcc and used
in various distributions. In fact, I think this kind of capability should be
the default - this would mean that the largest single class of attacks would
no longer enable attackers to take control in most cases.

As a related issue, in Linux you could modify the Linux kernel so that the
stack segment is not executable; such a patch to Linux does exist (see Solar
Designer's patch, which includes this, at [] However, as of this writing this is not built
into the Linux kernel. Part of the rationale is that this is less protection
than it seems; attackers can simply force the system to call other
``interesting'' locations already in the program (e.g., in its library, the
heap, or static data segments). Also, sometimes Linux does require executable
code in the stack, e.g., to implement signals and to implement GCC
``trampolines''. Solar Designer's patch does handle these cases, but this
does complicate the patch. Personally, I'd like to see this merged into the
main Linux distribution, since it does make attacks somewhat more difficult
and it defends against a range of existing attacks. However, I agree with
Linus Torvalds and others that this does not add the amount of protection it
would appear to and can be circumvented with relative ease. You can read
Linus Torvalds' explanation for not including this support at [http://]

In short, it's better to work first on developing a correct program that
defends itself against buffer overflows. Then, after you've done this, by all
means use techniques and tools like StackGuard as an additional safety net.
If you've worked hard to eliminate buffer overflows in the code itself, then
StackGuard (and tools like it) are are likely to be more effective because
there will be fewer ``chinks in the armor'' that StackGuard will be called on
to protect.

6.4. Other Languages

The problem of buffer overflows is an excellent argument for using other
programming languages such as Perl, Python, Java, and Ada95. After all,
nearly all other programming languages used today (other than assembly
language) protect against buffer overflows. Using those other languages does
not eliminate all problems, of course; in particular see the discussion in 
Section 8.3 regarding the NIL character. There is also the problem of
ensuring that those other languages' infrastructure (e.g., run-time library)
is available and secured. Still, you should certainly consider using other
programming languages when developing secure programs to protect against
buffer overflows.

Chapter 7. Structure Program Internals and Approach

                                       Like a city whose walls are broken    
                                       down is a man who lacks self-control. 
                                                         Proverbs 25:28 (NIV)

7.1. Follow Good Software Engineering Principles for Secure Programs

Saltzer [1974] and later Saltzer and Schroeder [1975] list the following
principles of the design of secure protection systems, which are still valid:

  * Least privilege. Each user and program should operate using the fewest
    privileges possible. This principle limits the damage from an accident,
    error, or attack. It also reduces the number of potential interactions
    among privileged programs, so unintentional, unwanted, or improper uses
    of privilege are less likely to occur. This idea can be extended to the
    internals of a program: only the smallest portion of the program which
    needs those privileges should have them. See Section 7.4 for more about
    how to do this.
  * Economy of mechanism/Simplicity. The protection system's design should be
    simple and small as possible. In their words, ``techniques such as
    line-by-line inspection of software and physical examination of hardware
    that implements protection mechanisms are necessary. For such techniques
    to be successful, a small and simple design is essential.'' This is
    sometimes described as the ``KISS'' principle (``keep it simple,
  * Open design. The protection mechanism must not depend on attacker
    ignorance. Instead, the mechanism should be public, depending on the
    secrecy of relatively few (and easily changeable) items like passwords or
    private keys. An open design makes extensive public scrutiny possible,
    and it also makes it possible for users to convince themselves that the
    system about to be used is adequate. Frankly, it isn't realistic to try
    to maintain secrecy for a system that is widely distributed; decompilers
    and subverted hardware can quickly expose any ``secrets'' in an
    implementation. Bruce Schneier argues that smart engineers should
    ``demand open source code for anything related to security'', as well as
    ensuring that it receives widespread review and that any identified
    problems are fixed [Schneier 1999].
  * Complete mediation. Every access attempt must be checked; position the
    mechanism so it cannot be subverted. For example, in a client-server
    model, generally the server must do all access checking because users can
    build or modify their own clients. This is the point of all of Chapter 5,
    as well as Section 7.2.
  * Fail-safe defaults (e.g., permission-based approach). The default should
    be denial of service, and the protection scheme should then identify
    conditions under which access is permitted. See Section 7.7 and Section
    7.9 for more.
  * Separation of privilege. Ideally, access to objects should depend on more
    than one condition, so that defeating one protection system won't enable
    complete access.
  * Least common mechanism. Minimize the amount and use of shared mechanisms
    (e.g. use of the /tmp or /var/tmp directories). Shared objects provide
    potentially dangerous channels for information flow and unintended
    interactions. See Section 7.10 for more information.
  * Psychological acceptability / Easy to use. The human interface must be
    designed for ease of use so users will routinely and automatically use
    the protection mechanisms correctly. Mistakes will be reduced if the
    security mechanisms closely match the user's mental image of his or her
    protection goals.


A good overview of various design principles for security is available in
Peter Neumann's []
Principled Assuredly Trustworthy Composable Architectures. 

7.2. Secure the Interface

Interfaces should be minimal (simple as possible), narrow (provide only the
functions needed), and non-bypassable. Trust should be minimized. Consider
limiting the data that the user can see.

7.3. Separate Data and Control

Any files you support should be designed to completely separate (passive)
data from programs that are executed. Applications and data viewers may be
used to display files developed externally, so in general don't allow them to
accept programs (also known as ``scripts'' or ``macros''). The most dangerous
kind is an auto-executing macro that executes when the application is loaded
and/or when the data is initially displayed; from a security point-of-view
this is generally a disaster waiting to happen.

If you truly must support programs downloaded remotely (e.g., to implement an
existing standard), make sure that you have extremely strong control over
what the macro can do (this is often called a ``sandbox''). Past experience
has shown that real sandboxes are hard to implement correctly. In fact, I
can't remember a single widely-used sandbox that hasn't been repeatedly
exploited (yes, that includes Java). If possible, at least have the programs
stored in a separate file, so that it's easier to block them out when another
sandbox flaw has been found but not yet fixed. Storing them separately also
makes it easier to reuse code and to cache it when helpful.

7.4. Minimize Privileges

As noted earlier, it is an important general principle that programs have the
minimal amount of privileges necessary to do its job (this is termed ``least
privilege''). That way, if the program is broken, its damage is limited. The
most extreme example is to simply not write a secure program at all - if this
can be done, it usually should be. For example, don't make your program
setuid or setgid if you can; just make it an ordinary program, and require
the administrator to log in as such before running it.

In Linux and Unix, the primary determiner of a process' privileges is the set
of id's associated with it: each process has a real, effective and saved id
for both the user and group (a few very old Unixes don't have a ``saved''
id). Linux also has, as a special extension, a separate filesystem UID and
GID for each process. Manipulating these values is critical to keeping
privileges minimized, and there are several ways to minimize them (discussed
below). You can also use chroot(2) to minimize the files visible to a
program, though using chroot() can be difficult to use correctly. There are a
few other values determining privilege in Linux and Unix, for example, POSIX
capabilities (supported by Linux 2.2 and greater, and by some other Unix-like

7.4.1. Minimize the Privileges Granted

Perhaps the most effective technique is to simply minimize the highest
privilege granted. In particular, avoid granting a program root privilege if
possible. Don't make a program setuid root if it only needs access to a small
set of files; consider creating separate user or group accounts for different

A common technique is to create a special group, change a file's group
ownership to that group, and then make the program setgid to that group. It's
better to make a program setgid instead of setuid where you can, since group
membership grants fewer rights (in particular, it does not grant the right to
change file permissions).

This is commonly done for game high scores. Games are usually setgid games,
the score files are owned by the group games, and the programs themselves and
their configuration files are owned by someone else (say root). Thus,
breaking into a game allows the perpetrator to change high scores but doesn't
grant the privilege to change the game's executable or configuration file.
The latter is important; if an attacker could change a game's executable or
its configuration files (which might control what the executable runs), then
they might be able to gain control of a user who ran the game.

If creating a new group isn't sufficient, consider creating a new pseudouser
(really, a special role) to manage a set of resources - often a new
pseudogroup (again, a special role) is also created just to run a program.
Web servers typically do this; often web servers are set up with a special
user (``nobody'') so that they can be isolated from other users. Indeed, web
servers are instructive here: web servers typically need root privileges to
start up (so they can attach to port 80), but once started they usually shed
all their privileges and run as the user ``nobody''. However, don't use the
``nobody'' account (unless you're writing a webserver); instead, create your
own pseudouser or new group. The purpose of this approach is to isolate
different programs, processes, and data from each other, by exploiting the
operating system's ability to keep users and groups separate. If different
programs shared the same account, then breaking into one program would also
grant privileges to the other. Usually the pseudouser should not own the
programs it runs; that way, an attack who breaks into the account cannot
change the program it runs. By isolating different parts of the system into
running separate users and groups, breaking one part will not necessarily
break the whole system's security.

If you're using a database system (say, by calling its query interface),
limit the rights of the database user that the application uses. For example,
don't give that user access to all of the system stored procedures if that
user only needs access to a handful of user-defined ones. Do everything you
can inside stored procedures. That way, even if someone does manage to force
arbitrary strings into the query, the damage that can be done is limited. If
you must directly pass a regular SQL query with client supplied data (and you
usually shouldn't), wrap it in something that limits its activities (e.g.,
sp_sqlexec). (My thanks to SPI Labs for these database system suggestions).

If you must give a program privileges usually reserved for root, consider
using POSIX capabilities as soon as your program can minimize the privileges
available to your program. POSIX capabilities are available in Linux 2.2 and
in many other Unix-like systems. By calling cap_set_proc(3) or the
Linux-specific capsetp(3) routines immediately after starting, you can
permanently reduce the abilities of your program to just those abilities it
actually needs. For example the network time daemon (ntpd) traditionally has
run as root, because it needs to modify the current time. However, patches
have been developed so ntpd only needs a single capability, CAP_SYS_TIME, so
even if an attacker gains control over ntpd it's somewhat more difficult to
exploit the program.

I say ``somewhat limited'' because, unless other steps are taken, retaining a
privilege using POSIX capabilities requires that the process continue to have
the root user id. Because many important files (configuration files,
binaries, and so on) are owned by root, an attacker controlling a program
with such limited capabilities can still modify key system files and gain
full root-level privilege. A Linux kernel extension (available in versions
2.4.X and 2.2.19+) provides a better way to limit the available privileges: a
program can start as root (with all POSIX capabilities), prune its
capabilities down to just what it needs, call prctl(PR_SET_KEEPCAPS,1), and
then use setuid() to change to a non-root process. The PR_SET_KEEPCAPS
setting marks a process so that when a process does a setuid to a nonzero
value, the capabilities aren't cleared (normally they are cleared). This
process setting is cleared on exec(). However, note that PR_SET_KEEPCAPS is a
Linux-unique extension for newer versions of the linux kernel.

One tool you can use to simplify minimizing granted privileges is the
``compartment'' tool developed by SuSE. This tool, which only works on Linux,
sets the filesystem root, uid, gid, and/or the capability set, then runs the
given program. This is particularly handy for running some other program
without modifying it. Here's the syntax of version 0.5:
|Syntax: compartment [options] /full/path/to/program                        |
|                                                                           |
|Options:                                                                   |
|  --chroot path   chroot to path                                           |
|  --user user     change UID to this user                                  |
|  --group group   change GID to this group                                 |
|  --init program  execute this program before doing anything               |
|  --cap capset    set capset name. You can specify several                 |
|  --verbose       be verbose                                               |
|  --quiet         do no logging (to syslog)                                |

Thus, you could start a more secure anonymous ftp server using:
|  compartment --chroot /home/ftp --cap CAP_NET_BIND_SERVICE anon-ftpd      |

At the time of this writing, the tool is immature and not available on
typical Linux distributions, but this may quickly change. You can download
the program via [] A
similar tool is dreamland; you can that at [

Note that not all Unix-like systems, implement POSIX capabilities, and
PR_SET_KEEPCAPS is currently a Linux-only extension. Thus, these approaches
limit portability. However, if you use it merely as an optional safeguard
only where it's available, using this approach will not really limit
portability. Also, while the Linux kernel version 2.2 and greater includes
the low-level calls, the C-level libraries to make their use easy are not
installed on some Linux distributions, slightly complicating their use in
applications. For more information on Linux's implementation of POSIX
capabilities, see [

FreeBSD has the jail() function for limiting privileges; see the jail
documentation for more information. There are a number of specialized tools
and extensions for limiting privileges; see Section 3.10. 

7.4.2. Minimize the Time the Privilege Can Be Used

As soon as possible, permanently give up privileges. Some Unix-like systems,
including Linux, implement ``saved'' IDs which store the ``previous'' value.
The simplest approach is to reset any supplemental groups if appropriate
(e.g., using setgroups(2)), and then set the other id's twice to an untrusted
id. In setuid/setgid programs, you should usually set the effective gid and
uid to the real ones, in particular right after a fork(2), unless there's a
good reason not to. Note that you have to change the gid first when dropping
from root to another privilege or it won't work - once you drop root
privileges, you won't be able to change much else. Note that in some systems,
just setting the group isn't enough, if the process belongs to supplemental
groups with privileges. For example, the ``rsync'' program didn't remove the
supplementary groups when it changed its uid and gid, which created a
potential exploit.

It's worth noting that there's a well-known related bug that uses POSIX
capabilities to interfere with this minimization. This bug affects Linux
kernel 2.2.0 through 2.2.15, and possibly a number of other Unix-like systems
with POSIX capabilities. See Bugtraq id 1322 on
for more information. Here is their summary:

    POSIX "Capabilities" have recently been implemented in the Linux kernel.
    These "Capabilities" are an additional form of privilege control to
    enable more specific control over what privileged processes can do.
    Capabilities are implemented as three (fairly large) bitfields, which
    each bit representing a specific action a privileged process can perform.
    By setting specific bits, the actions of privileged processes can be
    controlled -- access can be granted for various functions only to the
    specific parts of a program that require them. It is a security measure.
    The problem is that capabilities are copied with fork() execs, meaning
    that if capabilities are modified by a parent process, they can be
    carried over. The way that this can be exploited is by setting all of the
    capabilities to zero (meaning, all of the bits are off) in each of the
    three bitfields and then executing a setuid program that attempts to drop
    privileges before executing code that could be dangerous if run as root,
    such as what sendmail does. When sendmail attempts to drop privileges
    using setuid(getuid()), it fails not having the capabilities required to
    do so in its bitfields and with no checks on its return value . It
    continues executing with superuser privileges, and can run a users
    .forward file as root leading to a complete compromise.
One approach, used by sendmail, is to attempt to do setuid(0) after a setuid
(getuid()); normally this should fail. If it succeeds, the program should
stop. For more information, see In
the short term this might be a good idea in other programs, though clearly
the better long-term approach is to upgrade the underlying system.

7.4.3. Minimize the Time the Privilege is Active

Use setuid(2), seteuid(2), setgroups(2), and related functions to ensure that
the program only has these privileges active when necessary, and then
temporarily deactivate the privilege when it's not in use. As noted above,
you might want to ensure that these privileges are disabled while parsing
user input, but more generally, only turn on privileges when they're actually

Note that some buffer overflow attacks, if successful, can force a program to
run arbitrary code, and that code could re-enable privileges that were
temporarily dropped. Thus, there are many attacks that temporarily
deactivating a privilege won't counter - it's always much better to
completely drop privileges as soon as possible. There are many papers that
describe how to do this, such as "Designing Shellcode Demystified". Some
people even claim that ``seteuid() [is] considered harmful'' because of the
many attacks it doesn't counter. Still, temporarily deactivating these
permissions prevents a whole class of attacks, such as techniques to convince
a program to write into a file that perhaps it didn't intend to write into.
Since this technique prevents many attacks, it's worth doing if permanently
dropping the privilege can't be done at that point in the program.

7.4.4. Minimize the Modules Granted the Privilege

If only a few modules are granted the privilege, then it's much easier to
determine if they're secure. One way to do so is to have a single module use
the privilege and then drop it, so that other modules called later cannot
misuse the privilege. Another approach is to have separate commands in
separate executables; one command might be a complex tool that can do a vast
number of tasks for a privileged user (e.g., root), while the other tool is
setuid but is a small, simple tool that only permits a small command subset
(and does not trust its invoker). The small, simple tool checks to see if the
input meets various criteria for acceptability, and then if it determines the
input is acceptable, it passes the data on to the complex tool. Note that the
small, simple tool must do a thorough job checking its inputs and limiting
what it will pass along to the complex tool, or this can be a vulnerability.
The communication could be via shell invocation, or any IPC mechanism. These
approaches can even be layered several ways, for example, a complex user tool
could call a simple setuid ``wrapping'' program (that checks its inputs for
secure values) that then passes on information to another complex trusted

This approach is the normal approach for developing GUI-based applications
which requre privilege, but must be run by unprivileged users. The GUI
portion is run as a normal unprivileged user process; that process then
passes security-relevant requests on to another process that has the special
privileges (and does not trust the first process, but instead limits the
requests to whatever the user is allowed to do). Never develop a program that
is privileged (e.g., using setuid) and also directly invokes a graphical
toolkit: Graphical toolkits aren't designed to be used this way, and it would
be extremely difficult to audit graphical toolkits in a way to make this
possible. Fundamentally, graphical toolkits must be large, and it's extremely
unwise to place so much faith in the perfection of that much code, so there
is no point in trying to make them do what should never be done. Feel free to
create a small setuid program that invokes two separate programs: one without
privileges (but with the graphical interface), and one with privileges (and
without an external interface). Or, create a small setuid program that can be
invoked by the unprivileged GUI application. But never combine the two into a
single process. For more about this, see the statement by Owen Taylor about
GTK and setuid, discussing why GTK_MODULES is not a security hole.

Some applications can be best developed by dividing the problem into smaller,
mutually untrusting programs. A simple way is divide up the problem into
separate programs that do one thing (securely), using the filesystem and
locking to prevent problems between them. If more complex interactions are
needed, one approach is to fork into multiple processes, each of which has
different privilege. Communications channels can be set up in a variety of
ways; one way is to have a "master" process create communication channels
(say unnamed pipes or unnamed sockets), then fork into different processes
and have each process drop as many privileges as possible. If you're doing
this, be sure to watch for deadlocks. Then use a simple protocol to allow the
less trusted processes to request actions from the more trusted process(es),
and ensure that the more trusted processes only support a limited set of
requests. Setting user and group permissions so that no one else can even
start up the sub-programs makes it harder to break into.

Some operating systems have the concept of multiple layers of trust in a
single process, e.g., Multics' rings. Standard Unix and Linux don't have a
way of separating multiple levels of trust by function inside a single
process like this; a call to the kernel increases privileges, but otherwise a
given process has a single level of trust. This is one area where
technologies like Java 2, C# (which copies Java's approach), and Fluke (the
basis of security-enhanced Linux) have an advantage. For example, Java 2 can
specify fine-grained permissions such as the permission to only open a
specific file. However, general-purpose operating systems do not typically
have such abilities at this time; this may change in the near future. For
more about Java, see Section 10.6.

7.4.5. Consider Using FSUID To Limit Privileges

Each Linux process has two Linux-unique state values called filesystem user
id (FSUID) and filesystem group id (FSGID). These values are used when
checking against the filesystem permissions. If you're building a program
that operates as a file server for arbitrary users (like an NFS server), you
might consider using these Linux extensions. To use them, while holding root
privileges change just FSUID and FSGID before accessing files on behalf of a
normal user. This extension is fairly useful, and provides a mechanism for
limiting filesystem access rights without removing other (possibly necessary)
rights. By only setting the FSUID (and not the EUID), a local user cannot
send a signal to the process. Also, avoiding race conditions is much easier
in this situation. However, a disadvantage of this approach is that these
calls are not portable to other Unix-like systems.

7.4.6. Consider Using Chroot to Minimize Available Files

You can use chroot(2) to limit the files visible to your program. This
requires carefully setting up a directory (called the ``chroot jail'') and
correctly entering it. This can be a fairly effective technique for improving
a program's security - it's hard to interfere with files you can't see.
However, it depends on a whole bunch of assumptions, in particular, the
program must lack root privileges, it must not have any way to get root
privileges, and the chroot jail must be properly set up (e.g., be careful
what you put inside the chroot jail, and make sure that users can never
control its contents before calling chroot). I recommend using chroot(2)
where it makes sense to do so, but don't depend on it alone; instead, make it
part of a layered set of defenses. Here are a few notes about the use of

  * The program can still use non-filesystem objects that are shared across
    the entire machine (such as System V IPC objects and network sockets).
    It's best to also use separate pseudo-users and/or groups, because all
    Unix-like systems include the ability to isolate users; this will at
    least limit the damage a subverted program can do to other programs. Note
    that current most Unix-like systems (including Linux) won't isolate
    intentionally cooperating programs; if you're worried about malicious
    programs cooperating, you need to get a system that implements some sort
    of mandatory access control and/or limits covert channels.
  * Be sure to close any filesystem descriptors to outside files if you don't
    want them used later. In particular, don't have any descriptors open to
    directories outside the chroot jail, or set up a situation where such a
    descriptor could be given to it (e.g., via Unix sockets or an old
    implementation of /proc). If the program is given a descriptor to a
    directory outside the chroot jail, it could be used to escape out of the
    chroot jail.
  * The chroot jail has to be set up to be secure - it must never be
    controlled by a user and every file added must be carefully examined.
    Don't use a normal user's home directory, subdirectory, or other
    directory that can ever be controlled by a user as a chroot jail; use a
    separate directory directory specially set aside for the purpose. Using a
    directory controlled by a user is a disaster - for example, the user
    could create a ``lib'' directory containing a trojaned linker or libc
    (and could link a setuid root binary into that space, if the files you
    save don't use it). Place the absolute minimum number of files and
    directories there. Typically you'll have a /bin, /etc/, /lib, and maybe
    one or two others (e.g., /pub if it's an ftp server). Place in /bin only
    what you need to run after doing the chroot(); sometimes you need nothing
    at all (try to avoid placing a shell like /bin/sh there, though sometimes
    that can't be helped). You may need a /etc/passwd and /etc/group so file
    listings can show some correct names, but if so, try not to include the
    real system's values, and certainly replace all passwords with "*".
    In /lib, place only what you need; use ldd(1) to query each program in /
    bin to find out what it needs, and only include them. On Linux, you'll
    probably need a few basic libraries like, and not much
    else. Alternatively, recompile any necessary programs to be statically
    linked, so that they don't need dynamically loaded libraries at all.
    It's usually wiser to completely copy in all files, instead of making
    hard links; while this wastes some time and disk space, it makes it so
    that attacks on the chroot jail files do not automatically propagate into
    the regular system's files. Mounting a /proc filesystem, on systems where
    this is supported, is generally unwise. In fact, in very old versions of
    Linux (versions 2.0.x, at least up through 2.0.38) it's a known security
    flaw, since there are pseudo-directories in /proc that would permit a
    chroot'ed program to escape. Linux kernel 2.2 fixed this known problem,
    but there may be others; if possible, don't do it.
  * Chroot really isn't effective if the program can acquire root privilege.
    For example, the program could use calls like mknod(2) to create a device
    file that can view physical memory, and then use the resulting device
    file to modify kernel memory to give itself whatever privileges it
    desired. Another example of how a root program can break out of chroot is
    demonstrated at [] http:// In this example, the program opens a
    file descriptor for the current directory, creates and chroots into a
    subdirectory, sets the current directory to the previously-opened current
    directory, repeatedly cd's up from the current directory (which since it
    is outside the current chroot succeeds in moving up to the real
    filesystem root), and then calls chroot on the result. By the time you
    read this, these weaknesses may have been plugged, but the reality is
    that root privilege has traditionally meant ``all privileges'' and it's
    hard to strip them away. It's better to assume that a program requiring
    continuous root privileges will only be mildly helped using chroot(). Of
    course, you may be able to break your program into parts, so that at
    least part of it can be in a chroot jail.


7.4.7. Consider Minimizing the Accessible Data

Consider minimizing the amount of data that can be accessed by the user. For
example, in CGI scripts, place all data used by the CGI script outside of the
document tree unless there is a reason the user needs to see the data
directly. Some people have the false notion that, by not publicly providing a
link, no one can access the data, but this is simply not true.

7.4.8. Consider Minimizing the Resources Available

Consider minimizing the computer resources available to a given process so
that, even if it ``goes haywire,'' its damage can be limited. This is a
fundamental technique for preventing a denial of service. For network
servers, a common approach is to set up a separate process for each session,
and for each process limit the amount of CPU time (et cetera) that session
can use. That way, if an attacker makes a request that chews up memory or
uses 100% of the CPU, the limits will kick in and prevent that single session
from interfering with other tasks. Of course, an attacker can establish many
sessions, but this at least raises the bar for an attack. See Section 3.6 for
more information on how to set these limits (e.g., ulimit(1)).

7.5. Minimize the Functionality of a Component

In a related move, minimize the amount of functionality provided by your
component. If it does several functions, consider breaking its implementation
up into those smaller functions. That way, users who don't need some
functions can disable just those portions. This is particularly important
when a flaw is discovered - this way, users can disable just one component
and still use the other parts.

7.6. Avoid Creating Setuid/Setgid Scripts

Many Unix-like systems, in particular Linux, simply ignore the setuid and
setgid bits on scripts to avoid the race condition described earlier. Since
support for setuid scripts varies on Unix-like systems, they're best avoided
in new applications where possible. As a special case, Perl includes a
special setup to support setuid Perl scripts, so using setuid and setgid is
acceptable in Perl if you truly need this kind of functionality. If you need
to support this kind of functionality in your own interpreter, examine how
Perl does this. Otherwise, a simple approach is to ``wrap'' the script with a
small setuid/setgid executable that creates a safe environment (e.g., clears
and sets environment variables) and then calls the script (using the script's
full path). Make sure that the script cannot be changed by an attacker! Shell
scripting languages have additional problems, and really should not be setuid
/setgid; see Section 10.4 for more information about this.

7.7. Configure Safely and Use Safe Defaults

Configuration is considered to currently be the number one security problem.
Therefore, you should spend some effort to (1) make the initial installation
secure, and (2) make it easy to reconfigure the system while keeping it

Never have the installation routines install a working ``default'' password.
If you need to install new ``users'', that's fine - just set them up with an
impossible password, leaving time for administrators to set the password (and
leaving the system secure before the password is set). Administrators will
probably install hundreds of packages and almost certainly forget to set the
password - it's likely they won't even know to set it, if you create a
default password.

A program should have the most restrictive access policy until the
administrator has a chance to configure it. Please don't create ``sample''
working users or ``allow access to all'' configurations as the starting
configuration; many users just ``install everything'' (installing all
available services) and never get around to configuring many services. In
some cases the program may be able to determine that a more generous policy
is reasonable by depending on the existing authentication system, for
example, an ftp server could legitimately determine that a user who can log
into a user's directory should be allowed to access that user's files. Be
careful with such assumptions, however.

Have installation scripts install a program as safely as possible. By
default, install all files as owned by root or some other system user and
make them unwriteable by others; this prevents non-root users from installing
viruses. Indeed, it's best to make them unreadable by all but the trusted
user. Allow non-root installation where possible as well, so that users
without root privileges and administrators who do not fully trust the
installer can still use the program.

When installing, check to make sure that any assumptions necessary for
security are true. Some library routines are not safe on some platforms; see
the discussion of this in Section 8.1. If you know which platforms your
application will run on, you need not check their specific attributes, but in
that case you should check to make sure that the program is being installed
on only one of those platforms. Otherwise, you should require a manual
override to install the program, because you don't know if the result will be

Try to make configuration as easy and clear as possible, including
post-installation configuration. Make using the ``secure'' approach as easy
as possible, or many users will use an insecure approach without
understanding the risks. On Linux, take advantage of tools like linuxconf, so
that users can easily configure their system using an existing

If there's a configuration language, the default should be to deny access
until the user specifically grants it. Include many clear comments in the
sample configuration file, if there is one, so the administrator understands
what the configuration does.

7.8. Load Initialization Values Safely

Many programs read an initialization file to allow their defaults to be
configured. You must ensure that an attacker can't change which
initialization file is used, nor create or modify that file. Often you should
not use the current directory as a source of this information, since if the
program is used as an editor or browser, the user may be viewing the
directory controlled by someone else. Instead, if the program is a typical
user application, you should load any user defaults from a hidden file or
directory contained in the user's home directory. If the program is setuid/
setgid, don't read any file controlled by the user unless you carefully
filter it as an untrusted (potentially hostile) input. Trusted configuration
values should be loaded from somewhere else entirely (typically from a file
in /etc).

7.9. Fail Safe

A secure program should always ``fail safe'', that is, it should be designed
so that if the program does fail, the safest result should occur. For
security-critical programs, that usually means that if some sort of
misbehavior is detected (malformed input, reaching a ``can't get here''
state, and so on), then the program should immediately deny service and stop
processing that request. Don't try to ``figure out what the user wanted'':
just deny the service. Sometimes this can decrease reliability or useability
(from a user's perspective), but it increases security. There are a few cases
where this might not be desired (e.g., where denial of service is much worse
than loss of confidentiality or integrity), but such cases are quite rare.

Note that I recommend ``stop processing the request'', not ``fail
altogether''. In particular, most servers should not completely halt when
given malformed input, because that creates a trivial opportunity for a
denial of service attack (the attacker just sends garbage bits to prevent you
from using the service). Sometimes taking the whole server down is necessary,
in particular, reaching some ``can't get here'' states may signal a problem
so drastic that continuing is unwise.

Consider carefully what error message you send back when a failure is
detected. if you send nothing back, it may be hard to diagnose problems, but
sending back too much information may unintentionally aid an attacker.
Usually the best approach is to reply with ``access denied'' or
``miscellaneous error encountered'' and then write more detailed information
to an audit log (where you can have more control over who sees the

7.10. Avoid Race Conditions

A ``race condition'' can be defined as ``Anomalous behavior due to unexpected
critical dependence on the relative timing of events'' [FOLDOC]. Race
conditions generally involve one or more processes accessing a shared
resource (such a file or variable), where this multiple access has not been
properly controlled.

In general, processes do not execute atomically; another process may
interrupt it between essentially any two instructions. If a secure program's
process is not prepared for these interruptions, another process may be able
to interfere with the secure program's process. Any pair of operations in a
secure program must still work correctly if arbitrary amounts of another
process's code is executed between them.

Race condition problems can be notionally divided into two categories:

  * Interference caused by untrusted processes. Some security taxonomies call
    this problem a ``sequence'' or ``non-atomic'' condition. These are
    conditions caused by processes running other, different programs, which
    ``slip in'' other actions between steps of the secure program. These
    other programs might be invoked by an attacker specifically to cause the
    problem. This book will call these sequencing problems.
  * Interference caused by trusted processes (from the secure program's point
    of view). Some taxonomies call these deadlock, livelock, or locking
    failure conditions. These are conditions caused by processes running the
    ``same'' program. Since these different processes may have the ``same''
    privileges, if not properly controlled they may be able to interfere with
    each other in a way other programs can't. Sometimes this kind of
    interference can be exploited. This book will call these locking

7.10.1. Sequencing (Non-Atomic) Problems

In general, you must check your code for any pair of operations that might
fail if arbitrary code is executed between them.

Note that loading and saving a shared variable are usually implemented as
separate operations and are not atomic. This means that an ``increment
variable'' operation is usually converted into loading, incrementing, and
saving operation, so if the variable memory is shared the other process may
interfere with the incrementing.

Secure programs must determine if a request should be granted, and if so, act
on that request. There must be no way for an untrusted user to change
anything used in this determination before the program acts on it. This kind
of race condition is sometimes termed a ``time of check - time of use''
(TOCTOU) race condition.
----------------------------------------------------------------------------- Atomic Actions in the Filesystem

The problem of failing to perform atomic actions repeatedly comes up in the
filesystem. In general, the filesystem is a shared resource used by many
programs, and some programs may interfere with its use by other programs.
Secure programs should generally avoid using access(2) to determine if a
request should be granted, followed later by open(2), because users may be
able to move files around between these calls, possibly creating symbolic
links or files of their own choosing instead. A secure program should instead
set its effective id or filesystem id, then make the open call directly. It's
possible to use access(2) securely, but only when a user cannot affect the
file or any directory along its path from the filesystem root.

When creating a file, you should open it using the modes O_CREAT | O_EXCL and
grant only very narrow permissions (only to the current user); you'll also
need to prepare for having the open fail. If you need to be able to open the
file (e.g,. to prevent a denial-of-service), you'll need to repetitively (1)
create a ``random'' filename, (2) open the file as noted, and (3) stop
repeating when the open succeeds.

Ordinary programs can become security weaknesses if they don't create files
properly. For example, the ``joe'' text editor had a weakness called the
``DEADJOE'' symlink vulnerability. When joe was exited in a nonstandard way
(such as a system crash, closing an xterm, or a network connection going
down), joe would unconditionally append its open buffers to the file
"DEADJOE". This could be exploited by the creation of DEADJOE symlinks in
directories where root would normally use joe. In this way, joe could be used
to append garbage to potentially-sensitive files, resulting in a denial of
service and/or unintentional access.

As another example, when performing a series of operations on a file's
meta-information (such as changing its owner, stat-ing the file, or changing
its permission bits), first open the file and then use the operations on open
files. This means use the fchown( ), fstat( ), or fchmod( ) system calls,
instead of the functions taking filenames such as chown(), chgrp(), and chmod
(). Doing so will prevent the file from being replaced while your program is
running (a possible race condition). For example, if you close a file and
then use chmod() to change its permissions, an attacker may be able to move
or remove the file between those two steps and create a symbolic link to
another file (say /etc/passwd). Other interesting files include /dev/zero,
which can provide an infinitely-long data stream of input to a program; if an
attacker can ``switch'' the file midstream, the results can be dangerous.

But even this gets complicated - when creating files, you must give them as a
minimal set of rights as possible, and then change the rights to be more
expansive if you desire. Generally, this means you need to use umask and/or
open's parameters to limit initial access to just the user and user group.
For example, if you create a file that is initially world-readable, then try
to turn off the ``world readable'' bit, an attacker could try to open the
file while the permission bits said this was okay. On most Unix-like systems,
permissions are only checked on open, so this would result in an attacker
having more privileges than intended.

In general, if multiple users can write to a directory in a Unix-like system,
you'd better have the ``sticky'' bit set on that directory, and sticky
directories had better be implemented. It's much better to completely avoid
the problem, however, and create directories that only a trusted special
process can access (and then implement that carefully). The traditional Unix
temporary directories (/tmp and /var/tmp) are usually implemented as
``sticky'' directories, and all sorts of security problems can still surface,
as we'll see next.
----------------------------------------------------------------------------- Temporary Files

This issue of correctly performing atomic operations particularly comes up
when creating temporary files. Temporary files in Unix-like systems are
traditionally created in the /tmp or /var/tmp directories, which are shared
by all users. A common trick by attackers is to create symbolic links in the
temporary directory to some other file (e.g., /etc/passwd) while your secure
program is running. The attacker's goal is to create a situation where the
secure program determines that a given filename doesn't exist, the attacker
then creates the symbolic link to another file, and then the secure program
performs some operation (but now it actually opened an unintended file).
Often important files can be clobbered or modified this way. There are many
variations to this attack, such as creating normal files, all based on the
idea that the attacker can create (or sometimes otherwise access) file system
objects in the same directory used by the secure program for temporary files.

Michal Zalewski exposed in 2002 another serious problem with temporary
directories involving automatic cleaning of temporary directories. For more
information, see his posting to Bugtraq dated December 20, 2002, (subject "
[RAZOR] Problems with mkstemp()"). Basically, Zalewski notes that it's a
common practice to have a program automatically sweep temporary directories
like /tmp and /var/tmp and remove "old" files that have not been accessed for
a while (e.g., several days). Such programs are sometimes called "tmp
cleaners" (pronounced "temp cleaners"). Possibly the most common tmp cleaner
is "tmpwatch" by Erik Troan and Preston Brown of Red Hat Software; another
common one is 'stmpclean' by Stanislav Shalunov; many administrators roll
their own as well. Unfortunately, the existance of tmp cleaners creates an
opportunity for new security-critical race conditions; an attacker may be
able to arrange things so that the tmp cleaner interferes with the secure
program. For example, an attacker could create an "old" file, arrange for the
tmp cleaner to plan to delete the file, delete the file himself, and run a
secure program that creates the same file - now the tmp cleaner will delete
the secure program's file! Or, imagine that a secure program can have long
delays after using the file (e.g., a setuid program stopped with SIGSTOP and
resumed after many days with SIGCONT, or simply intentionally creating a lot
of work). If the temporary file isn't used for long enough, its temporary
files are likely to be removed by the tmp cleaner.

The general problem when creating files in these shared directories is that
you must guarantee that the filename you plan to use doesn't already exist at
time of creation, and atomically create the file. Checking ``before'' you
create the file doesn't work, because after the check occurs, but before
creation, another process can create that file with that filename. Using an
``unpredictable'' or ``unique'' filename doesn't work in general, because
another process can often repeatedly guess until it succeeds. Once you create
the file atomically, you must alway use the returned file descriptor (or file
stream, if created from the file descriptor using routines like fdopen()).
You must never re-open the file, or use any operations that use the filename
as a parameter - always use the file descriptor or associated stream.
Otherwise, the tmpwatch race issues noted above will cause problems. You
can't even create the file, close it, and re-open it, even if the permissions
limit who can open it. Note that comparing the descriptor and a reopened file
to verify inode numbers, creation times or file ownership is not sufficient -
please refer to "Symlinks and Cryogenic Sleep" by Olaf Kirch.

Fundamentally, to create a temporary file in a shared (sticky) directory, you
must repetitively: (1) create a ``random'' filename, (2) open it using
O_CREAT | O_EXCL and very narrow permissions (which atomically creates the
file and fails if it's not created), and (3) stop repeating when the open

According to the 1997 ``Single Unix Specification'', the preferred method for
creating an arbitrary temporary file (using the C interface) is tmpfile(3).
The tmpfile(3) function creates a temporary file and opens a corresponding
stream, returning that stream (or NULL if it didn't). Unfortunately, the
specification doesn't make any guarantees that the file will be created
securely. In earlier versions of this book, I stated that I was concerned
because I could not assure myself that all implementations do this securely.
I've since found that older System V systems have an insecure implementation
of tmpfile(3) (as well as insecure implementations of tmpnam(3) and tempnam
(3)), so on at least some systems it's absolutely useless. Library
implementations of tmpfile(3) should securely create such files, of course,
but users don't always realize that their system libraries have this security
flaw, and sometimes they can't do anything about it.

Kris Kennaway recommends using mkstemp(3) for making temporary files in
general. His rationale is that you should use well-known library functions to
perform this task instead of rolling your own functions, and that this
function has well-known semantics. This is certainly a reasonable position. I
would add that, if you use mkstemp(3), be sure to use umask(2) to limit the
resulting temporary file permissions to only the owner. This is because some
implementations of mkstemp(3) (basically older ones) make such files readable
and writable by all, creating a condition in which an attacker can read or
write private data in this directory. A minor nuisance is that mkstemp(3)
doesn't directly support the environment variables TMP or TMPDIR (as
discussed below), so if you want to support them you have to add code to do
so. Here's a program in C that demonstrates how to use mkstemp(3) for this
purpose, both directly and when adding support for TMP and TMPDIR:
#include <stdio.h>                                                              
#include <stdlib.h>                                                             
#include <sys/types.h>                                                          
#include <sys/stat.h>                                                           
void failure(msg) {                                                             
 fprintf(stderr, "%s\n", msg);                                                  
 * Given a "pattern" for a temporary filename                                   
 * (starting with the directory location and ending in XXXXXX),                 
 * create the file and return it.                                               
 * This routines unlinks the file, so normally it won't appear in               
 * a directory listing.                                                         
 * The pattern will be changed to show the final filename.                      
FILE *create_tempfile(char *temp_filename_pattern)                              
 int temp_fd;                                                                   
 mode_t old_mode;                                                               
 FILE *temp_file;                                                               
 old_mode = umask(077);  /* Create file with restrictive permissions */         
 temp_fd = mkstemp(temp_filename_pattern);                                      
 (void) umask(old_mode);                                                        
 if (temp_fd == -1) {                                                           
   failure("Couldn't open temporary file");                                     
 if (!(temp_file = fdopen(temp_fd, "w+b"))) {                                   
   failure("Couldn't create temporary file's file descriptor");                 
 if (unlink(temp_filename_pattern) == -1) {                                     
   failure("Couldn't unlink temporary file");                                   
 return temp_file;                                                              
 * Given a "tag" (a relative filename ending in XXXXXX),                        
 * create a temporary file using the tag.  The file will be created             
 * in the directory specified in the environment variables                      
 * TMPDIR or TMP, if defined and we aren't setuid/setgid, otherwise             
 * it will be created in /tmp.  Note that root (and su'd to root)               
 * _will_ use TMPDIR or TMP, if defined.                                        
FILE *smart_create_tempfile(char *tag)                                          
 char *tmpdir = NULL;                                                           
 char *pattern;                                                                 
 FILE *result;                                                                  
 if ((getuid()==geteuid()) && (getgid()==getegid())) {                          
   if (! ((tmpdir=getenv("TMPDIR")))) {                                         
 if (!tmpdir) {tmpdir = "/tmp";}                                                
 pattern = malloc(strlen(tmpdir)+strlen(tag)+2);                                
 if (!pattern) {                                                                
   failure("Could not malloc tempfile pattern");                                
 strcpy(pattern, tmpdir);                                                       
 strcat(pattern, "/");                                                          
 strcat(pattern, tag);                                                          
 result = create_tempfile(pattern);                                             
 return result;                                                                 
main() {                                                                        
 int c;                                                                         
 FILE *demo_temp_file1;                                                         
 FILE *demo_temp_file2;                                                         
 char demo_temp_filename1[] = "/tmp/demoXXXXXX";                                
 char demo_temp_filename2[] = "second-demoXXXXXX";                              
 demo_temp_file1 = create_tempfile(demo_temp_filename1);                        
 demo_temp_file2 = smart_create_tempfile(demo_temp_filename2);                  
 fprintf(demo_temp_file2, "This is a test.\n");                                 
 printf("Printing temporary file contents:\n");                                 
 while (  (c=fgetc(demo_temp_file2)) != EOF) {                                  
 printf("Exiting; you'll notice that there are no temporary files on exit.\n"); 

Kennaway states that if you can't use mkstemp(3), then make yourself a
directory using mkdtemp(3), which is protected from the outside world.
However, as Michal Zalewski notes, this is a bad idea if there are tmp
cleaners in use; instead, use a directory inside the user's HOME. Finally, if
you really have to use the insecure mktemp(3), use lots of X's - he suggests
10 (if your libc allows it) so that the filename can't easily be guessed
(using only 6 X's means that 5 are taken up by the PID, leaving only one
random character and allowing an attacker to mount an easy race condition).
Note that this is fundamentally insecure, so you should normally not do this.
I add that you should avoid tmpnam(3) as well - some of its uses aren't
reliable when threads are present, and it doesn't guarantee that it will work
correctly after TMP_MAX uses (yet most practical uses must be inside a loop).

In general, you should avoid using the insecure functions such as mktemp(3)
or tmpnam(3), unless you take specific measures to counter their insecurities
or test for a secure library implementation as part of your installation
routines. If you ever want to make a file in /tmp or a world-writable
directory (or group-writable, if you don't trust the group) and don't want to
use mk*temp() (e.g. you intend for the file to be predictably named), then 
always use the O_CREAT and O_EXCL flags to open() and check the return value.
If you fail the open() call, then recover gracefully (e.g. exit).

The GNOME programming guidelines recommend the following C code when creating
filesystem objects in shared (temporary) directories to securely open
temporary files [Quintero 2000]:
 char *filename;                                                             
 int fd;                                                                     
 do {                                                                        
   filename = tempnam (NULL, "foo");                                         
   fd = open (filename, O_CREAT | O_EXCL | O_TRUNC | O_RDWR, 0600);          
   free (filename);                                                          
 } while (fd == -1);                                                         
Note that, although the insecure function tempnam(3) is being used, it is
wrapped inside a loop using O_CREAT and O_EXCL to counteract its security
weaknesses, so this use is okay. Note that you need to free() the filename.
You should close() and unlink() the file after you are done. If you want to
use the Standard C I/O library, you can use fdopen() with mode "w+b" to
transform the file descriptor into a FILE *. Note that this approach won't
work over NFS version 2 (v2) systems, because older NFS doesn't correctly
support O_EXCL. Note that one minor disadvantage to this approach is that,
since tempnam can be used insecurely, various compilers and security scanners
may give you spurious warnings about its use. This isn't a problem with

If you need a temporary file in a shell script, you're probably best off
using pipes, using a local directory (e.g., something inside the user's home
directory), or in some cases using the current directory. That way, there's
no sharing unless the user permits it. If you really want/need the temporary
file to be in a shared directory like /tmp, do not use the traditional shell
technique of using the process id in a template and just creating the file
using normal operations like ">". Shell scripts can use "$$" to indicate the
PID, but the PID can be easily determined or guessed by an attacker, who can
then pre-create files or links with the same name. Thus the following
"typical" shell script is unsafe:
   echo "This is a test" > /tmp/test$$  # DON'T DO THIS.                     

If you need a temporary file or directory in a shell script, and you want it
in /tmp, a solution sometimes suggested is to use mktemp(1), which is
intended for use in shell scripts (note that mktemp(1) and mktemp(3) are
different things). However, as Michal Zalewski notes, this is insecure in
many environments that run tmp cleaners; the problem is that when a
privileged program sweeps through a temporary directory, it will probably
expose a race condition. Even if this weren't true, I do not recommend using
shell scripts that create temporary files in shared directories; creating
such files in private directories or using pipes instead is generally
preferable, even if you're sure your tmpwatch program is okay (or that you
have no local users). If you must use mktemp(1), note that mktemp(1) takes a
template, then creates a file or directory using O_EXCL and returns the
resulting name; thus, mktemp(1) won't work on NFS version 2 filesystems. Here
are some examples of correct use of mktemp(1) in Bourne shell scripts; these
examples are straight from the mktemp(1) man page:
 # Simple use of mktemp(1), where the script should quit                     
 # if it can't get a safe temporary file.                                    
 # Note that this will be INSECURE on many systems, since they use           
 # tmpwatch-like programs that will erase "old" files and expose race        
 # conditions.                                                               
   TMPFILE=`mktemp /tmp/$0.XXXXXX` || exit 1                                 
   echo "program output" >> $TMPFILE                                         
  # Simple example, if you want to catch the error:                          
   TMPFILE=`mktemp -q /tmp/$0.XXXXXX`                                        
   if [ $? -ne 0 ]; then                                                     
      echo "$0: Can't create temp file, exiting..."                          
      exit 1                                                                 

Perl programmers should use File::Temp, which tries to provide a
cross-platform means of securely creating temporary files. However, read the
documentation carefully on how to use it properly first; it includes
interfaces to unsafe functions as well. I suggest explicitly setting its
safe_level to HIGH; this will invoke additional security checks. [http://] The Perl 5.8
documentation of File::Temp is available on-line.

Don't reuse a temporary filename (i.e. remove and recreate it), no matter how
you obtained the ``secure'' temporary filename in the first place. An
attacker can observe the original filename and hijack it before you recreate
it the second time. And of course, always use appropriate file permissions.
For example, only allow world/group access if you need the world or a group
to access the file, otherwise keep it mode 0600 (i.e., only the owner can
read or write it).

Clean up after yourself, either by using an exit handler, or making use of
UNIX filesystem semantics and unlink()ing the file immediately after creation
so the directory entry goes away but the file itself remains accessible until
the last file descriptor pointing to it is closed. You can then continue to
access it within your program by passing around the file descriptor.
Unlinking the file has a lot of advantages for code maintenance: the file is
automatically deleted, no matter how your program crashes. It also decreases
the likelihood that a maintainer will insecurely use the filename (they need
to use the file descriptor instead). The one minor problem with immediate
unlinking is that it makes it slightly harder for administrators to see how
disk space is being used, since they can't simply look at the file system by

You might consider ensuring that your code for Unix-like systems respects the
environment variables TMP or TMPDIR if the provider of these variable values
is trusted. By doing so, you make it possible for users to move their
temporary files into an unshared directory (and eliminating the problems
discussed here), such as a subdirectory inside their home directory. Recent
versions of Bastille can set these variables to reduce the sharing between
users. Unfortunately, many users set TMP or TMPDIR to a shared directory (say
/tmp), so your secure program must still correctly create temporary files
even if these environment variables are set. This is one advantage of the
GNOME approach, since at least on some systems tempnam(3) automatically uses
TMPDIR, while the mkstemp(3) approach requires more code to do this. Please
don't create yet more environment variables for temporary directories (such
as TEMP), and in particular don't create a different environment name for
each application (e.g., don't use "MYAPP_TEMP"). Doing so greatly complicates
managing systems, and users wanting a special temporary directory for a
specific application can just set the environment variable specially when
running that particular application. Of course, if these environment
variables might have been set by an untrusted source, you should ignore them
- which you'll do anyway if you follow the advice in Section 5.2.3.

These techniques don't work if the temporary directory is remotely mounted
using NFS version 2 (NFSv2), because NFSv2 doesn't properly support O_EXCL.
See Section for more information. NFS version 3 and later properly
support O_EXCL; the simple solution is to ensure that temporary directories
are either local or, if mounted using NFS, mounted using NFS version 3 or
later. There is a technique for safely creating temporary files on NFS v2,
involving the use of link(2) and stat(2), but it's complex; see Section which has more information about this.

As an aside, it's worth noting that FreeBSD has recently changed the mk*temp
() family to get rid of the PID component of the filename and replace the
entire thing with base-62 encoded randomness. This drastically raises the
number of possible temporary files for the "default" usage of 6 X's, meaning
that even mktemp(3) with 6 X's is reasonably (probabilistically) secure
against guessing, except under very frequent usage. However, if you also
follow the guidance here, you'll eliminate the problem they're addressing.

Much of this information on temporary files was derived from Kris Kennaway's
posting to Bugtraq about temporary files on December 15, 2000.

I should note that the Openwall Linux patch from [
linux/] includes an optional ``temporary file
directory'' policy that counters many temporary file based attacks. The Linux
Security Module (LSM) project includes an "owlsm" module that implements some
of the OpenWall ideas, so Linux Kernels with LSM can quickly insert these
rules into a running system. When enabled, it has two protections:

  * Hard links: Processes may not make hard links to files in certain cases.
    The OpenWall documentation states that "Processes may not make hard links
    to files they do not have write access to." In the LSM version, the rules
    are as follows: if both the process' uid and fsuid (usually the same as
    the euid) is is different from the linked-to-file's uid, the process uid
    is not root, and the process lacks the FOWNER capability, then the hard
    link is forbidden. The check against the process uid may be dropped
    someday (they are work-arounds for the atd(8) program), at which point
    the rules would be: if both the process' fsuid (usually the same as the
    euid) is is different from the linked-to-file's uid and and the process
    lacks the FOWNER capability, then the hard link is forbidden. In other
    words, you can only create hard links to files you own, unless you have
    the FOWNER capability. 
  * Symbolic links (symlinks): Certain symlinks are not followed. The
    original OpenWall documentation states that "root processes may not
    follow symlinks that are not owned by root", but the actual rules (from
    looking at the code) are more complicated. In the LSM version, if the
    directory is sticky ("+t" mode, used in shared directories like /tmp),
    symlinks are not followed if the symlink was created by anyone other than
    either the owner of the directory or the current process' fsuid (which is
    usually the effective uid).

Many systems do not implement this openwall policy, so you can't depend on
this in general protecting your system. However, I encourage using this
policy on your own system, and please make sure that your application will
work when this policy is in place.

7.10.2. Locking

There are often situations in which a program must ensure that it has
exclusive rights to something (e.g., a file, a device, and/or existence of a
particular server process). Any system which locks resources must deal with
the standard problems of locks, namely, deadlocks (``deadly embraces''),
livelocks, and releasing ``stuck'' locks if a program doesn't clean up its
locks. A deadlock can occur if programs are stuck waiting for each other to
release resources. For example, a deadlock would occur if process 1 locks
resources A and waits for resource B, while process 2 locks resource B and
waits for resource A. Many deadlocks can be prevented by simply requiring all
processes that lock multiple resources to lock them in the same order (e.g.,
alphabetically by lock name).
----------------------------------------------------------------------------- Using Files as Locks

On Unix-like systems resource locking has traditionally been done by creating
a file to indicate a lock, because this is very portable. It also makes it
easy to ``fix'' stuck locks, because an administrator can just look at the
filesystem to see what locks have been set. Stuck locks can occur because the
program failed to clean up after itself (e.g., it crashed or malfunctioned)
or because the whole system crashed. Note that these are ``advisory'' (not
``mandatory'') locks - all processes needed the resource must cooperate to
use these locks.

However, there are several traps to avoid. First, don't use the technique
used by very old Unix C programs, which is calling creat() or its open()
equivalent, the open() mode O_WRONLY | O_CREAT | O_TRUNC, with the file mode
set to 0 (no permissions). For normal users on normal file systems, this
works, but this approach fails to lock the file when the user has root
privileges. Root can always perform this operation, even when the file
already exists. In fact, old versions of Unix had this particular problem in
the old editor ``ed'' -- the symptom was that occasionally portions of the
password file would be placed in user's files [Rochkind 1985, 22]! Instead,
if you're creating a lock for processes that are on the local filesystem, you
should use open() with the flags O_WRONLY | O_CREAT | O_EXCL (and again, no
permissions, so that other processes with the same owner won't get the lock).
Note the use of O_EXCL, which is the official way to create ``exclusive''
files; this even works for root on a local filesystem. [Rochkind 1985, 27].

Second, if the lock file may be on an NFS-mounted filesystem, then you have
the problem that NFS version 2 doesn't completely support normal file
semantics. This can even be a problem for work that's supposed to be
``local'' to a client, since some clients don't have local disks and may have
all files remotely mounted via NFS. The manual for open(2) explains how to
handle things in this case (which also handles the case of root programs):

"... programs which rely on [the O_CREAT and O_EXCL flags of open(2) to work
on filesystems accessed via NFS version 2] for performing locking tasks will
contain a race condition. The solution for performing atomic file locking
using a lockfile is to create a unique file on the same filesystem (e.g.,
incorporating hostname and pid), use link(2) to make a link to the lockfile
and use stat(2) on the unique file to check if its link count has increased
to 2. Do not use the return value of the link(2) call."

Obviously, this solution only works if all programs doing the locking are
cooperating, and if all non-cooperating programs aren't allowed to interfere.
In particular, the directories you're using for file locking must not have
permissive file permissions for creating and removing files.

NFS version 3 added support for O_EXCL mode in open(2); see IETF RFC 1813, in
particular the "EXCLUSIVE" value to the "mode" argument of "CREATE". Sadly,
not everyone has switched to NFS version 3 or higher at the time of this
writing, so you can't depend on this yet in portable programs. Still, in the
long run there's hope that this issue will go away.

If you're locking a device or the existence of a process on a local machine,
try to use standard conventions. I recommend using the Filesystem Hierarchy
Standard (FHS); it is widely referenced by Linux systems, but it also tries
to incorporate the ideas of other Unix-like systems. The FHS describes
standard conventions for such locking files, including naming, placement, and
standard contents of these files [FHS 1997]. If you just want to be sure that
your server doesn't execute more than once on a given machine, you should
usually create a process identifier as /var/run/ with the pid as its
contents. In a similar vein, you should place lock files for things like
device lock files in /var/lock. This approach has the minor disadvantage of
leaving files hanging around if the program suddenly halts, but it's standard
practice and that problem is easily handled by other system tools.

It's important that the programs which are cooperating using files to
represent the locks use the same directory, not just the same directory name.
This is an issue with networked systems: the FHS explicitly notes that /var/
run and /var/lock are unshareable, while /var/mail is shareable. Thus, if you
want the lock to work on a single machine, but not interfere with other
machines, use unshareable directories like /var/run (e.g., you want to permit
each machine to run its own server). However, if you want all machines
sharing files in a network to obey the lock, you need to use a directory that
they're sharing; /var/mail is one such location. See FHS section 2 for more
information on this subject.
----------------------------------------------------------------------------- Other Approaches to Locking

Of course, you need not use files to represent locks. Network servers often
need not bother; the mere act of binding to a port acts as a kind of lock,
since if there's an existing server bound to a given port, no other server
will be able to bind to that port.

Another approach to locking is to use POSIX record locks, implemented through
fcntl(2) as a ``discretionary lock''. These are discretionary, that is, using
them requires the cooperation of the programs needing the locks (just as the
approach to using files to represent locks does). There's a lot to recommend
POSIX record locks: POSIX record locking is supported on nearly all Unix-like
platforms (it's mandated by POSIX.1), it can lock portions of a file (not
just a whole file), and it can handle the difference between read locks and
write locks. Even more usefully, if a process dies, its locks are
automatically removed, which is usually what is desired.

You can also use mandatory locks, which are based on System V's mandatory
locking scheme. These only apply to files where the locked file's setgid bit
is set, but the group execute bit is not set. Also, you must mount the
filesystem to permit mandatory file locks. In this case, every read(2) and
write(2) is checked for locking; while this is more thorough than advisory
locks, it's also slower. Also, mandatory locks don't port as widely to other
Unix-like systems (they're available on Linux and System V-based systems, but
not necessarily on others). Note that processes with root privileges can be
held up by a mandatory lock, too, making it possible that this could be the
basis of a denial-of-service attack.

7.11. Trust Only Trustworthy Channels

In general, only trust information (input or results) from trustworthy
channels. For example, the routines getlogin(3) and ttyname(3) return
information that can be controlled by a local user, so don't trust them for
security purposes.

In most computer networks (and certainly for the Internet at large), no
unauthenticated transmission is trustworthy. For example, packets sent over
the public Internet can be viewed and modified at any point along their path,
and arbitrary new packets can be forged. These forged packets might include
forged information about the sender (such as their machine (IP) address and
port) or receiver. Therefore, don't use these values as your primary criteria
for security decisions unless you can authenticate them (say using

This means that, except under special circumstances, two old techniques for
authenticating users in TCP/IP should often not be used as the sole
authentication mechanism. One technique is to limit users to ``certain
machines'' by checking the ``from'' machine address in a data packet; the
other is to limit access by requiring that the sender use a ``trusted'' port
number (a number less that 1024). The problem is that in many environments an
attacker can forge these values.

In some environments, checking these values (e.g., the sending machine IP
address and/or port) can have some value, so it's not a bad idea to support
such checking as an option in a program. For example, if a system runs behind
a firewall, the firewall can't be breached or circumvented, and the firewall
stops external packets that claim to be from the inside, then you can claim
that any packet saying it's from the inside really does. Note that you can't
be sure the packet actually comes from the machine it claims it comes from -
so you're only countering external threats, not internal threats. However,
broken firewalls, alternative paths, and mobile code make even these
assumptions suspect.

The problem is supporting untrustworthy information as the only way to
authenticate someone. If you need a trustworthy channel over an untrusted
network, in general you need some sort of cryptologic service (at the very
least, a cryptologically safe hash). See Section 11.5 for more information on
cryptographic algorithms and protocols. If you're implementing a standard and
inherently insecure protocol (e.g., ftp and rlogin), provide safe defaults
and document the assumptions clearly.

The Domain Name Server (DNS) is widely used on the Internet to maintain
mappings between the names of computers and their IP (numeric) addresses. The
technique called ``reverse DNS'' eliminates some simple spoofing attacks, and
is useful for determining a host's name. However, this technique is not
trustworthy for authentication decisions. The problem is that, in the end, a
DNS request will be sent eventually to some remote system that may be
controlled by an attacker. Therefore, treat DNS results as an input that
needs validation and don't trust it for serious access control.

Arbitrary email (including the ``from'' value of addresses) can be forged as
well. Using digital signatures is a method to thwart many such attacks. A
more easily thwarted approach is to require emailing back and forth with
special randomly-created values, but for low-value transactions such as
signing onto a public mailing list this is usually acceptable.

Note that in any client/server model, including CGI, that the server must
assume that the client (or someone interposing between the client and server)
can modify any value. For example, so-called ``hidden fields'' and cookie
values can be changed by the client before being received by CGI programs.
These cannot be trusted unless special precautions are taken. For example,
the hidden fields could be signed in a way the client cannot forge as long as
the server checks the signature. The hidden fields could also be encrypted
using a key only the trusted server could decrypt (this latter approach is
the basic idea behind the Kerberos authentication system). InfoSec labs has
further discussion about hidden fields and applying encryption at [http://]
mschff.htm. In general, you're better off keeping data you care about at the
server end in a client/server model. In the same vein, don't depend on
HTTP_REFERER for authentication in a CGI program, because this is sent by the
user's browser (not the web server).

This issue applies to data referencing other data, too. For example, HTML or
XML allow you to include by reference other files (e.g., DTDs and style
sheets) that may be stored remotely. However, those external references could
be modified so that users see a very different document than intended; a
style sheet could be modified to ``white out'' words at critical locations,
deface its appearance, or insert new text. External DTDs could be modified to
prevent use of the document (by adding declarations that break validation) or
insert different text into documents [St. Laurent 2000].

7.12. Set up a Trusted Path

The counterpart to needing trustworthy channels (see Section 7.11) is
assuring users that they really are working with the program or system they
intended to use.

The traditional example is a ``fake login'' program. If a program is written
to look like the login screen of a system, then it can be left running. When
users try to log in, the fake login program can then capture user passwords
for later use.

A solution to this problem is a ``trusted path.'' A trusted path is simply
some mechanism that provides confidence that the user is communicating with
what the user intended to communicate with, ensuring that attackers can't
intercept or modify whatever information is being communicated.

If you're asking for a password, try to set up trusted path. Unfortunately,
stock Linux distributions and many other Unixes don't have a trusted path
even for their normal login sequence. One approach is to require pressing an
unforgeable key before login, e.g., Windows NT/2000 uses
``control-alt-delete'' before logging in; since normal programs in Windows
can't intercept this key pattern, this approach creates a trusted path.
There's a Linux equivalent, termed the Secure Attention Key (SAK); it's
recommended that this be mapped to ``control-alt-pause''. Unfortunately, at
the time of this writing SAK is immature and not well-supported by Linux
distributions. Another approach for implementing a trusted path locally is to
control a separate display that only the login program can perform. For
example, if only trusted programs could modify the keyboard lights (the LEDs
showing Num Lock, Caps Lock, and Scroll Lock), then a login program could
display a running pattern to indicate that it's the real login program.
Unfortunately, since in current Linux normal users can change the LEDs, the
LEDs can't currently be used to confirm a trusted path.

Sadly, the problem is much worse for network applications. Although setting
up a trusted path is desirable for network applications, completely doing so
is quite difficult. When sending a password over a network, at the very least
encrypt the password between trusted endpoints. This will at least prevent
eavesdropping of passwords by those not connected to the system, and at least
make attacks harder to perform. If you're concerned about trusted path for
the actual communication, make sure that the communication is encrypted and
authenticated (or at least authenticated).

It turns out that this isn't enough to have a trusted path to networked
applications, in particular for web-based applications. There are documented
methods for fooling users of web browsers into thinking that they're at one
place when they are really at another. For example, Felten [1997] discusses
``web spoofing'', where users believe they're viewing one web page when in
fact all the web pages they view go through an attacker's site (who can then
monitor all traffic and modify any data sent in either direction). This is
accomplished by rewriting URL. The rewritten URLs can be made nearly
invisible by using other technology (such as Javascript) to hide any possible
evidence in the status line, location line, and so on. See their paper for
more details. Another technique for hiding such URLs is exploiting
rarely-used URL syntax, for example, the URL ``'' is actually a request to view ``'' (a
potentially malevolent site) using the unusual username ``'.
If the URL is long enough, the real material won't be displayed and users are
unlikely to notice the exploit anyway. Yet another approach is to create
sites with names deliberately similar to the ``real'' site - users may not
know the difference. In all of these cases, simply encrypting the line
doesn't help - the attacker can be quite content in encrypting data while
completely controlling what's shown.

Countering these problems is more difficult; at this time I have no good
technical solution for fully preventing ``fooled'' web users. I would
encourage web browser developers to counter such ``fooling'', making it
easier to spot. If it's critical that your users correctly connect to the
correct site, have them use simple procedures to counter the threat. Examples
include having them halt and restart their browser, and making sure that the
web address is very simple and not normally misspelled (so misspelling it is
unlikely). You might also want to gain ownership of some ``similar'' sounding
DNS names, and search for other such DNS names and material to find

7.13. Use Internal Consistency-Checking Code

The program should check to ensure that its call arguments and basic state
assumptions are valid. In C, macros such as assert(3) may be helpful in doing

7.14. Self-limit Resources

In network daemons, shed or limit excessive loads. Set limit values (using
setrlimit(2)) to limit the resources that will be used. At the least, use
setrlimit(2) to disable creation of ``core'' files. For example, by default
Linux will create a core file that saves all program memory if the program
fails abnormally, but such a file might include passwords or other sensitive

7.15. Prevent Cross-Site (XSS) Malicious Content

Some secure programs accept data from one untrusted user (the attacker) and
pass that data on to a different user's application (the victim). If the
secure program doesn't protect the victim, the victim's application (e.g.,
their web browser) may then process that data in a way harmful to the victim.
This is a particularly common problem for web applications using HTML or XML,
where the problem goes by several names including ``cross-site scripting'',
``malicious HTML tags'', and ``malicious content.'' This book will call this
problem ``cross-site malicious content,'' since the problem isn't limited to
scripts or HTML, and its cross-site nature is fundamental. Note that this
problem isn't limited to web applications, but since this is a particular
problem for them, the rest of this discussion will emphasize web
applications. As will be shown in a moment, sometimes an attacker can cause a
victim to send data from the victim to the secure program, so the secure
program must protect the victim from himself.

7.15.1. Explanation of the Problem

Let's begin with a simple example. Some web applications are designed to
permit HTML tags in data input from users that will later be posted to other
readers (e.g., in a guestbook or ``reader comment'' area). If nothing is done
to prevent it, these tags can be used by malicious users to attack other
users by inserting scripts, Java references (including references to hostile
applets), DHTML tags, early document endings (via </HTML>), absurd font size
requests, and so on. This capability can be exploited for a wide range of
effects, such as exposing SSL-encrypted connections, accessing restricted web
sites via the client, violating domain-based security policies, making the
web page unreadable, making the web page unpleasant to use (e.g., via
annoying banners and offensive material), permit privacy intrusions (e.g., by
inserting a web bug to learn exactly who reads a certain page), creating
denial-of-service attacks (e.g., by creating an ``infinite'' number of
windows), and even very destructive attacks (by inserting attacks on security
vulnerabilities such as scripting languages or buffer overflows in browsers).
By embedding malicious FORM tags at the right place, an intruder may even be
able to trick users into revealing sensitive information (by modifying the
behavior of an existing form). Or, by embedding scripts, an intruder can
cause no end of problems. This is by no means an exhaustive list of problems,
but hopefully this is enough to convince you that this is a serious problem.

Most ``discussion boards'' have already discovered this problem, and most
already take steps to prevent it in text intended to be part of a multiperson
discussion. Unfortunately, many web application developers don't realize that
this is a much more general problem. Every data value that is sent from one
user to another can potentially be a source for cross-site malicious posting,
even if it's not an ``obvious'' case of an area where arbitrary HTML is
expected. The malicious data can even be supplied by the user himself, since
the user may have been fooled into supplying the data via another site.
Here's an example (from CERT) of an HTML link that causes the user to send
malicious data to another site:
 <A HREF="<SCRIPT                   
 SRC='http://bad-site/badfile'></SCRIPT>"> Click here</A>                    

In short, a web application cannot accept input (including any form data)
without checking, filtering, or encoding it. You can't even pass that data
back to the same user in many cases in web applications, since another user
may have surreptitiously supplied the data. Even if permitting such material
won't hurt your system, it will enable your system to be a conduit of attacks
to your users. Even worse, those attacks will appear to be coming from your

CERT describes the problem this way in their advisory:

    A web site may inadvertently include malicious HTML tags or script in a
    dynamically generated page based on unvalidated input from untrustworthy
    sources (CERT Advisory CA-2000-02, Malicious HTML Tags Embedded in Client
    Web Requests).
More information from CERT about this is available at [

7.15.2. Solutions to Cross-Site Malicious Content

Fundamentally, this means that all web application output impacted by any
user must be filtered (so characters that can cause this problem are
removed), encoded (so the characters that can cause this problem are encoded
in a way to prevent the problem), or validated (to ensure that only ``safe''
data gets through). This includes all output derived from input such as URL
parameters, form data, cookies, database queries, CORBA ORB results, and data
from users stored in files. In many cases, filtering and validation should be
done at the input, but encoding can be done during either input validation or
output generation. If you're just passing the data through without analysis,
it's probably better to encode the data on input (so it won't be forgotten).
However, if your program processes the data, it can be easier to encode it on
output instead. CERT recommends that filtering and encoding be done during
data output; this isn't a bad idea, but there are many cases where it makes
sense to do it at input instead. The critical issue is to make sure that you
cover all cases for every output, which is not an easy thing to do regardless
of approach.

Warning - in many cases these techniques can be subverted unless you've also
gained control over the character encoding of the output. Otherwise, an
attacker could use an ``unexpected'' character encoding to subvert the
techniques discussed here. Thankfully, this isn't hard; gaining control over
output character encoding is discussed in Section 9.5.

One minor defense, that's often worth doing, is the "HttpOnly" flag for
cookies. Scripts that run in a web browser cannot access cookie values that
have the HttpOnly flag set (they just get an empty value instead). This is
currently implemented in Microsoft Internet Explorer, and I expect Mozilla/
Netscape to implement this soon too. You should set HttpOnly on for any
cookie you send, unless you have scripts that need the cookie, to counter
certain kinds of cross-site scripting (XSS) attacks. However, the HttpOnly
flag can be circumvented in a variety of ways, so using as your primary
defense is inappropriate. Instead, it's a helpful secondary defense that may
help save you in case your application is written incorrectly.

The first subsection below discusses how to identify special characters that
need to be filtered, encoded, or validated. This is followed by subsections
describing how to filter or encode these characters. There's no subsection
discussing how to validate data in general, however, for input validation in
general see Chapter 5, and if the input is straight HTML text or a URI, see 
Section 5.11. Also note that your web application can receive malicious
cross-postings, so non-queries should forbid the GET protocol (see Section
----------------------------------------------------------------------------- Identifying Special Characters

Here are the special characters for a variety of circumstances (my thanks to
the CERT, who developed this list):

  * In the content of a block-level element (e.g., in the middle of a
    paragraph of text in HTML or a block in XML):
      + "<" is special because it introduces a tag.
      + "&" is special because it introduces a character entity.
      + ">" is special because some browsers treat it as special, on the
        assumption that the author of the page really meant to put in an
        opening "<", but omitted it in error.
  * In attribute values:
      + In attribute values enclosed with double quotes, the double quotes
        are special because they mark the end of the attribute value.
      + In attribute values enclosed with single quote, the single quotes are
        special because they mark the end of the attribute value. XML's
        definition allows single quotes, but I've been told that some XML
        parsers don't handle them correctly, so you might avoid using single
        quotes in XML.
      + Attribute values without any quotes make the white-space characters
        such as space and tab special. Note that these aren't legal in XML
        either, and they make more characters special. Thus, I recommend
        against unquoted attributes if you're using dynamically generated
        values in them.
      + "&" is special when used in conjunction with some attributes because
        it introduces a character entity.
  * In URLs, for example, a search engine might provide a link within the
    results page that the user can click to re-run the search. This can be
    implemented by encoding the search query inside the URL. When this is
    done, it introduces additional special characters:
      + Space, tab, and new line are special because they mark the end of the
      + "&" is special because it introduces a character entity or separates
        CGI parameters.
      + Non-ASCII characters (that is, everything above 128 in the ISO-8859-1
        encoding) aren't allowed in URLs, so they are all special here.
      + The "%" must be filtered from input anywhere parameters encoded with
        HTTP escape sequences are decoded by server-side code. The percent
        must be filtered if input such as "%68%65%6C%6C%6F" becomes "hello"
        when it appears on the web page in question.
  * Within the body of a <SCRIPT> </SCRIPT> the semicolon, parenthesis, curly
    braces, and new line should be filtered in situations where text could be
    inserted directly into a preexisting script tag.
  * Server-side scripts that convert any exclamation characters (!) in input
    to double-quote characters (") on output might require additional

Note that, in general, the ampersand (&) is special in HTML and XML.
----------------------------------------------------------------------------- Filtering

One approach to handling these special characters is simply eliminating them
(usually during input or output).

If you're already validating your input for valid characters (and you
generally should), this is easily done by simply omitting the special
characters from the list of valid characters. Here's an example in Perl of a
filter that only accepts legal characters, and since the filter doesn't
accept any special characters other than the space, it's quite acceptable for
use in areas such as a quoted attribute:
 # Accept only legal characters:                                             
 $summary =~ tr/A-Za-z0-9\ \.\://dc;                                         

However, if you really want to strip away only the smallest number of
characters, then you could create a subroutine to remove just those
 sub remove_special_chars {                                                  
  local($s) = @_;                                                            
  $s =~ s/[\<\>\"\'\%\;\(\)\&\+]//g;                                         
  return $s;                                                                 
 # Sample use:                                                               
 $data = &remove_special_chars($data);                                       
----------------------------------------------------------------------------- Encoding (Quoting)

An alternative to removing the special characters is to encode them so that
they don't have any special meaning. This has several advantages over
filtering the characters, in particular, it prevents data loss. If the data
is "mangled" by the process from the user's point of view, at least when the
data is encoded it's possible to reconstruct the data that was originally

HTML, XML, and SGML all use the ampersand ("&") character as a way to
introduce encodings in the running text; this encoding is often called ``HTML
encoding.'' To encode these characters, simply transform the special
characters in your circumstance. Usually this means '<' becomes '&lt;', '>'
becomes '&gt;', '&' becomes '&amp;', and '"' becomes '&quot;'. As noted
above, although in theory '>' doesn't need to be quoted, because some
browsers act on it (and fill in a '<') it needs to be quoted. There's a minor
complexity with the double-quote character, because '&quot;' only needs to be
used inside attributes, and some extremely old browsers don't properly render
it. If you can handle the additional complexity, you can try to encode '"'
only when you need to, but it's easier to simply encode it and ask users to
upgrade their browsers. Few users will use such ancient browsers, and the
double-quote character encoding has been a standard for a long time.

Scripting languages may consider implementing specialized auto-quoting types,
the interesting approach developed in the web application framework [http://] Quixote. Quixote includes a
"template" feature which allows easy mixing of HTML text and Python code;
text generated by a template is passed back to the web browser as an HTML
document. As of version 0.6, Quixote has two kinds of text (instead of a
single kind as most such languages). Anything which appears in a literal,
quoted string is of type "htmltext," and it is assumed to be exactly as the
programmer wanted it to be (this is reasoble, since the programmer wrote it).
Anything which takes the form of an ordinary Python string, however, is
automatically quoted as the template is executed. As a result, text from a
database or other external source is automatically quoted, and cannot be used
for a cross-site scripting attack. Thus, Quixote implements a safe default -
programmers no longer need to worry about quoting every bit of text that
passes through the application (bugs involving too much quoting are less
likely to be a security problem, and will be obvious in testing). Quixote
uses an open source software license, but because of its venue identification
it is probably GPL-incompatible, and is used by organizations such as the
[] Linux Weekly News.

This approach to HTML encoding isn't quite enough encoding in some
circumstances. As discussed in Section 9.5, you need to specify the output
character encoding (the ``charset''). If some of your data is encoded using a
different character encoding than the output character encoding, then you'll
need to do something so your output uses a consistent and correct encoding.
Also, you've selected an output encoding other than ISO-8859-1, then you need
to make sure that any alternative encodings for special characters (such as "
<") can't slip through to the browser. This is a problem with several
character encodings, including popular ones like UTF-7 and UTF-8; see Section
5.9 for more information on how to prevent ``alternative'' encodings of
characters. One way to deal with incompatible character encodings is to first
translate the characters internally to ISO 10646 (which has the same
character values as Unicode), and then using either numeric character
references or character entity references to represent them:

  * A numeric character reference looks like "&#D;", where D is a decimal
    number, or "&#xH;" or "&#XH;", where H is a hexadecimal number. The
    number given is the ISO 10646 character id (which has the same character
    values as Unicode). Thus &#1048; is the Cyrillic capital letter "I". The
    hexadecimal system isn't supported in the SGML standard (ISO 8879), so
    I'd suggest using the decimal system for output. Also, although SGML
    specification permits the trailing semicolon to be omitted in some
    circumstances, in practice many systems don't handle it - so always
    include the trailing semicolon.
  * A character entity reference does the same thing but uses mnemonic names
    instead of numbers. For example, "&lt;" represents the < sign. If you're
    generating HTML, see the [] HTML specification which
    lists all mnemonic names.

Either system (numeric or character entity) works; I suggest using character
entity references for '<', '>', '&', and '"' because it makes your code (and
output) easier for humans to understand. Other than that, it's not clear that
one or the other system is uniformly better. If you expect humans to edit the
output by hand later, use the character entity references where you can,
otherwise I'd use the decimal numeric character references just because
they're easier to program. This encoding scheme can be quite inefficient for
some languages (especially Asian languages); if that is your primary content,
you might choose to use a different character encoding (charset), filter on
the critical characters (e.g., "<") and ensure that no alternative encodings
for critical characters are allowed.

URIs have their own encoding scheme, commonly called ``URL encoding.'' In
this system, characters not permitted in URLs are represented using a percent
sign followed by its two-digit hexadecimal value. To handle all of ISO 10646
(Unicode), it's recommended to first translate the codes to UTF-8, and then
encode it. See Section 5.11.4 for more about validating URIs.

7.16. Foil Semantic Attacks

A ``semantic attack'' is an attack in which the attacker uses the computing
infrastructure/system in a way that fools the victim into thinking they are
doing something, but are doing something different, yet the computing
infrastructure/system is working exactly as it was designed to do. Semantic
attacks often involve financial scams, where the attacker is trying to fool
the victim into giving the attacker large sums of money (e.g., thinking
they're investing in something). For example, the attacker may try to
convince the user that they're looking at a trusted website, even if they

Semantic attacks are difficult to counter, because they're exploiting the
correct operation of the computer. The way to deal with semantic attacks is
to help give the human additional information, so that when ``odd'' things
happen the human will have more information or a warning will be presented
that something may not be what it appears to be.

One example is URIs that, while legitimate, may fool users into thinking they
have a different meaning. For example, look at this URI:                                    
If a user clicked on that URI, they might think that they're going to
Bloomberg (who provide financial commodities news), but instead they're going
to (and providing the username, which will conveniently ignore). If the website then
imitated the site, a user might be convinced that they're
seeing the real thing (and make investment decisions based on
attacker-controlled information). This depends on URIs being used in an
unusual way - clickable URIs can have usernames, but usually don't. One
solution for this case is for the web browser to detect such unusual URIs and
create a pop-up confirmation widget, saying ``You are about to log into as user; do you wish to proceed?'' If the
widget allows the user to change these entries, it provides additional
functionality to the user as well as providing protection against that

Another example is homographs, particularly international homographs. Certain
letters look similar to each other, and these can be exploited as well. For
example, since 0 (zero) and O (the letter O) look similar to each other,
users may not realize that WWW.BLOOMBERG.COM and WWW.BL00MBERG.COM are
different web addresses. Other similar-looking letters include 1 (one) and l
(lower-case L). If international characters are allowed, the situation is
worse. For example, many Cyrillic letters look essentially the same as Roman
letters, but the computer will treat them differently. Currently most systems
don't allow international characters in host names, but for various good
reasons it's widely agreed that support for them will be necessary in the
future. One proposed solution has been to diplay letters from different code
regions using different colors - that way, users get more information
visually. If the users look at URI, they will hopefully notice the strange
coloring. [Gabrilovich 2002] However, this does show the essence of a
semantic attack - it's difficult to defend against, precisely because the
computers are working correctly.

7.17. Be Careful with Data Types

Be careful with the data types used, in particular those used in interfaces.
For example, ``signed'' and ``unsigned'' values are treated differently in
many languages (such as C or C++).

Chapter 8. Carefully Call Out to Other Resources

                                       Do not put your trust in princes, in  
                                       mortal men, who cannot save.          
                                                           Psalms 146:3 (NIV)

Practically no program is truly self-contained; nearly all programs call out
to other programs for resources, such as programs provided by the operating
system, software libraries, and so on. Sometimes this calling out to other
resources isn't obvious or involves a great deal of ``hidden'' infrastructure
which must be depended on, e.g., the mechanisms to implement dynamic
libraries. Clearly, you must be careful about what other resources your
program trusts and you must make sure that the way you send requests to them.

8.1. Call Only Safe Library Routines

Sometimes there is a conflict between security and the development principles
of abstraction (information hiding) and reuse. The problem is that some
high-level library routines may or may not be implemented securely, and their
specifications won't tell you. Even if a particular implementation is secure,
it may not be possible to ensure that other versions of the routine will be
safe, or that the same interface will be safe on other platforms. 

In the end, if your application must be secure, you must sometimes
re-implement your own versions of library routines. Basically, you have to
re-implement routines if you can't be sure that the library routines will
perform the necessary actions you require for security. Yes, in some cases
the library's implementation should be fixed, but it's your users who will be
hurt if you choose a library routine that is a security weakness. If can, try
to use the high-level interfaces when you must re-implement something - that
way, you can switch to the high-level interface on systems where its use is

If you can, test to see if the routine is secure or not, and use it if it's
secure - ideally you can perform this test as part of compilation or
installation (e.g., as part of an ``autoconf'' script). For some conditions
this kind of run-time testing is impractical, but for other conditions, this
can eliminate many problems. If you don't want to bother to re-implement the
library, at least test to make sure it's safe and halt installation if it
isn't. That way, users will not accidentally install an insecure program and
will know what the problem is.

8.2. Limit Call-outs to Valid Values

Ensure that any call out to another program only permits valid and expected
values for every parameter. This is more difficult than it sounds, because
many library calls or commands call lower-level routines in potentially
surprising ways. For example, many system calls are implemented indirectly by
calling the shell, which means that passing characters which are shell
metacharacters can have dangerous effects. So, let's discuss metacharacters.

8.3. Handle Metacharacters

Many systems, such as the command line shell and SQL interpreters, have
``metacharacters'', that is, characters in their input that are not
interpreted as data. Such characters might commands, or delimit data from
commands or other data. If there's a language specification for that system's
interface that you're using, then it certainly has metacharacters. If your
program invokes those other systems and allows attackers to insert such
metacharacters, the usual result is that an attacker can completely control
your program.

One of the most pervasive metacharacter problems are those involving shell
metacharacters. The standard Unix-like command shell (stored in /bin/sh)
interprets a number of characters specially. If these characters are sent to
the shell, then their special interpretation will be used unless escaped;
this fact can be used to break programs. According to the WWW Security FAQ
[Stein 1999, Q37], these metacharacters are:
|& ; ` ' \ " | * ? ~ < > ^ ( ) [ ] { } $ \n \r                              |

I should note that in many situations you'll also want to escape the tab and
space characters, since they (and the newline) are the default parameter
separators. The separator values can be changed by setting the IFS
environment variable, but if you can't trust the source of this variable you
should have thrown it out or reset it anyway as part of your environment
variable processing.

Unfortunately, in real life this isn't a complete list. Here are some other
characters that can be problematic:

  * '!' means ``not'' in an expression (as it does in C); if the return value
    of a program is tested, prepending ! could fool a script into thinking
    something had failed when it succeeded or vice versa. In some shells, the
    "!" also accesses the command history, which can cause real problems. In
    bash, this only occurs for interactive mode, but tcsh (a csh clone found
    in some Linux distributions) uses "!" even in scripts.
  * '#' is the comment character; all further text on the line is ignored.
  * '-' can be misinterpreted as leading an option (or, as - -, disabling all
    further options). Even if it's in the ``middle'' of a filename, if it's
    preceded by what the shell considers as whitespace you may have a
  * ' ' (space), '\t' (tab), '\n' (newline), '\r' (return), '\v' (vertical
    space), '\f' (form feed), and other whitespace characters can have many
    dangerous effects. They can may turn a ``single'' filename into multiple
    arguments, for example, or turn a single parameter into multiple
    parameter when stored. Newline and return have a number of additional
    dangers, for example, they can be used to create ``spoofed'' log entries
    in some programs, or inserted just before a separate command that is then
    executed (if an underlying protocol uses newlines or returns as command
  * Other control characters (in particular, NIL) may cause problems for some
    shell implementations.
  * Depending on your usage, it's even conceivable that ``.'' (the ``run in
    current shell'') and ``='' (for setting variables) might be worrisome
    characters. However, any example I've found so far where these are issues
    have other (much worse) security problems.


What makes the shell metacharacters particularly pervasive is that several
important library calls, such as popen(3) and system(3), are implemented by
calling the command shell, meaning that they will be affected by shell
metacharacters too. Similarly, execlp(3) and execvp(3) may cause the shell to
be called. Many guidelines suggest avoiding popen(3), system(3), execlp(3),
and execvp(3) entirely and use execve(3) directly in C when trying to spawn a
process [Galvin 1998b]. At the least, avoid using system(3) when you can use
the execve(3); since system(3) uses the shell to expand characters, there is
more opportunity for mischief in system(3). In a similar manner the Perl and
shell backtick (`) also call a command shell; for more information on Perl
see Section 10.2.

Since SQL also has metacharacters, a similar issue revolves around calls to
SQL. When metacharacters are provided as input to trigger SQL metacharacters,
it's often called "SQL injection". See [
SQLInjectionWhitePaper.pdf] SPI Dynamic's paper ``SQL Injection: Are your Web
Applications Vulnerable?'' for further discussion on this. As discussed in 
Chapter 5, define a very limited pattern and only allow data matching that
pattern to enter; if you limit your pattern to ^[0-9]$ or ^[0-9A-Za-z]*$ then
you won't have a problem. If you must handle data that may include SQL
metacharacters, a good approach is to convert it (as early as possible) to
some other encoding before storage, e.g., HTML encoding (in which case you'll
need to encode any ampersand characters too). Also, prepend and append a
quote to all user input, even if the data is numeric; that way, insertions of
white space and other kinds of data won't be as dangerous.

Forgetting one of these characters can be disastrous, for example, many
programs omit backslash as a shell metacharacter [rfp 1999]. As discussed in
the Chapter 5, a recommended approach by some is to immediately escape at
least all of these characters when they are input. But again, by far and away
the best approach is to identify which characters you wish to permit, and use
a filter to only permit those characters.

A number of programs, especially those designed for human interaction, have
``escape'' codes that perform ``extra'' activities. One of the more common
(and dangerous) escape codes is one that brings up a command line. Make sure
that these ``escape'' commands can't be included (unless you're sure that the
specific command is safe). For example, many line-oriented mail programs
(such as mail or mailx) use tilde (~) as an escape character, which can then
be used to send a number of commands. As a result, apparently-innocent
commands such as ``mail admin < file-from-user'' can be used to execute
arbitrary programs. Interactive programs such as vi, emacs, and ed have
``escape'' mechanisms that allow users to run arbitrary shell commands from
their session. Always examine the documentation of programs you call to
search for escape mechanisms. It's best if you call only programs intended
for use by other programs; see Section 8.4.

The issue of avoiding escape codes even goes down to low-level hardware
components and emulators of them. Most modems implement the so-called
``Hayes'' command set. Unless the command set is disabled, inducing a delay,
the phrase ``+++'', and then another delay forces the modem to interpret any
following text as commands to the modem instead. This can be used to
implement denial-of-service attacks (by sending ``ATH0'', a hang-up command)
or even forcing a user to connect to someone else (a sophisticated attacker
could re-route a user's connection through a machine under the attacker's
control). For the specific case of modems, this is easy to counter (e.g., add
"ATS2-255" in the modem initialization string), but the general issue still
holds: if you're controlling a lower-level component, or an emulation of one,
make sure that you disable or otherwise handle any escape codes built into

Many ``terminal'' interfaces implement the escape codes of ancient, long-gone
physical terminals like the VT100. These codes can be useful, for example,
for bolding characters, changing font color, or moving to a particular
location in a terminal interface. However, do not allow arbitrary untrusted
data to be sent directly to a terminal screen, because some of those codes
can cause serious problems. On some systems you can remap keys (e.g., so when
a user presses "Enter" or a function key it sends the command you want them
to run). On some you can even send codes to clear the screen, display a set
of commands you'd like the victim to run, and then send that set ``back'',
forcing the victim to run the commands of the attacker's choosing without
even waiting for a keystroke. This is typically implemented using ``page-mode
buffering''. This security problem is why emulated tty's (represented as
device files, usually in /dev/) should only be writeable by their owners and
never anyone else - they should never have ``other write'' permission set,
and unless only the user is a member of the group (i.e., the ``user-private
group'' scheme), the ``group write'' permission should not be set either for
the terminal [Filipski 1986]. If you're displaying data to the user at a
(simulated) terminal, you probably need to filter out all control characters
(characters with values less than 32) from data sent back to the user unless
they're identified by you as safe. Worse comes to worse, you can identify tab
and newline (and maybe carriage return) as safe, removing all the rest.
Characters with their high bits set (i.e., values greater than 127) are in
some ways trickier to handle; some old systems implement them as if they
weren't set, but simply filtering them inhibits much international use. In
this case, you need to look at the specifics of your situation.

A related problem is that the NIL character (character 0) can have surprising
effects. Most C and C++ functions assume that this character marks the end of
a string, but string-handling routines in other languages (such as Perl and
Ada95) can handle strings containing NIL. Since many libraries and kernel
calls use the C convention, the result is that what is checked is not what is
actually used [rfp 1999].

When calling another program or referring to a file always specify its full
path (e.g, /usr/bin/sort). For program calls, this will eliminate possible
errors in calling the ``wrong'' command, even if the PATH value is
incorrectly set. For other file referents, this reduces problems from ``bad''
starting directories.

8.4. Call Only Interfaces Intended for Programmers

Call only application programming interfaces (APIs) that are intended for use
by programs. Usually a program can invoke any other program, including those
that are really designed for human interaction. However, it's usually unwise
to invoke a program intended for human interaction in the same way a human
would. The problem is that programs's human interfaces are intentionally rich
in functionality and are often difficult to completely control. As discussed
in Section 8.3, interactive programs often have ``escape'' codes, which might
enable an attacker to perform undesirable functions. Also, interactive
programs often try to intuit the ``most likely'' defaults; this may not be
the default you were expecting, and an attacker may find a way to exploit

Examples of programs you shouldn't normally call directly include mail,
mailx, ed, vi, and emacs. At the very least, don't call these without
checking their input first.

Usually there are parameters to give you safer access to the program's
functionality, or a different API or application that's intended for use by
programs; use those instead. For example, instead of invoking a text editor
to edit some text (such as ed, vi, or emacs), use sed where you can.

8.5. Check All System Call Returns

Every system call that can return an error condition must have that error
condition checked. One reason is that nearly all system calls require limited
system resources, and users can often affect resources in a variety of ways.
Setuid/setgid programs can have limits set on them through calls such as
setrlimit(3) and nice(2). External users of server programs and CGI scripts
may be able to cause resource exhaustion simply by making a large number of
simultaneous requests. If the error cannot be handled gracefully, then fail
safe as discussed earlier.

8.6. Avoid Using vfork(2)

The portable way to create new processes in Unix-like systems is to use the
fork(2) call. BSD introduced a variant called vfork(2) as an optimization
technique. In vfork(2), unlike fork(2), the child borrows the parent's memory
and thread of control until a call to execve(2V) or an exit occurs; the
parent process is suspended while the child is using its resources. The
rationale is that in old BSD systems, fork(2) would actually cause memory to
be copied while vfork(2) would not. Linux never had this problem; because
Linux used copy-on-write semantics internally, Linux only copies pages when
they changed (actually, there are still some tables that have to be copied;
in most circumstances their overhead is not significant). Nevertheless, since
some programs depend on vfork(2), recently Linux implemented the BSD vfork(2)
semantics (previously vfork(2) had been an alias for fork(2)).

There are a number of problems with vfork(2). From a portability
point-of-view, the problem with vfork(2) is that it's actually fairly tricky
for a process to not interfere with its parent, especially in high-level
languages. The ``not interfering'' requirement applies to the actual machine
code generated, and many compilers generate hidden temporaries and other code
structures that cause unintended interference. The result: programs using
vfork(2) can easily fail when the code changes or even when compiler versions

For secure programs it gets worse on Linux systems, because Linux (at least
2.2 versions through 2.2.17) is vulnerable to a race condition in vfork()'s
implementation. If a privileged process uses a vfork(2)/execve(2) pair in
Linux to execute user commands, there's a race condition while the child
process is already running as the user's UID, but hasn`t entered execve(2)
yet. The user may be able to send signals, including SIGSTOP, to this
process. Due to the semantics of vfork(2), the privileged parent process
would then be blocked as well. As a result, an unprivileged process could
cause the privileged process to halt, resulting in a denial-of-service of the
privileged process' service. FreeBSD and OpenBSD, at least, have code to
specifically deal with this case, so to my knowledge they are not vulnerable
to this problem. My thanks to Solar Designer, who noted and documented this
problem in Linux on the ``security-audit'' mailing list on October 7, 2000.

The bottom line with vfork(2) is simple: don't use vfork(2) in your programs.
This shouldn't be difficult; the primary use of vfork(2) is to support old
programs that needed vfork's semantics.

8.7. Counter Web Bugs When Retrieving Embedded Content

Some data formats can embed references to content that is automatically
retrieved when the data is viewed (not waiting for a user to select it). If
it's possible to cause this data to be retrieved through the Internet (e.g.,
through the World Wide Wide), then there is a potential to use this
capability to obtain information about readers without the readers'
knowledge, and in some cases to force the reader to perform activities
without the reader's consent. This privacy concern is sometimes called a
``web bug.''

In a web bug, a reference is intentionally inserted into a document and used
by the content author to track who, where, and how often a document is read.
The author can also essentially watch how a ``bugged'' document is passed
from one person to another or from one organization to another.

The HTML format has had this issue for some time. According to the [http://] Privacy Foundation:

    Web bugs are used extensively today by Internet advertising companies on
    Web pages and in HTML-based email messages for tracking. They are
    typically 1-by-1 pixel in size to make them invisible on the screen to
    disguise the fact that they are used for tracking. However, they could be
    any image (using the img tag); other HTML tags that can implement web
    bugs, e.g., frames, form invocations, and scripts. By itself, invoking
    the web bug will provide the ``bugging'' site the reader IP address, the
    page that the reader visited, and various information about the browser;
    by also using cookies it's often possible to determine the specific
    identify of the reader. A survey about web bugs is available at [http://] http://
What is more concerning is that other document formats seem to have such a
capability, too. When viewing HTML from a web site with a web browser, there
are other ways of getting information on who is browsing the data, but when
viewing a document in another format from an email few users expect that the
mere act of reading the document can be monitored. However, for many formats,
reading a document can be monitored. For example, it has been recently
determined that Microsoft Word can support web bugs; see [http://] the Privacy Foundation
advisory for more information . As noted in their advisory, recent versions
of Microsoft Excel and Microsoft Power Point can also be bugged. In some
cases, cookies can be used to obtain even more information.

Web bugs are primarily an issue with the design of the file format. If your
users value their privacy, you probably will want to limit the automatic
downloading of included files. One exception might be when the file itself is
being downloaded (say, via a web browser); downloading other files from the
same location at the same time is much less likely to concern users.

8.8. Hide Sensitive Information

Sensitive information should be hidden from prying eyes, both while being
input and output, and when stored in the system. Sensitive information
certainly includes credit card numbers, account balances, and home addresses,
and in many applications also includes names, email addressees, and other
private information.

Web-based applications should encrypt all communication with a user that
includes sensitive information; the usual way is to use the "https:" protocol
(HTTP on top of SSL or TLS). According to the HTTP 1.1 specification (IETF
RFC 2616 section 15.1.3), authors of services which use the HTTP protocol 
should not use GET based forms for the submission of sensitive data, because
this will cause this data to be encoded in the Request-URI. Many existing
servers, proxies, and user agents will log the request URI in some place
where it might be visible to third parties. Instead, use POST-based
submissions, which are intended for this purpose.

Databases of such sensitive data should also be encrypted on any storage
device (such as files on a disk). Such encryption doesn't protect against an
attacker breaking the secure application, of course, since obviously the
application has to have a way to access the encrypted data too. However, it 
does provide some defense against attackers who manage to get backup disks of
the data but not of the keys used to decrypt them. It also provides some
defense if an attacker doesn't manage to break into an application, but does
manage to partially break into a related system just enough to view the
stored data - again, they now have to break the encryption algorithm to get
the data. There are many circumstances where data can be transferred
unintentionally (e.g., core files), which this also prevents. It's worth
noting, however, that this is not as strong a defense as you'd think, because
often the server itself can be subverted or broken.

Chapter 9. Send Information Back Judiciously

                                       Do not answer a fool according to his 
                                       folly, or you will be like him        
                                                          Proverbs 26:4 (NIV)

9.1. Minimize Feedback

Avoid giving much information to untrusted users; simply succeed or fail, and
if it fails just say it failed and minimize information on why it failed.
Save the detailed information for audit trail logs. For example:

  * If your program requires some sort of user authentication (e.g., you're
    writing a network service or login program), give the user as little
    information as possible before they authenticate. In particular, avoid
    giving away the version number of your program before authentication.
    Otherwise, if a particular version of your program is found to have a
    vulnerability, then users who don't upgrade from that version advertise
    to attackers that they are vulnerable.
  * If your program accepts a password, don't echo it back; this creates
    another way passwords can be seen.


9.2. Don't Include Comments

When returning information, don't include any ``comments'' unless you're sure
you want the receiving user to be able to view them. This is a particular
problem for web applications that generate files (such as HTML). Often web
application programmers wish to comment their work (which is fine), but
instead of simply leaving the comment in their code, the comment is included
as part of the generated file (usually HTML or XML) that is returned to the
user. The trouble is that these comments sometimes provide insight into how
the system works in a way that aids attackers.

9.3. Handle Full/Unresponsive Output

It may be possible for a user to clog or make unresponsive a secure program's
output channel back to that user. For example, a web browser could be
intentionally halted or have its TCP/IP channel response slowed. The secure
program should handle such cases, in particular it should release locks
quickly (preferably before replying) so that this will not create an
opportunity for a Denial-of-Service attack. Always place time-outs on
outgoing network-oriented write requests.

9.4. Control Data Formatting (Format Strings/Formatation)

A number of output routines in computer languages have a parameter that
controls the generated format. In C, the most obvious example is the printf()
family of routines (including printf(), sprintf(), snprintf(), fprintf(), and
so on). Other examples in C include syslog() (which writes system log
information) and setproctitle() (which sets the string used to display
process identifier information). Many functions with names beginning with
``err'' or ``warn'', containing ``log'' , or ending in ``printf'' are worth
considering. Python includes the "%" operation, which on strings controls
formatting in a similar manner. Many programs and libraries define formatting
functions, often by calling built-in routines and doing additional processing
(e.g., glib's g_snprintf() routine).

Format languages are essentially little programming languages - so developers
who let attackers control the format string are essentially running programs
written by attackers! Surprisingly, many people seem to forget the power of
these formatting capabilities, and use data from untrusted users as the
formatting parameter. The guideline here is clear - never use unfiltered data
from an untrusted user as the format parameter. Failing to follow this
guideline usually results in a format string vulnerability (also called a
formatation vulnerability). Perhaps this is best shown by example:
  /* Wrong way: */                                                           
  /* Right ways: */                                                          
  printf("%s", string_from_untrusted_user); /* safe */                       
  fputs(string_from_untrusted_user); /* better for simple strings */         

If an attacker controls the formatting information, an attacker can cause all
sorts of mischief by carefully selecting the format. The case of C's printf()
is a good example - there are lots of ways to possibly exploit
user-controlled format strings in printf(). These include buffer overruns by
creating a long formatting string (this can result in the attacker having
complete control over the program), conversion specifications that use
unpassed parameters (causing unexpected data to be inserted), and creating
formats which produce totally unanticipated result values (say by prepending
or appending awkward data, causing problems in later use). A particularly
nasty case is printf's %n conversion specification, which writes the number
of characters written so far into the pointer argument; using this, an
attacker can overwrite a value that was intended for printing! An attacker
can even overwrite almost arbitrary locations, since the attacker can specify
a ``parameter'' that wasn't actually passed. The %n conversion specification
has been standard part of C since its beginning, is required by all C
standards, and is used by real programs. In 2000, Greg KH did a quick search
of source code and identified the programs BitchX (an irc client), Nedit (a
program editor), and SourceNavigator (a program editor / IDE / Debugger) as
using %n, and there are doubtless many more. Deprecating %n would probably be
a good idea, but even without %n there can be significant problems. Many
papers discuss these attacks in more detail, for example, you can see 
Avoiding security holes when developing an application - Part 4: format

Since in many cases the results are sent back to the user, this attack can
also be used to expose internal information about the stack. This information
can then be used to circumvent stack protection systems such as StackGuard
and ProPolice; StackGuard uses constant ``canary'' values to detect attacks,
but if the stack's contents can be displayed, the current value of the canary
will be exposed, suddenly making the software vulnerable again to stack
smashing attacks.

A formatting string should almost always be a constant string, possibly
involving a function call to implement a lookup for internationalization
(e.g., via gettext's _()). Note that this lookup must be limited to values
that the program controls, i.e., the user must be allowed to only select from
the message files controlled by the program. It's possible to filter user
data before using it (e.g., by designing a filter listing legal characters
for the format string such as [A-Za-z0-9]), but it's usually better to simply
prevent the problem by using a constant format string or fputs() instead.
Note that although I've listed this as an ``output'' problem, this can cause
problems internally to a program before output (since the output routines may
be saving to a file, or even just generating internal state such as via

The problem of input formatting causing security problems is not an idle
possibility; see CERT Advisory CA-2000-13 for an example of an exploit using
this weakness. For more information on how these problems can be exploited,
see Pascal Bouchareine's email article titled ``[Paper] Format bugs'',
published in the July 18, 2000 edition of []
Bugtraq. As of December 2000, developmental versions of the gcc compiler
support warning messages for insecure format string usages, in an attempt to
help developers avoid these problems.

Of course, this all begs the question as to whether or not the
internationalization lookup is, in fact, secure. If you're creating your own
internationalization lookup routines, make sure that an untrusted user can
only specify a legal locale and not something else like an arbitrary path.

Clearly, you want to limit the strings created through internationalization
to ones you can trust. Otherwise, an attacker could use this ability to
exploit the weaknesses in format strings, particularly in C/C++ programs.
This has been an item of discussion in Bugtraq (e.g., see John Levon's
Bugtraq post on July 26, 2000). For more information, see the discussion on
permitting users to only select legal language values in Section 5.8.3.

Although it's really a programming bug, it's worth mentioning that different
countries notate numbers in different ways, in particular, both the period
(.) and comma (,) are used to separate an integer from its fractional part.
If you save or load data, you need to make sure that the active locale does
not interfere with data handling. Otherwise, a French user may not be able to
exchange data with an English user, because the data stored and retrieved
will use different separators. I'm unaware of this being used as a security
problem, but it's conceivable.

9.5. Control Character Encoding in Output

In general, a secure program must ensure that it synchronizes its clients to
any assumptions made by the secure program. One issue often impacting web
applications is that they forget to specify the character encoding of their
output. This isn't a problem if all data is from trusted sources, but if some
of the data is from untrusted sources, the untrusted source may sneak in data
that uses a different encoding than the one expected by the secure program.
This opens the door for a cross-site malicious content attack; see Section
5.10 for more information.

[] CERT's tech
tip on malicious code mitigation explains the problem of unspecified
character encoding fairly well, so I quote it here:

    Many web pages leave the character encoding ("charset" parameter in HTTP)
    undefined. In earlier versions of HTML and HTTP, the character encoding
    was supposed to default to ISO-8859-1 if it wasn't defined. In fact, many
    browsers had a different default, so it was not possible to rely on the
    default being ISO-8859-1. HTML version 4 legitimizes this - if the
    character encoding isn't specified, any character encoding can be used.
    If the web server doesn't specify which character encoding is in use, it
    can't tell which characters are special. Web pages with unspecified
    character encoding work most of the time because most character sets
    assign the same characters to byte values below 128. But which of the
    values above 128 are special? Some 16-bit character-encoding schemes have
    additional multi-byte representations for special characters such as "<".
    Some browsers recognize this alternative encoding and act on it. This is
    "correct" behavior, but it makes attacks using malicious scripts much
    harder to prevent. The server simply doesn't know which byte sequences
    represent the special characters.
    For example, UTF-7 provides alternative encoding for "<" and ">", and
    several popular browsers recognize these as the start and end of a tag.
    This is not a bug in those browsers. If the character encoding really is
    UTF-7, then this is correct behavior. The problem is that it is possible
    to get into a situation in which the browser and the server disagree on
    the encoding.
Thankfully, though explaining the issue is tricky, its resolution in HTML is
easy. In the HTML header, simply specify the charset, like this example from
<META http-equiv="Content-Type"                                              
content="text/html; charset=ISO-8859-1">                                     
<TITLE>HTML SAMPLE</TITLE>                                                   
<P>This is a sample HTML page                                                

From a technical standpoint, an even better approach is to set the character
encoding as part of the HTTP protocol output, though some libraries make this
more difficult. This is technically better because it doesn't force the
client to examine the header to determine a character encoding that would
enable it to read the META information in the header. Of course, in practice
a browser that couldn't read the META information given above and use it
correctly would not succeed in the marketplace, but that's a different issue.
In any case, this just means that the server would need to send as part of
the HTTP protocol, a ``charset'' with the desired value. Unfortunately, it's
hard to heartily recommend this (technically better) approach, because some
older HTTP/1.0 clients did not deal properly with an explicit charset
parameter. Although the HTTP/1.1 specification requires clients to obey the
parameter, it's suspicious enough that you probably ought to use it as an
adjunct to forcing the use of the correct character encoding, and not your
sole mechanism.

9.6. Prevent Include/Configuration File Access

When developing web based applications, do not allow users to access (read)
files such as the program include and configuration files. This data may
provide enough information (e.g., passwords) to break into the system. Note
that this guideline sometimes also applies to other kinds of applications.
There are several actions you can take to do this, including:

  * Place the include/configuration files outside of the web documentation
    root (so that the web server will never serve the files). Really, this is
    the best approach unless there's some reason the files have to be inside
    the document root.
  * Configure the web server so it will not serve include files as text. For
    example, if you're using Apache, you can add a handler or an action for
    .inc files like so:
     <Files *.inc>                                                   
       Order allow,deny                                              
       Deny from all                                                 
  * Place the include files in a protected directory (using .htaccess), and
    designate them as files that won't be served.
  * Use a filter to deny access to the files. For Apache, this can be done
     <Files ~ "\.phpincludes">                                       
        Order allow,deny                                             
        Deny from all                                                
    If you need full regular expressions to match filenames, in Apache you
    could use the FilesMatch directive.
  * If your include file is a valid script file, which your server will
    parse, make sure that it doesn't act on user-supplied parameters and that
    it's designed to be secure.

These approaches won't protect you from users who have access to the
directories your files are in if they are world-readable. You could change
the permissions of the files so that only the uid/gid of the webserver can
read these files. However, this approach won't work if the user can get the
web server to run his own scripts (the user can just write scripts to access
your files). Fundamentally, if your site is being hosted on a server shared
with untrusted people, it's harder to secure the system. One approach is to
run multiple web serving programs, each with different permissions; this
provides more security but is painful in practice. Another approach is to set
these files to be read only by your uid/gid, and have the server run scripts
at ``your'' permission. This latter approach has its own problems: it means
that certain parts of the server must have root privileges, and that the
script may have more permissions than necessary.

Chapter 10. Language-Specific Issues

                                       Undoubtedly there are all sorts of    
                                       languages in the world, yet none of   
                                       them is without meaning.              
                                                    1 Corinthians 14:10 (NIV)

There are many language-specific security issues. Many of them can be
summarized as follows:

  * Turn on all relevant warnings and protection mechanisms available to you
    where practical. For compiled languages, this includes both compile-time
    mechanisms and run-time mechanisms. In general, security-relevant
    programs should compile cleanly with all warnings turned on.
  * If you can use a ``safe mode'' (e.g., a mode that limits the activities
    of the executable), do so. Many interpreted languages include such a
    mode. In general, don't depend on the safe mode to provide absolute
    protection; most language's safe modes have not been sufficiently
    analyzed for their security, and when they are, people usually discover
    many ways to exploit it. However, by writing your code so that it's
    secure out of safe mode, and then adding the safe mode, you end up with
    defense-in-depth (since in many cases, an attacker has to break both your
    application code and the safe mode).
  * Avoid dangerous and deprecated operations in the language. By
    ``dangerous'', I mean operations which are difficult to use correctly.
    For example, many languages include some mechanisms or functions that are
    ``magical'', that is, they try to infer the ``right'' thing to do using a
    heuristic - generally you should avoid them, because an attacker may be
    able to exploit the heuristic and do something dangerous instead of what
    was intended. A common error is an ``off-by-one'' error, in which the
    bound is off by one, and sometimes these result in exploitable errors. In
    general, write code in a way that minimizes the likelihood of off-by-one
    errors. If there are standard conventions in the language (e.g., for
    writing loops), use them.
  * Ensure that the languages' infrastructure (e.g., run-time library) is
    available and secured.
  * Languages that automatically garbage-collect strings should be especially
    careful to immediately erase secret data (in particular secret keys and
  * Know precisely the semantics of the operations that you are using. Look
    up each operation's semantics in its documentation. Do not ignore return
    values unless you're sure they cannot be relevant. Don't ignore the
    difference between ``signed'' and ``unsigned'' values. This is
    particularly difficult in languages which don't support exceptions, like
    C, but that's the way it goes.

10.1. C/C++

It is possible to develop secure code using C or C++, but both languages
include fundamental design decisions that make it more difficult to write
secure code. C and C++ easily permit buffer overflows, force programmers to
do their own memory management, and are fairly lax in their typing systems.
For systems programs (such as an operating system kernel), C and C++ are fine
choices. For applications, C and C++ are often over-used. Strongly consider
using an even higher-level language, at least for the majority of the
application. But clearly, there are many existing programs in C and C++ which
won't get completely rewritten, and many developers may choose to develop in
C and C++.

One of the biggest security problems with C and C++ programs is buffer
overflow; see Chapter 6 for more information. C has the additional weakness
of not supporting exceptions, which makes it easy to write programs that
ignore critical error situations.

Another problem with C and C++ is that developers have to do their own memory
management (e.g., using malloc(), alloc(), free(), new, and delete), and
failing to do it correctly may result in a security flaw. The more serious
problem is that programs may erroneously free memory that should not be freed
(e.g., because it's already been freed). This can result in an immediate
crash or be exploitable, allowing an attacker to cause arbitrary code to be
executed; see [Anonymous Phrack 2001]. Some systems (such as many GNU/Linux
systems) don't protect against double-freeing at all by default, and it is
not clear that those systems which attempt to protect themselves are truly
unsubvertable. Although I haven't seen anything written on the subject, I
suspect that using the incorrect call in C++ (e.g., mixing new and malloc())
could have similar effects. For example, on March 11, 2002, it was announced
that the zlib library had this problem, affecting the many programs that use
it. Thus, when testing programs on GNU/Linux, you should set the environment
variable MALLOC_CHECK_ to 1 or 2, and you might consider executing your
program with that environment variable set with 0, 1, 2. The reason for this
variable is explained in GNU/Linux malloc(3) man page:

    Recent versions of Linux libc (later than 5.4.23) and GNU libc (2.x)
    include a malloc implementation which is tunable via environment
    variables. When MALLOC_CHECK_ is set, a special (less efficient)
    implementation is used which is designed to be tolerant against simple
    errors, such as double calls of free() with the same argument, or
    overruns of a single byte (off-by-one bugs). Not all such errors can be
    protected against, however, and memory leaks can result. If MALLOC_CHECK_
    is set to 0, any detected heap corruption is silently ignored; if set to
    1, a diagnostic is printed on stderr; if set to 2, abort() is called
    immediately. This can be useful because otherwise a crash may happen much
    later, and the true cause for the problem is then very hard to track
There are various tools to deal with this, such as Electric Fence and
Valgrind; see Section 11.7 for more information. If unused memory is not
free'd, (e.g., using free()), that unused memory may accumulate - and if
enough unused memory can accumulate, the program may stop working. As a
result, the unused memory may be exploitable by attackers to create a denial
of service. It's theoretically possible for attackers to cause memory to be
fragmented and cause a denial of service, but usually this is a fairly
impractical and low-risk attack.

Be as strict as you reasonably can when you declare types. Where you can, use
``enum'' to define enumerated values (and not just a ``char'' or ``int'' with
special values). This is particularly useful for values in switch statements,
where the compiler can be used to determine if all legal values have been
covered. Where it's appropriate, use ``unsigned'' types if the value can't be

One complication in C and C++ is that the character type ``char'' can be
signed or unsigned (depending on the compiler and machine). When a signed
char with its high bit set is saved in an integer, the result will be a
negative number; in some cases this can be exploitable. In general, use
``unsigned char'' instead of char or signed char for buffers, pointers, and
casts when dealing with character data that may have values greater than 127

C and C++ are by definition rather lax in their type-checking support, but
you can at least increase their level of checking so that some mistakes can
be detected automatically. Turn on as many compiler warnings as you can and
change the code to cleanly compile with them, and strictly use ANSI
prototypes in separate header (.h) files to ensure that all function calls
use the correct types. For C or C++ compilations using gcc, use at least the
following as compilation flags (which turn on a host of warning messages) and
try to eliminate all warnings (note that -O2 is used since some warnings can
only be detected by the data flow analysis performed at higher optimization
|gcc -Wall -Wpointer-arith -Wstrict-prototypes -O2                          |
You might want ``-W -pedantic'' too.

Many C/C++ compilers can detect inaccurate format strings. For example, gcc
can warn about inaccurate format strings for functions you create if you use
its __attribute__() facility (a C extension) to mark such functions, and you
can use that facility without making your code non-portable. Here is an
example of what you'd put in your header (.h) file:
 /* in header.h */                                                           
 #ifndef __GNUC__                                                            
 #  define __attribute__(x) /*nothing*/                                      
 extern void logprintf(const char *format, ...)                              
 extern void logprintva(const char *format, va_list args)                    
The "format" attribute takes either "printf" or "scanf", and the numbers that
follow are the parameter number of the format string and the first variadic
parameter (respectively). The GNU docs talk about this well. Note that there
are other __attribute__ facilities as well, such as "noreturn" and "const".

Avoid common errors made by C/C++ developers. For example, be careful about
not using ``='' when you mean ``==''.

10.2. Perl

Perl programmers should first read the man page perlsec(1), which describes a
number of issues involved with writing secure programs in Perl. In
particular, perlsec(1) describes the ``taint'' mode, which most secure Perl
programs should use. Taint mode is automatically enabled if the real and
effective user or group IDs differ, or you can use the -T command line flag
(use the latter if you're running on behalf of someone else, e.g., a CGI
script). Taint mode turns on various checks, such as checking path
directories to make sure they aren't writable by others.

The most obvious affect of taint mode, however, is that you may not use data
derived from outside your program to affect something else outside your
program by accident. In taint mode, all externally-obtained input is marked
as ``tainted'', including command line arguments, environment variables,
locale information (see perllocale(1)), results of certain system calls
(readdir, readlink, the gecos field of getpw* calls), and all file input.
Tainted data may not be used directly or indirectly in any command that
invokes a sub-shell, nor in any command that modifies files, directories, or
processes. There is one important exception: If you pass a list of arguments
to either system or exec, the elements of that list are NOT checked for
taintedness, so be especially careful with system or exec while in taint

Any data value derived from tainted data becomes tainted also. There is one
exception to this; the way to untaint data is to extract a substring of the
tainted data. Don't just use ``.*'' blindly as your substring, though, since
this would defeat the tainting mechanism's protections. Instead, identify
patterns that identify the ``safe'' pattern allowed by your program, and use
them to extract ``good'' values. After extracting the value, you may still
need to check it (in particular for its length).

The open, glob, and backtick functions call the shell to expand filename wild
card characters; this can be used to open security holes. You can try to
avoid these functions entirely, or use them in a less-privileged ``sandbox''
as described in perlsec(1). In particular, backticks should be rewritten
using the system() call (or even better, changed entirely to something

The perl open() function comes with, frankly, ``way too much magic'' for most
secure programs; it interprets text that, if not carefully filtered, can
create lots of security problems. Before writing code to open or lock a file,
consult the perlopentut(1) man page. In most cases, sysopen() provides a
safer (though more convoluted) approach to opening a file. The new Perl 5.6
adds an open() call with 3 parameters to turn off the magic behavior without
requiring the convolutions of sysopen().

Perl programs should turn on the warning flag (-w), which warns of
potentially dangerous or obsolete statements.

You can also run Perl programs in a restricted environment. For more
information see the ``Safe'' module in the standard Perl distribution. I'm
uncertain of the amount of auditing that this has undergone, so beware of
depending on this for security. You might also investigate the ``Penguin
Model for Secure Distributed Internet Scripting'', though at the time of this
writing the code and documentation seems to be unavailable.

Many installations include a setuid root version of perl named ``suidperl''.
However, the perldelta man page version 5.6.1 recommends using sudo instead,
stating the following:

    "Note that suidperl is neither built nor installed by default in any
    recent version of perl. Use of suidperl is highly discouraged. If you
    think you need it, try alternatives such as sudo first. See http://".
10.3. Python

As with any language, beware of any functions which allow data to be executed
as parts of a program, to make sure an untrusted user can't affect their
input. This includes exec(), eval(), and execfile() (and frankly, you should
check carefully any call to compile()). The input() statement is also
surprisingly dangerous. [Watters 1996, 150].

Python programs with privileges that can be invoked by unprivileged users
(e.g., setuid/setgid programs) must not import the ``user'' module. The user
module causes the file to be read and executed. Since this file
would be under the control of an untrusted user, importing the user module
allows an attacker to force the trusted program to run arbitrary code.

Python does very little compile-time checking -- it has essentially no
compile-time type information, and it doesn't even check that the number of
parameters passed are legal for a given function or method. This is
unfortunate, resulting in a lot of latent bugs (both John Viega and I have
experienced this problem). Hopefully someday Python will implement optional
static typing and type-checking, an idea that's been discussed for some time.
A partial solution for now is PyChecker, a lint-like program that checks for
common bugs in Python source code. You can get PyChecker from [http://]

Python includes support for ``Restricted Execution'' through its RExec class.
This is primarily intended for executing applets and mobile code, but it can
also be used to limit privilege in a program even when the code has not been
provided externally. By default, a restricted execution environment permits
reading (but not writing) of files, and does not include operations for
network access or GUI interaction. These defaults can be changed, but beware
of creating loopholes in the restricted environment. In particular, allowing
a user to unrestrictedly add attributes to a class permits all sorts of ways
to subvert the environment because Python's implementation calls many
``hidden'' methods. Note that, by default, most Python objects are passed by
reference; if you insert a reference to a mutable value into a restricted
program's environment, the restricted program can change the object in a way
that's visible outside the restricted environment! Thus, if you want to give
access to a mutable value, in many cases you should copy the mutable value or
use the Bastion module (which supports restricted access to another object).
For more information, see Kuchling [2000]. I'm uncertain of the amount of
auditing that the restricted execution capability has undergone, so
programmer beware.

10.4. Shell Scripting Languages (sh and csh Derivatives)

I strongly recommend against using standard command shell scripting languages
(such as csh, sh, and bash) for setuid/setgid secure code. Some systems (such
as Linux) completely disable setuid/setgid shell scripts, so creating setuid/
setgid shell scripts creates an unnecessary portability problem. On some old
systems they are fundamentally insecure due to a race condition (as discussed
in Section 3.1.3). Even for other systems, they're not really a good idea.

In fact, there are a vast number of circumstances where shell scripting
languages shouldn't be used at all for secure programs. Standard command
shells are notorious for being affected by nonobvious inputs - generally
because command shells were designed to try to do things ``automatically''
for an interactive user, not to defend against a determined attacker. Shell
programs are fine for programs that don't need to be secure (e.g., they run
at the same privilege as the unprivileged user and don't accept ``untrusted''
data). They can also be useful when they're running with privilege, as long
as all the input (e.g., files, directories, command line, environment, etc.)
are all from trusted users - which is why they're often used quite
successfully in startup/shutdown scripts.

Writing secure shell programs in the presence of malicious input is harder
than in many other languages because of all the things that shells are
affected by. For example, ``hidden'' environment variables (e.g., the ENV,
BASH_ENV, and IFS values) can affect how they operate or even execute
arbitrary user-defined code before the script can even execute. Even things
like filenames of the executable or directory contents can affect execution.
If an attacker can create filenames containing some control characters (e.g.,
newline), or whitespace, or shell metacharacters, or begin with a dash (the
option flag syntax), there are often ways to exploit them. For example, on
many Bourne shell implementations, doing the following will grant root access
(thanks to NCSA for describing this exploit):
 % ln -s /usr/bin/setuid-shell /tmp/-x                                       
 % cd /tmp                                                                   
 % -x                                                                        
Some systems may have closed this hole, but the point still stands: most
command shells aren't intended for writing secure setuid/setgid programs. For
programming purposes, avoid creating setuid shell scripts, even on those
systems that permit them. Instead, write a small program in another language
to clean up the environment, then have it call other executables (some of
which might be shell scripts).

If you still insist on using shell scripting languages, at least put the
script in a directory where it cannot be moved or changed. Set PATH and IFS
to known values very early in your script; indeed, the environment should be
cleaned before the script is called. Also, very early on, ``cd'' to a safe
directory. Use data only from directories that is controlled by trusted
users, e.g., /etc, so that attackers can't insert maliciously-named files
into those directories. Be sure to quote every filename passed on a command
line, e.g., use "$1" not $1, because filenames with whitespace will be split.
Call commands using "--" to disable additional options where you can, because
attackers may create or pass filenames beginning with dash in the hope of
tricking the program into processing it as an option. Be especially careful
of filenames embedding other characters (e.g., newlines and other control
characters). Examine input filenames especially carefully and be very
restrictive on what filenames are permitted.

If you don't mind limiting your program to only work with GNU tools (or if
you detect and optionally use the GNU tools instead when they are available),
you might want to use NIL characters as the filename terminator instead of
newlines. By using NIL characters, rather than whitespace or newlines,
handling nasty filenames (e.g., those with embedded newlines) is much
simpler. Several GNU tools that output or input filenames can use this format
instead of the more common ``one filename per line'' format. Unfortunately,
the name of this option isn't consistent between tools; for many tools the
name of this option is ``--null'' or ``-0''. GNU programs xargs and cpio
allow using either --null or -0, tar uses --null, find uses -print0, grep
uses either --null or -Z, and sort uses either -z or --zero-terminated. Those
who find this inconsistency particularly disturbing are invited to supply
patches to the GNU authors; I would suggest making sure every program
supported ``--null'' since that seems to be the most common option name. For
example, here's one way to move files to a target directory, even if there
may be a vast number of files and some may have awkward names with embedded
newlines (thanks to Jim Dennis for reminding me of this):
 find . -print0 | xargs --null mv --target-dir=$TARG                         

In a similar vein, I recommend not trusting ``restricted shells'' to
implement secure policies. Restricted shells are shells that intentionally
prevent users from performing a large set of activities - their goal is to
force users to only run a small set of programs. A restricted shell can be
useful as a defense-in-depth measure, but restricted shells are notoriously
hard to configure correctly and as configured are often subvertable. For
example, some restricted shells will start by running some file in an
unrestricted mode (e.g., ``.profile'') - if a user can change this file, they
can force execution of that code. A restricted shell should be set up to only
run a few programs, but if any of those programs have ``shell escapes'' to
let users run more programs, attackers can use those shell escapes to escape
the restricted shell. Even if the programs don't have shell escapes, it's
quite likely that the various programs can be used together (along with the
shell's capabilities) to escape the restrictions. Of course, if you don't set
the PATH of a restricted shell (and allow any program to run), then an
attacker can use the shell escapes of many programs (including text editors,
mailers, etc.). The problem is that the purpose of a shell is to run other
programs, but those other programs may allow unintended operations -- and the
shell doesn't interpose itself to prevent these operations.

10.5. Ada

In Ada95, the Unbounded_String type is often more flexible than the String
type because it is automatically resized as necessary. However, don't store
especially sensitive secret values such as passwords or secret keys in an
Unbounded_String, since core dumps and page areas might still hold them
later. Instead, use the String type for this data, lock it into memory while
it's used, and overwrite the data as soon as possible with some constant
value such as (others => ' '). Use the Ada pragma Inspection_Point on the
object holding the secret after erasing the memory. That way, you can be
certain that the object containing the secret will really be erased (and that
the the overwriting won't be optimized away).

It's common for beginning Ada programmers to believe that the String type's
first index value is always 1, but this isn't true if the string is sliced.
Avoid this error.

It's worth noting that SPARK is a ``high-integrity subset of the Ada
programming language''; SPARK users use a tool called the ``SPARK Examiner''
to check conformance to SPARK rules, including flow analysis, and there are
various supports for full formal proof of the code if desired. See the SPARK
website for more information. To my knowledge, there are no OSS/FS SPARK
tools. If you're storing passwords and private keys you should still lock
them into memory if appropriate and overwrite them as soon as possible. Note
that SPARK is often used in environments where paging does not occur.

10.6. Java

If you're developing secure programs using Java, frankly your first step
(after learning Java) is to read the two primary texts for Java security,
namely Gong [1999] and McGraw [1999] (for the latter, look particularly at
section 7.1). You should also look at Sun's posted security code guidelines
at []
security/seccodeguide.html, and there's a nice [
developerworks/java/library/j-staticsec.html?loc=dwmain] article by Sahu et
al [2002] A set of slides describing Java's security model are freely
available at []
javasec. You can also see McGraw [1998].

Obviously, a great deal depends on the kind of application you're developing.
Java code intended for use on the client side has a completely different
environment (and trust model) than code on a server side. The general
principles apply, of course; for example, you must check and filter any input
from an untrusted source. However, in Java there are some ``hidden'' inputs
or potential inputs that you need to be wary of, as discussed below.
Johnathan Nightingale [2000] made an interesting statement summarizing many
of the issues in Java programming:

    ... the big thing with Java programming is minding your inheritances. If
    you inherit methods from parents, interfaces, or parents' interfaces, you
    risk opening doors to your code.
The following are a few key guidelines, based on Gong [1999], McGraw [1999],
Sun's guidance, and my own experience:

 1. Do not use public fields or variables; declare them as private and
    provide accessors to them so you can limit their accessibility.
 2. Make methods private unless there is a good reason to do otherwise (and
    if you do otherwise, document why). These non-private methods must
    protect themselves, because they may receive tainted data (unless you've
    somehow arranged to protect them).
 3. The JVM may not actually enforce the accessibility modifiers (e.g.,
    ``private'') at run-time in an application (as opposed to an applet). My
    thanks to John Steven (Cigital Inc.), who pointed this out on the
    ``Secure Programming'' mailing list on November 7, 2000. The issue is
    that it all depends on what class loader the class requesting the access
    was loaded with. If the class was loaded with a trusted class loader
    (including the null/ primordial class loader), the access check returns
    "TRUE" (allowing access). For example, this works (at least with Sun's
    1.2.2 VM ; it might not work with other implementations):
     a. write a victim class (V) with a public field, compile it.
     b. write an 'attack' class (A) that accesses that field, compile it
     c. change V's public field to private, recompile
     d. run A - it'll access V's (now private) field.
    However, the situation is different with applets. If you convert A to an
    applet and run it as an applet (e.g., with appletviewer or browser), its
    class loader is no longer a trusted (or null) class loader. Thus, the
    code will throw java.lang.IllegalAccessError, with the message that
    you're trying to access a field V.secret from class A.
 4. Avoid using static field variables. Such variables are attached to the
    class (not class instances), and classes can be located by any other
    class. As a result, static field variables can be found by any other
    class, making them much more difficult to secure.
 5. Never return a mutable object to potentially malicious code (since the
    code may decide to change it). Note that arrays are mutable (even if the
    array contents aren't), so don't return a reference to an internal array
    with sensitive data.
 6. Never store user given mutable objects (including arrays of objects)
    directly. Otherwise, the user could hand the object to the secure code,
    let the secure code ``check'' the object, and change the data while the
    secure code was trying to use the data. Clone arrays before saving them
    internally, and be careful here (e.g., beware of user-written cloning
 7. Don't depend on initialization. There are several ways to allocate
    uninitialized objects.
 8. Make everything final, unless there's a good reason not to. If a class or
    method is non-final, an attacker could try to extend it in a dangerous
    and unforeseen way. Note that this causes a loss of extensibility, in
    exchange for security.
 9. Don't depend on package scope for security. A few classes, such as
    java.lang, are closed by default, and some Java Virtual Machines (JVMs)
    let you close off other packages. Otherwise, Java classes are not closed.
    Thus, an attacker could introduce a new class inside your package, and
    use this new class to access the things you thought you were protecting.
10. Don't use inner classes. When inner classes are translated into byte
    codes, the inner class is translated into a class accesible to any class
    in the package. Even worse, the enclosing class's private fields silently
    become non-private to permit access by the inner class!
11. Minimize privileges. Where possible, don't require any special
    permissions at all. McGraw goes further and recommends not signing any
    code; I say go ahead and sign the code (so users can decide to ``run only
    signed code by this list of senders''), but try to write the program so
    that it needs nothing more than the sandbox set of privileges. If you
    must have more privileges, audit that code especially hard.
12. If you must sign your code, put it all in one archive file. Here it's
    best to quote McGraw [1999]:
        The goal of this rule is to prevent an attacker from carrying out a
        mix-and-match attack in which the attacker constructs a new applet or
        library that links some of your signed classes together with
        malicious classes, or links together signed classes that you never
        meant to be used together. By signing a group of classes together,
        you make this attack more difficult. Existing code-signing systems do
        an inadequate job of preventing mix-and-match attacks, so this rule
        cannot prevent such attacks completely. But using a single archive
        can't hurt.
13. Make your classes uncloneable. Java's object-cloning mechanism allows an
    attacker to instantiate a class without running any of its constructors.
    To make your class uncloneable, just define the following method in each
    of your classes:
    public final Object clone() throws java.lang.CloneNotSupportedException { 
       throw new java.lang.CloneNotSupportedException();                      
    If you really need to make your class cloneable, then there are some
    protective measures you can take to prevent attackers from redefining
    your clone method. If you're defining your own clone method, just make it
    final. If you're not, you can at least prevent the clone method from
    being maliciously overridden by adding the following:
    public final void clone() throws java.lang.CloneNotSupportedException { 
14. Make your classes unserializeable. Serialization allows attackers to view
    the internal state of your objects, even private portions. To prevent
    this, add this method to your classes:
    private final void writeObject(ObjectOutputStream out)             
      throws {                                     
         throw new"Object cannot be serialized"); 
    Even in cases where serialization is okay, be sure to use the transient
    keyword for the fields that contain direct handles to system resources
    and that contain information relative to an address space. Otherwise,
    deserializing the class may permit improper access. You may also want to
    identify sensitive information as transient.
    If you define your own serializing method for a class, it should not pass
    an internal array to any DataInput/DataOuput method that takes an array.
    The rationale: All DataInput/DataOutput methods can be overridden. If a
    Serializable class passes a private array directly to a DataOutput(write
    (byte [] b)) method, then an attacker could subclass ObjectOutputStream
    and override the write(byte [] b) method to enable him to access and
    modify the private array. Note that the default serialization does not
    expose private byte array fields to DataInput/DataOutput byte array
15. Make your classes undeserializeable. Even if your class is not
    serializeable, it may still be deserializeable. An attacker can create a
    sequence of bytes that happens to deserialize to an instance of your
    class with values of the attacker's choosing. In other words,
    deserialization is a kind of public constructor, allowing an attacker to
    choose the object's state - clearly a dangerous operation! To prevent
    this, add this method to your classes:
    private final void readObject(ObjectInputStream in)                
      throws {                                     
        throw new"Class cannot be deserialized"); 
16. Don't compare classes by name. After all, attackers can define classes
    with identical names, and if you're not careful you can cause confusion
    by granting these classes undesirable privileges. Thus, here's an example
    of the wrong way to determine if an object has a given class:
      if (obj.getClass().getName().equals("Foo")) {                  
    If you need to determine if two objects have exactly the same class,
    instead use getClass() on both sides and compare using the == operator,
    Thus, you should use this form:
      if (a.getClass() == b.getClass()) {                            
    If you truly need to determine if an object has a given classname, you
    need to be pedantic and be sure to use the current namespace (of the
    current class's ClassLoader). Thus, you'll need to use this format:
      if (obj.getClass() == this.getClassLoader().loadClass("Foo")) {
    This guideline is from McGraw and Felten, and it's a good guideline. I'll
    add that, where possible, it's often a good idea to avoid comparing class
    values anyway. It's often better to try to design class methods and
    interfaces so you don't need to do this at all. However, this isn't
    always practical, so it's important to know these tricks.
17. Don't store secrets (cryptographic keys, passwords, or algorithm) in the
    code or data. Hostile JVMs can quickly view this data. Code obfuscation
    doesn't really hide the code from serious attackers.


10.7. Tcl

Tcl stands for ``tool command language'' and is pronounced ``tickle.'' Tcl is
divided into two parts: a language and a library. The language is a simple
language, originally intended for issuing commands to interactive programs
and including basic programming capabilities. The library can be embedded in
application programs. You can find more information about Tcl at sites such
as the [] and the [
/Tcl.html] Tcl WWW Info web page and the comp.lang.tcl FAQ launch page at
tcl-faq. My thanks go to Wojciech Kocjan for providing some of this detailed
information on using Tcl in secure applications.

For some security applications, especially interesting components of Tcl are
Safe-Tcl (which creates a sandbox in Tcl) and Safe-TK (which implements a
sandboxed portable GUI for Safe Tcl), as well as the WebWiseTclTk Toolkit
which permits Tcl packages to be automatically located and loaded from
anywhere on the World Wide Web. You can find more about the latter from
software/WebWiseTclTk. It's not clear to me how much code review this has

Tcl's original design goal to be a small, simple language resulted in a
language that was originally somewhat limiting and slow. For an example of
the limiting weaknesses in the original language, see [
/~jchapin/6853-FT97/Papers/stallman-tcl.html] Richard Stallman's ``Why You
Should Not Use Tcl''. For example, Tcl was originally designed to really
support only one data type (string). Thankfully, these issues have been
addressed over time. In particular, version 8.0 added support for more data
types (integers are stored internally as integers, lists as lists and so on).
This improves its capabilities, and in particular improves its speed.

As with essentially all scripting languages, Tcl has an "eval" command that
parses and executes arbitrary Tcl commands. And like all such scripting
languages, this eval command needs to be used especially carefully, or an
attacker could insert characters in the input to cause malicious things to
occur. For example, an attackers may be able insert characters with special
meaning to Tcl such as embedded whitespace (including space and newline),
double-quote, curly braces, square brackets, dollar signs, backslash,
semicolon, or pound sign (or create input to cause these characters to be
created during processing). This also applies to any function that passes
data to eval as well (depending on how eval is called).

Here is a small example that may make this concept clearer; first, let's
define a small function and then interactively invoke it directly - note that
these uses are fine:
 proc something {a b c d e} {                                                
       puts "A='$a'"                                                         
       puts "B='$b'"                                                         
       puts "C='$c'"                                                         
       puts "D='$d'"                                                         
       puts "E='$e'"                                                         
 % # This works normally:                                                    
 % something "test 1" "test2" "t3" "t4" "t5"                                 
 A='test 1'                                                                  
 % # Imagine that str1 is set by an attacker:                                
 % set str1 {test 1 [puts HELLOWORLD]}                                       
 % # This works as well                                                      
 % something $str1 t2 t3 t4 t5                                               
 A='test 1 [puts HELLOWORLD]'                                                
However, continuing the example, let's see how "eval" can be incorrectly and
correctly called. If you call eval in an incorrect (dangerous) way, it allows
attackers to misuse it. However, by using commands like list or lrange to
correctly group the input, you can avoid this problem:
 % # This is the WRONG way - str1 is interpreted.                            
 % eval something $str1 t2 t3                                                
 % # Here's one solution, using "list".                                      
 % eval something [list $str1 t2 t3 t4 t5]                                   
 A='test 1 [puts HELLOWORLD]'                                                
 % # Here's another solution, using lrange:                                  
 % eval something [lrange $str1 0 end] t2                                    
Using lrange is useful when concatenating arguments to a called function,
e.g., with more complex libraries using callbacks. In Tcl, eval is often used
to create a one-argument version of a function that takes a variable number
of arguments, and you need to be careful when using it this way. Here's
another example (presuming that you've defined a "printf" function):
 proc vprintf {str arglist} {                                                
      eval printf [list $str] [lrange $arglist 0 end]                        
 % printf "1+1=%d  2+2=%d" 2 4                                               
 % vprintf "1+1=%d  2+2=%d" {2 4}                                            

Fundamentally, when passing a command that will be eventually evaluated, you
must pass Tcl commands as a properly built list, and not as a (possibly
concatentated) string. For example, the "after" command runs a Tcl command
after a given number of milliseconds; if the data in $param1 can be
controlled by an attacker, this Tcl code is dangerously wrong:
  # DON'T DO THIS if param1 can be controlled by an attacker                 
  after 1000 "someCommand someparam $param1"                                 
This is wrong, because if an attacker can control the value of $param1, the
attacker can control the program. For example, if the attacker can cause
$param1 to have '[exit]', then the program will exit. Also, if $param1 would
be '; exit', it would also exit.

Thus, the proper alternative would be:
 after 1000 [list someCommand someparam $param1]                             
Even better would be something like the following:
 set cmd [list someCommand someparam]                                        
 after 1000 [concat $cmd $param1]                                            

Here's another example showing what you shouldn't do, pretending that $params
is data controlled by possibly malicious user:
 set params "%-20s TESTSTRING"                                               
 puts "'[eval format $params]'"                                              
will result in:
 'TESTSTRING       '                                                         
But, when if the untrusted user sends data with an embedded newline, like
 set params "%-20s TESTSTRING\nputs HELLOWORLD"                              
 puts "'[eval format $params]'"                                              
The result will be this (notice that the attacker's code was executed!):
 'TESTINGSTRING       '                                                      
Wojciech Kocjan suggests that the simplest solution in this case is to
convert this to a list using lrange, doing this:
 set params "%-20s TESTINGSTRING\nputs HELLOWORLD"                           
 puts "'[eval format [lrange $params 0 end]]'"                               
The result would be:
 'TESTINGSTRING       '                                                      
Note that this solution presumes that the potentially malicious text is
concatenated to the end of the text; as with all languages, make sure the
attacker cannot control the format text.

As a matter of style always use curly braces when using if, while, for, expr,
and any other command which parses an argument using expr/eval/subst. Doing
this will avoid a common error when using Tcl called unintended double
substitution (aka double substitution). This is best explained by example;
the following code is incorrect:
 while ![eof $file] {                                                        
     set line [gets $file]                                                   
The code is incorrect because the "![eof $file]" text will be evaluated by
the Tcl parser when the while command is executed the first time, and not
re-evaluated in every iteration as it should be. Instead, do this:
 while {![eof $file]} {                                                      
      set line [gets $file]                                                  
Note that both the condition, and the action to be performed, are surrounded
by curly braces. Although there are cases where the braces are redundant,
they never hurt, and when you fail to include the curly braces where they're
needed (say, when making a minor change) subtle and hard-to-find errors often

More information on good Tcl style can be found in documents such as [http://] Ray Johnson's Tcl Style Guide.

In the past, I have stated that I don't recommend Tcl for writing programs
which must mediate a security boundary. Tcl seems to have improved since that
time, so while I cannot guarantee Tcl will work for your needs, I can't
guarantee that any other language will work for you either. Again, my thanks
to Wojciech Kocjan who provided some of these suggestions on how to write Tcl
code for secure applications.

10.8. PHP

SecureReality has put out a very interesting paper titled ``A Study In
Scarlet - Exploiting Common Vulnerabilities in PHP'' [Clowes 2001], which
discusses some of the problems in writing secure programs in PHP,
particularly in versions before PHP 4.1.0. Clowes concludes that ``it is very
hard to write a secure PHP application (in the default configuration of PHP),
even if you try''.

Granted, there are security issues in any language, but one particular issue
stands out in older versions of PHP that arguably makes older PHP versions
less secure than most languages: the way it loads data into its namespace. By
default, in PHP (versions 4.1.0 and lower) all environment variables and
values sent to PHP over the web are automatically loaded into the same
namespace (global variables) that normal variables are loaded into - so
attackers can set arbitrary variables to arbitrary values, which keep their
values unless explicitly reset by a PHP program. In addition, PHP
automatically creates variables with a default value when they're first
requested, so it's common for PHP programs to not initialize variables. If
you forget to set a variable, PHP can report it, but by default PHP won't -
and note that this simply an error report, it won't stop an attacker who
finds an unusual way to cause it. Thus, by default PHP allows an attacker to
completely control the values of all variables in a program unless the
program takes special care to override the attacker. Once the program takes
over, it can reset these variables, but failing to reset any variable (even
one not obvious) might open a vulnerability in the PHP program.

For example, the following PHP program (an example from Clowes) intends to
only let those who know the password to get some important information, but
an attacker can set ``auth'' in their web browser and subvert the
authorization check:
  if ($pass == "hello")                                                      
   $auth = 1;                                                                
  if ($auth == 1)                                                            
   echo "some important information";                                        

I and many others have complained about this particularly dangerous problem;
it's particularly a problem because PHP is widely used. A language that's
supposed to be easy to use better make it easy to write secure programs in,
after all. It's possible to disable this misfeature in PHP by turning the
setting ``register_globals'' to ``off'', but by default PHP versions up
through 4.1.0 default set this to ``on'' and PHP before 4.1.0 is harder to
use with register_globals off. The PHP developers warned in their PHP 4.1.0
announcenment that ``as of the next semi-major version of PHP, new
installations of PHP will default to having register_globals set to off.''
This has now happened; as of PHP version 4.2.0, External variables (from the
environment, the HTTP request, cookies or the web server) are no longer
registered in the global scope by default. The preferred method of accessing
these external variables is by using the new Superglobal arrays, introduced
in PHP 4.1.0.

PHP with ``register_globals'' set to ``on'' is a dangerous choice for
nontrivial programs - it's just too easy to write insecure programs. However,
once ``register_globals'' is set to ``off'', PHP is quite a reasonable
language for development.

The secure default should include setting ``register_globals'' to ``off'',
and also including several functions to make it much easier for users to
specify and limit the input they'll accept from external sources. Then web
servers (such as Apache) could separately configure this secure PHP
installation. Routines could be placed in the PHP library to make it easy for
users to list the input variables they want to accept; some functions could
check the patterns these variables must have and/or the type that the
variable must be coerced to. In my opinion, PHP is a bad choice for secure
web development if you set register_globals on.

As I suggested in earlier versions of this book, PHP has been trivially
modified to become a reasonable choice for secure web development. However,
note that PHP doesn't have a particularly good security vulnerability track
record (e.g., register_globals, a file upload problem, and a format string
problem in the error reporting library); I believe that security issues were
not considered sufficiently in early editions of PHP; I also think that the
PHP developers are now emphasizing security and that these security issues
are finally getting worked out. One evidence is the major change that the PHP
developers have made to get turn off register_globals; this had a significant
impact on PHP users, and their willingness to make this change is a good
sign. Unfortunately, it's not yet clear how secure PHP really is; PHP just
hasn't had much of a track record now that the developers of PHP are
examining it seriously for security issues. Hopefully this will become clear

If you've decided to use PHP, here are some of my recommendations (many of
these recommendations are based on ways to counter the issues that Clowes

  * Set the PHP configuration option ``register_globals'' off, and use PHP
    4.2.0 or greater. PHP 4.1.0 adds several special arrays, particularly
    $_REQUEST, which makes it far simpler to develop software in PHP when
    ``register_globals'' is off. Setting register_globals off, which is the
    default in PHP 4.2.0, completely eliminates the most common PHP attacks.
    If you're assuming that register_globals is off, you should check for
    this first (and halt if it's not true) - that way, people who install
    your program will quickly know there's a problem. Note that many
    third-party PHP applications cannot work with this setting, so it can be
    difficult to keep it off for an entire website. It's possible to set
    register_globals off for only some programs. For example, for Apache, you
    could insert these lines into the file .htaccess in the PHP directory (or
    use Directory directives to control it further):
     php_flag register_globals Off                                   
     php_flag track_vars On                                          
    However, the .htaccess file itself is ignored unless the Apache web
    server is configured to permit overrides; often the Apache global
    configuration is set so that AllowOverride is set to None. So, for Apache
    users, if you can convince your web hosting service to set
    ``AllowOverride Options'' in their configuration file (often /etc/http/
    conf/http.conf) for your host, do that. Then write helper functions to
    simplify loading the data you need (and only that data).
  * If you must develop software where register_globals might be on while
    running (e.g., a widely-deployed PHP application), always set values not
    provided by the user. Don't depend on PHP default values, and don't trust
    any variable you haven't explicitly set. Note that you have to do this
    for every entry point (e.g., every PHP program or HTML file using PHP).
    The best approach is to begin each PHP program by setting all variables
    you'll be using, even if you're simply resetting them to the usual
    default values (like "" or 0). This includes global variables referenced
    in included files, even all libraries, transitively. Unfortunately, this
    makes this recommendation hard to do, because few developers truly know
    and understand all global variables that may be used by all functions
    they call. One lesser alternative is to search through HTTP_GET_VARS,
    provided the data - but programmers often forget to check all sources,
    and what happens if PHP adds a new data source (e.g., HTTP_POST_FILES
    wasn't in old versions of PHP). Of course, this simply tells you how to
    make the best of a bad situation; in case you haven't noticed yet, turn
    off register_globals!
  * Set the error reporting level to E_ALL, and resolve all errors reported
    by it during testing. Among other things, this will complain about
    un-initialized variables, which are a key issues in PHP. This is a good
    idea anyway whenever you start using PHP, because this helps debug
    programs, too. There are many ways to set the error reporting level,
    including in the ``php.ini'' file (global), the ``.htttpd.conf'' file
    (single-host), the ``.htaccess'' file (multi-host), or at the top of the
    script through the error_reporting function. I recommend setting the
    error reporting level in both the php.ini file and also at the top of the
    script; that way, you're protected if (1) you forget to insert the
    command at the top of the script, or (2) move the program to another
    machine and forget to change the php.ini file. Thus, every PHP program
    should begin like this:
      <?php error_reporting(E_ALL);?>                                
    It could be argued that this error reporting should be turned on during
    development, but turned off when actually run on a real site (since such
    error message could give useful information to an attacker). The problem
    is that if they're disabled during ``actual use'' it's all too easy to
    leave them disabled during development. So for the moment, I suggest the
    simple approach of simply including it in every entrance. A much better
    approach is to record all errors, but direct the error reports so they're
    only included in a log file (instead of having them reported to the
  * Filter any user information used to create filenames carefully, in
    particular to prevent remote file access. PHP by default comes with
    ``remote files'' functionality -- that means that file-opening commands
    like fopen(), that in other languages can only open local files, can
    actually be used to invoke web or ftp requests from another site.
  * Do not use old-style PHP file uploads; use the HTTP_POST_FILES array and
    related functions. PHP supports file uploads by uploading the file to
    some temporary directory with a special filename. PHP originally set a
    collection of variables to indicate where that filename was, but since an
    attacker can control variable names and their values, attackers could use
    that ability to cause great mischief. Instead, always use HTTP_POST_FILES
    and related functions to access uploaded files. Note that even in this
    case, PHP's approach permits attackers to temporarily upload files to you
    with arbitrary content, which is risky by itself.
  * Only place protected entry points in the document tree; place all other
    code (which should be most of it) outside the document tree. PHP has a
    history of unfortunate advice on this topic. Originally, PHP users were
    supposed to use the ``.inc'' (include) extension for ``included'' files,
    but these included files often had passwords and other information, and
    Apache would just give requesters the contents of the ``.inc'' files when
    asked to do so when they were in the document tree. Then developers gave
    all files a ``.php'' extension - which meant that the contents weren't
    seen, but now files never meant to be entry points became entry points
    and were sometimes exploitable. As mentioned earlier, the usual security
    advice is the best: place only the proected entry points (files) in the
    document tree, and place other code (e.g., libraries) outside the
    document tree. There shouldn't be any ``.inc'' files in the document tree
    at all.
  * Avoid the session mechanism. The ``session'' mechanism is handy for
    storing persistent data, but its current implementation has many
    problems. First, by default sessions store information in temporary files
    - so if you're on a multi-hosted system, you open yourself up to many
    attacks and revelations. Even those who aren't currently multi-hosted may
    find themselves multi-hosted later! You can "tie" this information into a
    database instead of the filesystem, but if others on a multi-hosted
    database can access that database with the same permissions, the problem
    is the same. There are also ambiguities if you're not careful (``is this
    the session value or an attacker's value''?) and this is another case
    where an attacker can force a file or key to reside on the server with
    content of their choosing - a dangerous situation - and the attacker can
    even control to some extent the name of the file or key where this data
    will be placed.
  * For all inputs, check that they match a pattern for acceptability (as
    with any language), and then use type casting to coerce non-string data
    into the type it should have. Develop ``helper'' functions to easily
    check and import a selected list of (expected) inputs. PHP is loosely
    typed, and this can cause trouble. For example, if an input datum has the
    value "000", it won't be equal to "0" nor is it empty(). This is
    particularly important for associative arrays, because their indexes are
    strings; this means that $data["000"] is different than $data["0"]. For
    example, to make sure $bar has type double (after making sure it only has
    the format legal for a double):
      $bar = (double) $bar;                                          
  * Be especially careful of risky functions. This includes those that
    perform PHP code execution (e.g., require(), include(), eval(),
    preg_replace()), command execution (e.g., exec(), passthru(), the
    backtick operator, system(), and popen()), and open files (e.g., fopen(),
    readfile(), and file()). This is not an exhaustive list!
  * Use magic_quotes_gpc() where appropriate - this eliminates many kinds of
  * Avoid file uploads, and consider modifying the php.ini file to disable
    them (file_uploads = Off). File uploads have had security holes in the
    past, so on older PHP's this is a necessity, and until more experience
    shows that they're safe this isn't a bad thing to remove. Remember, in
    general, to secure a system you should disable or remove anything you
    don't need.

Chapter 11. Special Topics

                                       Understanding is a fountain of life to
                                       those who have it, but folly brings   
                                       punishment to fools.                  
                                                         Proverbs 16:22 (NIV)

11.1. Passwords

Where possible, don't write code to handle passwords. In particular, if the
application is local, try to depend on the normal login authentication by a
user. If the application is a CGI script, try to depend on the web server to
provide the protection as much as possible - but see below about handling
authentication in a web server. If the application is over a network, avoid
sending the password as cleartext (where possible) since it can be easily
captured by network sniffers and reused later. ``Encrypting'' a password
using some key fixed in the algorithm or using some sort of shrouding
algorithm is essentially the same as sending the password as cleartext.

For networks, consider at least using digest passwords. Digest passwords are
passwords developed from hashes; typically the server will send the client
some data (e.g., date, time, name of server), the client combines this data
with the user password, the client hashes this value (termed the ``digest
pasword'') and replies just the hashed result to the server; the server
verifies this hash value. This works, because the password is never actually
sent in any form; the password is just used to derive the hash value. Digest
passwords aren't considered ``encryption'' in the usual sense and are usually
accepted even in countries with laws constraining encryption for
confidentiality. Digest passwords are vulnerable to active attack threats but
protect against passive network sniffers. One weakness is that, for digest
passwords to work, the server must have all the unhashed passwords, making
the server a very tempting target for attack.

If your application permits users to set their passwords, check the passwords
and permit only ``good'' passwords (e.g., not in a dictionary, having certain
minimal length, etc.). You may want to look at information such as [http://]
writeup/security/security_3.html on how to choose a good password. You should
use PAM if you can, because it supports pluggable password checkers.

11.2. Authenticating on the Web

On the web, a web server is usually authenticated to users by using SSL or
TLS and a server certificate - but it's not as easy to authenticate who the
users are. SSL and TLS do support client-side certificates, but there are
many practical problems with actually using them (e.g., web browsers don't
support a single user certificate format and users find it difficult to
install them). You can learn about how to set up digital certificates from
many places, e.g., [
file=index] Petbrain. Using Java or Javascript has its own problems, since
many users disable them, some firewalls filter them out, and they tend to be
slow. In most cases, requiring every user to install a plug-in is impractical
too, though if the system is only for an intranet for a relatively small
number of users this may be appropriate.

If you're building an intranet application, you should generally use whatever
authentication system is used by your users. Unix-like systems tend to use
Kerberos, NIS+, or LDAP. You may also need to deal with a Windows-based
authentication schemes (which can be viewed as proprietary variants of
Kerberos and LDAP). Thus, if your organization depend on Kerberos, design
your system to use Kerberos. Try to separate the authentication system from
the rest of your application, since the organization may (will!) change their
authentication system over time.

Many techniques don't work or don't work very well. One approach that works
in some cases is to use ``basic authentication'', which is built into
essentially all browsers and servers. Unfortunately, basic authentication
sends passwords unencrypted, so it makes passwords easy to steal; basic
authentication by itself is really useful only for worthless information. You
could store authentication information in the URLs selected by the users, but
for most circumstances you should never do this - not only are the URLs sent
unprotected over the wire (as with basic authentication), but there are too
many other ways that this information can leak to others (e.g., through the
browser history logs stored by many browsers, logs of proxies, and to other
web sites through the Referer: field). You could wrap all communication with
a web server using an SSL/TLS connection (which would encrypt it); this is
secure (depending on how you do it), and it's necessary if you have important
data, but note that this is costly in terms of performance. You could also
use ``digest authentication'', which exposes the communication but at least
authenticates the user without exposing the underlying password used to
authenticate the user. Digest authentication is intended to be a simple
partial solution for low-value communications, but digest authentication is
not widely supported in an interoperable way by web browsers and servers. In
fact, as noted in a March 18, 2002 eWeek article, Microsoft's web client
(Internet Explorer) and web server (IIS) incorrectly implement the standard
(RFC 2617), and thus won't work with other servers or browsers. Since
Microsoft don't view this incorrect implementation as a serious problem, it
will be a very long time before most of their customers have a
correctly-working program.

Thus, the most common technique for authenticating on the web today is
through cookies. Cookies weren't really designed for this purpose, but they
can be used for authentication - but there are many wrong ways to use them
that create security vulnerabilities, so be careful. For more information
about cookies, see IETF RFC 2965, along with the older specifications about
them. Note that to use cookies, some browsers (e.g., Microsoft Internet
Explorer 6) may insist that you have a privacy profile (named p3p.xml on the
root directory of the server).

Note that some users don't accept cookies, so this solution still has some
problems. If you want to support these users, you should send this
authentication information back and forth via HTML form hidden fields (since
nearly all browsers support them without concern). You'd use the same
approach as with cookies - you'd just use a different technology to have the
data sent from the user to the server. Naturally, if you implement this
approach, you need to include settings to ensure that these pages aren't
cached for use by others. However, while I think avoiding cookies is
preferable, in practice these other approaches often require much more
development effort. Since it's so hard to implement this on a large scale for
many application developers, I'm not currently stressing these approaches. I
would rather describe an approach that is reasonably secure and reasonably
easy to implement, than emphasize approaches that are too hard to implement
correctly (by either developers or users). However, if you can do so without
much effort, by all means support sending the authentication information
using form hidden fields and an encrypted link (e.g., SSL/TLS). As with all
cookies, for these cookies you should turn on the HttpOnly flag unless you
have a web browser script that must be able to read the cookie.

Fu [2001] discusses client authentication on the web, along with a suggested
approach, and this is the approach I suggest for most sites. The basic idea
is that client authentication is split into two parts, a ``login procedure''
and ``subsequent requests.'' In the login procedure, the server asks for the
user's username and password, the user provides them, and the server replies
with an ``authentication token''. In the subsequent requests, the client (web
browser) sends the authentication token to the server (along with its
request); the server verifies that the token is valid, and if it is, services
the request. Another good source of information about web authentication is
Seifried [2001].

One serious problem with some web authentication techniques is that they are
vulnerable to a problem called "session fixation". In a session fixation
attack, the attacker fixes the user's session ID before the user even logs
into the target server, thus eliminating the need to obtain the user's
session ID afterwards. Basically, the attacker obtains an account, and then
tricks another user into using the attacker's account - often by creating a
special hypertext link and tricking the user into clicking on it. A good
paper describing session fixation is the paper by [
/session_fixation.pdf] Mitja Kolsek [2002]. A web authentication system you
use should be resistant to session fixation.

11.2.1. Authenticating on the Web: Logging In

The login procedure is typically implemented as an HTML form; I suggest using
the field names ``username'' and ``password'' so that web browsers can
automatically perform some useful actions. Make sure that the password is
sent over an encrypted connection (using SSL or TLS, through an https:
connection) - otherwise, eavesdroppers could collect the password. Make sure
all password text fields are marked as passwords in the HTML, so that the
password text is not visible to anyone who can see the user's screen.

If both the username and password fields are filled in, do not try to
automatically log in as that user. Instead, display the login form with the
user and password fields; this lets the user verify that they really want to
log in as that user. If you fail to do this, attackers will be able to
exploit this weakness to perform a session fixation attack. Paranoid systems
might want simply ignore the password field and make the user fill it in, but
this interferes with browsers which can store passwords for users.

When the user sends username and password, it must be checked against the
user account database. This database shouldn't store the passwords ``in the
clear'', since if someone got a copy of the this database they'd suddenly get
everyone's password (and users often reuse passwords). Some use crypt() to
handle this, but crypt can only handle a small input, so I recommend using a
different approach (this is my approach - Fu [2001] doesn't discuss this).
Instead, the user database should store a username, salt, and the password
hash for that user. The ``salt'' is just a random sequence of characters,
used to make it harder for attackers to determine a password even if they get
the password database - I suggest an 8-character random sequence. It doesn't
need to be cryptographically random, just different from other users. The
password hash should be computed by concatenating ``server key1'', the user's
password, and the salt, and then running a cryptographically secure hash
algorithm. Server key1 is a secret key unique to this server - keep it
separate from the password database. Someone who has server key1 could then
run programs to crack user passwords if they also had the password database;
since it doesn't need to be memorized, it can be a long and complex password.
Most secure would be HMAC-SHA-1 or HMAC-MD5; you could use SHA-1 (most web
sites aren't really worried about the attacks it allows) or MD5 (but MD5
would be poorer choice; see the discussion about MD5).

Thus, when users create their accounts, the password is hashed and placed in
the password database. When users try to log in, the purported password is
hashed and compared against the hash in the database (they must be equal).
When users change their password, they should type in both the old and new
password, and the new password twice (to make sure they didn't mistype it);
and again, make sure none of these password's characters are visible on the

By default, don't save the passwords themselves on the client's web browser
using cookies - users may sometimes use shared clients (say at some coffee
shop). If you want, you can give users the option of ``saving the password''
on their browser, but if you do, make sure that the password is set to only
be transmitted on ``secure'' connections, and make sure the user has to
specifically request it (don't do this by default).

Make sure that the page is marked to not be cached, or a proxy server might
re-serve that page to other users.

Once a user successfully logs in, the server needs to send the client an
``authentication token'' in a cookie, which is described next.

11.2.2. Authenticating on the Web: Subsequent Actions

Once a user logs in, the server sends back to the client a cookie with an
authentication token that will be used from then on. A separate
authentication token is used, so that users don't need to keep logging in, so
that passwords aren't continually sent back and forth, and so that
unencrypted communication can be used if desired. A suggested token (ignoring
session fixation attacks) would look like this:
Where t is the expiration time of the token (say, in several hours), and data
s identifies the user (say, the user name or session id). The digest is a
keyed digest of the other fields. Feel free to change the field name of
``data'' to be more descriptive (e.g., username and/or sessionid). If you
have more than one field of data (e.g., both a username and a sessionid),
make sure the digest uses both the field names and data values of all fields
you're authenticating; concatenate them with a pattern (say ``%%'', ``+'', or
``&'') that can't occur in any of the field data values. As described in a
moment, it would be a good idea to include a username. The keyed digest
should be a cryptographic hash of the other information in the token, keyed
using a different server key2. The keyed digest should use HMAC-MD5 or
HMAC-SHA1, using a different server key (key2), though simply using SHA1
might be okay for some purposes (or even MD5, if the risks are low). Key2 is
subject to brute force guessing attacks, so it should be long (say 12+
characters) and unguessable; it does NOT need to be easily remembered. If
this key2 is compromised, anyone can authenticate to the server, but it's
easy to change key2 - when you do, it'll simply force currently ``logged in''
users to re-authenticate. See Fu [2001] for more details.

There is a potential weakness in this approach. I have concerns that Fu's
approach, as originally described, is weak against session fixation attacks
(from several different directions, which I don't want to get into here).
Thus, I now suggest modifying Fu's approach and using this token format
This is the same as the original Fu aproach, and older versions of this book
(before December 2002) didn't suggest it. This modification adds a new
"client" field to uniquely identify the client's current location/identity.
The data in the client field should be something that should change if
someone else tries to use the account; ideally, its new value should be
unguessable, though that's hard to accomplish in practice. Ideally the client
field would be the client's SSL client certificate, but currently that's a
suggest that is hard to meet. At the least, it should be the user's IP
address (as perceived from the server, and remember to plan for IPv6's longer
addresses). This modification doesn't completely counter session fixation
attacks, unfortunately (since if an attacker can determine what the user
would send, the attacker may be able to make a request to a server and
convince the client to accept those values). However, it does add resistance
to the attack. Again, the digest must now include all the other data.

Here's an example. If a user logs into sucessfully, you might
establish the expiration date as 2002-12-30T1800 (let's assume we'll transmit
as ASCII text in this format for the moment), the username as "fred", the
client session as "1234", and you might determine that the client's IP
address was If you use a simple SHA-1 keyed digest (and use a key
prefixing the rest of the data), with the server key2 value of "rM!V^m~v*
Dzx", the digest could be computed over:
A keyed digest can be computed by running a cryptographic hash code over,
say, the server key2, then the data; in this case, the digest would be:

From then on, the server must check the expiration time and recompute the
digest of this authentication token, and only accept client requests if the
digest is correct. If there's no token, the server should reply with the user
login page (with a hidden form field to show where the successful login
should go afterwards).

It would be prudent to display the username, especially on important screens,
to help counter session fixation attacks. If users are given feedback on
their username, they may notice if they don't have their expected username.
This is helpful anyway if it's possible to have an unexpected username (e.g.,
a family that shares the same machine). Examples of important screens include
those when a file is uploaded that should be kept private.

One odd implementation issue: although the specifications for the "Expires:"
(expiration time) field for cookies permit time zones, it turns out that some
versions of Microsoft's Internet Explorer don't implement time zones
correctly for cookie expiration. Thus, you need to always use UTC time (also
called Zulu time) in cookie expiration times for maximum portability. It's a
good idea in general to use UTC time for time values, and convert when
necessary for human display, since this eliminates other time zone and
daylight savings time issues.

If you include a sessionid in the authentication token, you can limit access
further. Your server could ``track'' what pages a user has seen in a given
session, and only permit access to other appropriate pages from that point
(e.g., only those directly linked from those page(s)). For example, if a user
is granted access to page foo.html, and page foo.html has pointers to
resources bar1.jpg and bar2.png, then accesses to bar4.cgi can be rejected.
You could even kill the session, though only do this if the authentication
information is valid (otherwise, this would make it possible for attackers to
cause denial-of-service attacks on other users). This would somewhat limit
the access an attacker has, even if they successfully hijack a session,
though clearly an attacker with time and an authentication token could
``walk'' the links just as a normal user would.

One decision is whether or not to require the authentication token and/or
data to be sent over a secure connection (e.g., SSL). If you send an
authentication token in the clear (non-secure), someone who intercepts the
token could do whatever the user could do until the expiration time. Also,
when you send data over an unencrypted link, there's the risk of unnoticed
change by an attacker; if you're worried that someone might change the data
on the way, then you need to authenticate the data being transmitted.
Encryption by itself doesn't guarantee authentication, but it does make
corruption more likely to be detected, and typical libraries can support both
encryption and authentication in a TLS/SSL connection. In general, if you're
encrypting a message, you should also authenticate it. If your needs vary,
one alternative is to create two authentication tokens - one is used only in
a ``secure'' connection for important operations, while the other used for
less-critical operations. Make sure the token used for ``secure'' connections
is marked so that only secure connections (typically encrypted SSL/TLS
connections) are used. If users aren't really different, the authentication
token could omit the ``data'' entirely.

Again, make sure that the pages with this authentication token aren't cached.
There are other reasonable schemes also; the goal of this text is to provide
at least one secure solution. Many variations are possible.

11.2.3. Authenticating on the Web: Logging Out

You should always provide users with a mechanism to ``log out'' - this is
especially helpful for customers using shared browsers (say at a library).
Your ``logout'' routine's task is simple - just unset the client's
authentication token.

11.3. Random Numbers

In many cases secure programs must generate ``random'' numbers that cannot be
guessed by an adversary. Examples include session keys, public or private
keys, symmetric keys, nonces and IVs used in many protocols, salts, and so
on. Ideally, you should use a truly random source of data for random numbers,
such as values based on radioactive decay (through precise timing of Geiger
counter clicks), atmospheric noise, or thermal noise in electrical circuits.
Some computers have a hardware component that functions as a real random
value generator, and if it's available you should use it.

However, most computers don't have hardware that generates truly random
values, so in most cases you need a way to generate random numbers that is
sufficiently random that an adversary can't predict it. In general, this
means that you'll need three things:

  * An ``unguessable'' state; typically this is done by measuring variances
    in timing of low-level devices (keystrokes, disk drive arm jitter, etc.)
    in a way that an adversary cannot control.
  * A cryptographically strong pseudo-random number generator (PRNG), which
    uses the state to generate ``random'' numbers.
  * A large number of bits (in both the seed and the resulting value used).
    There's no point in having a strong PRNG if you only have a few possible
    values, because this makes it easy for an attacker to use brute force
    attacks. The number of bits necessary varies depending on the
    circumstance, however, since these are often used as cryptographic keys,
    the normal rules of thumb for keys apply. For a symmetric key (result),
    I'd use at least 112 bits (3DES), 128 bits is a little better, and 160
    bits or more is even safer.

Typically the PRNG uses the state to generate some values, and then some of
its values and other unguessable inputs are used to update the state. There
are lots of ways to attack these systems. For example, if an attacker can
control or view inputs to the state (or parts of it), the attacker may be
able to determine your supposedly ``random'' number.

A real danger with PRNGs is that most computer language libraries include a
large set of pseudo-random number generators (PRNGs) which are inappropriate
for security purposes. Let me say it again: do not use typical random number
generators for security purposes. Typical library PRNGs are intended for use
in simulations, games, and so on; they are not sufficiently random for use in
security functions such as key generation. Most non-cryptographic library
PRNGs are some variation of ``linear congruential generators'', where the
``next'' random value is computed as "(aX+b) mod m" (where X is the previous
value). Good linear congruential generators are fast and have useful
statistical properties, making them appropriate for their intended uses. The
problem with such PRNGs is that future values can be easily deduced by an
attacker (though they may appear random). Other algorithms for generating
random numbers quickly, such as quadratic generators and cubic generators,
have also been broken [Schneier 1996]. In short, you have to use
cryptographically strong PRNGs to generate random numbers in secure
applications - ordinary random number libraries are not sufficient.

Failing to correctly generate truly random values for keys has caused a
number of problems, including holes in Kerberos, the X window system, and NFS
[Venema 1996].

If possible, you should use system services (typically provided by the
operating system) that are expressly designed to create cryptographically
secure random values. For example, the Linux kernel (since 1.3.30) includes a
random number generator, which is sufficient for many security purposes. This
random number generator gathers environmental noise from device drivers and
other sources into an entropy pool. When accessed as /dev/random, random
bytes are only returned within the estimated number of bits of noise in the
entropy pool (when the entropy pool is empty, the call blocks until
additional environmental noise is gathered). When accessed as /dev/urandom,
as many bytes as are requested are returned even when the entropy pool is
exhausted. If you are using the random values for cryptographic purposes
(e.g., to generate a key) on Linux, use /dev/random. *BSD systems also
include /dev/random. Solaris users with the SUNWski package also have /dev/
random. Note that if a hardware random number generator is available and its
driver is installed, it will be used instead. More information is available
in the system documentation random(4).

On other systems, you'll need to find another way to get truly random
results. One possibility for other Unix-like systems is the Entropy Gathering
Daemon (EGD), which monitors system activity and hashes it into random
values; you can get it at [] http:// You might consider using a cryptographic hash
functions (e.g., SHA-1) on PRNG outputs. By using a hash algorithm, even if
the PRNG turns out to be guessable, this means that the attacker must now
also break the hash function.

If you have to implement a strong PRNG yourself, a good choice for a
cryptographically strong (and patent-unencumbered) PRNG is the Yarrow
algorithm; you can learn more about Yarrow from [
yarrow.html] Some other PRNGs can be
useful, but many widely-used ones have known weaknesses that may or may not
matter depending on your application. Before implementing a PRNG yourself,
consult the literature, such as [Kelsey 1998] and [McGraw 2000a]. You should
also examine [] IETF RFC 1750. NIST has
some useful information; see the [
800-22/sp-800-22-051501.pdf] NIST publication 800-22 and [http://] NIST errata. You
should know about the [] diehard tests
too. You might want to examine the paper titled "how Intel checked its PRNG",
but unfortunately that paper appears to be unavailable now.

11.4. Specially Protect Secrets (Passwords and Keys) in User Memory

If your application must handle passwords or non-public keys (such as session
keys, private keys, or secret keys), try to hide them and overwrite them
immediately after using them so they have minimal exposure.

Systems such as Linux support the mlock() and mlockall() calls to keep memory
from being paged to disk (since someone might acquire the kep later from the
swap file). Note that on Linux this is a privileged system call, which causes
its own issues (do I grant the program superuser privileges so it can call
mlock, if it doesn't need them otherwise?).

Also, if your program handles such secret values, be sure to disable creating
core dumps (via ulimit). Otherwise, an attacker may be able to halt the
program and find the secret value in the data dump.

Beware - normally processes can monitor other processes through the calls for
debuggers (e.g., via ptrace(2) and the /proc pseudo-filesystem) [Venema 1996]
Kernels usually protect against these monitoring routines if the process is
setuid or setgid (on the few ancient ones that don't, there really isn't a
good way to defend yourself other than upgrading). Thus, if your process
manages secret values, you probably should make it setgid or setuid (to a
different unprivileged group or user) to forceably inhibit this kind of
monitoring. Unless you need it to be setuid, use setgid (since this grants
fewer privileges).

Then there's the problem of being able to actually overwrite the value, which
often becomes language and compiler specific. In many languages, you need to
make sure that you store such information in mutable locations, and then
overwrite those locations. For example, in Java, don't use the type String to
store a password because Strings are immutable (they will not be overwritten
until garbage-collected and then reused, possibly a far time in the future).
Instead, in Java use char[] to store a password, so it can be immediately
overwritten. In Ada, use type String (an array of characters), and not type
Unbounded_String, to make sure that you have control over the contents.

In many languages (including C and C++), be careful that the compiler doesn't
optimize away the "dead code" for overwriting the value - since in this case
it's not dead code. Many compilers, including many C/C++ compilers, remove
writes to stores that are no longer used - this is often referred to as "dead
store removal." Unfortunately, if the write is really to overwrite the value
of a secret, this means that code that appears to be correct will be silently
discareded. Ada provides the pragma Inspection_Point; place this after the
code erasing the memory, and that way you can be certain that the object
containing the secret will really be erased (and that the the overwriting
won't be optimized away).

A Bugtraq post by Andy Polyakov (November 7, 2002) reported that the C/C++
compilers gcc version 3 or higher, SGI MIPSpro, and the Microsoft compilers
eliminated simple inlined calls to memset intended to overwrite secrets. This
is allowed by the C and C++ standards. Other C/C++ compilers (such as gcc
less than version 3) preserved the inlined call to memset at all optimization
levels, showing that the issue is compiler-specific. Simply declaring that
the destination data is volatile doesn't help on all compilers; both the
MIPSpro and Microsoft compilers ignored simple "volatilization". Simply
"touching" the first byte of the secret data doesn't help either; he found
that the MIPSpro and GCC>=3 cleverly nullify only the first byte and leave
the rest intact (which is actually quite clever - the problem is that the
compiler's cleverness is interfering with our goals). One approach that seems
to work on all platforms is to write your own implementation of memset with
internal "volatilization" of the first argument (this code is based on a
workaround proposed by Michael Howard):
 void *guaranteed_memset(void *v,int c,size_t n)                             
  { volatile char *p=v; while (n--) *p++=c; return v; }                      
Then place this definition into an external file to force the function to be
external (define the function in a corresponding .h file, and #include the
file in the callers, as is usual). This approach appears to be safe at any
optimization level (even if the function gets inlined).

11.5. Cryptographic Algorithms and Protocols

Often cryptographic algorithms and protocols are necessary to keep a system
secure, particularly when communicating through an untrusted network such as
the Internet. Where possible, use cryptographic techniques to authenticate
information and keep the information private (but don't assume that simple
encryption automatically authenticates as well). Generally you'll need to use
a suite of available tools to secure your application.

For background information and code, you should probably look at the classic
text ``Applied Cryptography'' [Schneier 1996]. The newsgroup ``sci.crypt''
has a series of FAQ's; you can find them at many locations, including [http:/
cryptography-faq. Linux-specific resources include the Linux Encryption HOWTO
at []
Encryption-HOWTO/. A discussion on how protocols use the basic algorithms can
be found in [Opplinger 1998]. A useful collection of papers on how to apply
cryptography in protocols can be found in [Stallings 1996]. What follows here
is just a few comments; these areas are rather specialized and covered more
thoroughly elsewhere.

Cryptographic protocols and algorithms are difficult to get right, so do not
create your own. Instead, where you can, use protocols and algorithms that
are widely-used, heavily analyzed, and accepted as secure. When you must
create anything, give the approach wide public review and make sure that
professional security analysts examine it for problems. In particular, do not
create your own encryption algorithms unless you are an expert in cryptology,
know what you're doing, and plan to spend years in professional review of the
algorithm. Creating encryption algorithms (that are any good) is a task for
experts only.

A number of algorithms are patented; even if the owners permit ``free use''
at the moment, without a signed contract they can always change their minds
later, putting you at extreme risk later. In general, avoid all patented
algorithms - in most cases there's an unpatented approach that is at least as
good or better technically, and by doing so you avoid a large number of legal

Another complication is that many counties regulate or restrict cryptography
in some way. A survey of legal issues is available at the ``Crypto Law
Survey'' site, []

Often, your software should provide a way to reject ``too small'' keys, and
let the user set what ``too small'' is. For RSA keys, 512 bits is too small
for use. There is increasing evidence that 1024 bits for RSA keys is not
enough either; Bernstein has suggested techniques that simplify brute-forcing
RSA, and other work based on it (such as Shamir and Tromer's "Factoring Large
Numbers with the TWIRL device") now suggests that 1024 bit keys can be broken
in a year by a $10 Million device. You may want to make 2048 bits the minimum
for RSA if you really want a secure system, and you should certainly do so if
you plan to use those keys after 2015. For more about RSA specifically, see 
RSA's commentary on Bernstein's work. For a more general discussion of key
length and other general cryptographic algorithm issues, see [http://] NIST's
key management workshop in November 2001.

11.5.1. Cryptographic Protocols

When you need a security protocol, try to use standard-conforming protocols
such as IPSec, SSL (soon to be TLS), SSH, S/MIME, OpenPGP/GnuPG/PGP, and
Kerberos. Each has advantages and disadvantages; many of them overlap
somewhat in functionality, but each tends to be used in different areas:

  * Internet Protocol Security (IPSec). IPSec provides encryption and/or
    authentication at the IP packet level. However, IPSec is often used in a
    way that only guarantees authenticity of two communicating hosts, not of
    the users. As a practical matter, IPSec usually requires low-level
    support from the operating system (which not all implement) and an
    additional keyring server that must be configured. Since IPSec can be
    used as a "tunnel" to secure packets belonging to multiple users and
    multiple hosts, it is especially useful for building a Virtual Private
    Network (VPN) and connecting a remote machine. As of this time, it is
    much less often used to secure communication from individual clients to
    servers. The new version of the Internet Protocol, IPv6, comes with IPSec
    ``built in,'' but IPSec also works with the more common IPv4 protocol.
    Note that if you use IPSec, don't use the encryption mode without the
    authentication, because the authentication also acts as integrity
  * Secure Socket Layer (SSL) / TLS. SSL/TLS works over TCP and tunnels other
    protocols using TCP, adding encryption, authentication of the server, and
    optional authentication of the client (but authenticating clients using
    SSL/TLS requires that clients have configured X.509 client certificates,
    something rarely done). SSL version 3 is widely used; TLS is a later
    adjustment to SSL that strengthens its security and improves its
    flexibility. Currently there is a slow transition going on from SSLv3 to
    TLS, aided because implementations can easily try to use TLS and then
    back off to SSLv3 without user intervention. Unfortunately, a few bad
    SSLv3 implementations cause problems with the backoff, so you may need a
    preferences setting to allow users to skip using TLS if necessary. Don't
    use SSL version 2, it has some serious security weaknesses.
    SSL/TLS is the primary method for protecting http (web) transactions. Any
    time you use an "https://" URL, you're using SSL/TLS. Other protocols
    that often use SSL/TLS include POP3 and IMAP. SSL/TLS usually use a
    separate TCP/IP port number from the unsecured port, which the IETF is a
    little unhappy about (because it consumes twice as many ports; there are
    solutions to this). SSL is relatively easy to use in programs, because
    most library implementations allow programmers to use operations similar
    to the operations on standard sockets like SSL_connect(), SSL_write(),
    SSL_read(), etc. A widely used OSS/FS implementation of SSL (as well as
    other capabilities) is OpenSSL, available at []
  * OpenPGP and S/MIME. There are two competing, essentially incompatible
    standards for securing email: OpenPGP and S/MIME. OpenPHP is based on the
    PGP application; an OSS/FS implementation is GNU Privacy Guard from
    [] Currently, their
    certificates are often not interchangeable; work is ongoing to repair
  * SSH. SSH is the primary method of securing ``remote terminals'' over an
    internet, and it also includes methods for tunelling X Windows sessions.
    However, it's been extended to support single sign-on and general secure
    tunelling for TCP streams, so it's often used for securing other data
    streams too (such as CVS accesses). The most popular implementation of
    SSH is OpenSSH [], which is
    OSS/FS. Typical uses of SSH allows the client to authenticate that the
    server is truly the server, and then the user enters a password to
    authenticate the user (the password is encrypted and sent to the other
    system for verification). Current versions of SSH can store private keys,
    allowing users to not enter the password each time. To prevent
    man-in-the-middle attacks, SSH records keying information about servers
    it talks to; that means that typical use of SSH is vulnerable to a
    man-in-the-middle attack during the very first connection, but it can
    detect problems afterwards. In contrast, SSL generally uses a certificate
    authority, which eliminates the first connection problem but requires
    special setup (and payment!) to the certificate authority.
  * Kerberos. Kerberos is a protocol for single sign-on and authenticating
    users against a central authentication and key distribution server.
    Kerberos works by giving authenticated users "tickets", granting them
    access to various services on the network. When clients then contact
    servers, the servers can verify the tickets. Kerberos is a primary method
    for securing and supporting authentication on a LAN, and for establishing
    shared secrets (thus, it needs to be used with other algorithms for the
    actual protection of communication). Note that to use Kerberos, both the
    client and server have to include code to use it, and since not everyone
    has a Kerberos setup, this has to be optional - complicating the use of
    Kerberos in some programs. However, Kerberos is widely used.

Many of these protocols allow you to select a number of different algorithms,
so you'll still need to pick reasonable defaults for algorithms (e.g., for

11.5.2. Symmetric Key Encryption Algorithms

The use, export, and/or import of implementations of encryption algorithms
are restricted in many countries, and the laws can change quite rapidly. Find
out what the rules are before trying to build applications using

For secret key (bulk data) encryption algorithms, use only encryption
algorithms that have been openly published and withstood years of attack, and
check on their patent status. I would recommend using the new Advanced
Encryption Standard (AES), also known as Rijndahl -- a number of
cryptographers have analyzed it and not found any serious weakness in it, and
I believe it has been through enough analysis to be trustworthy now. However,
in August 2002 researchers Fuller and Millar discovered a mathematical
property of the cipher that, while not an attack, might be exploitable into
an attack (the approach may actually has serious consequences for some other
algorithms, too). Thus, it's worth staying tuned to future work. A good
alternative to AES is the Serpent algorithm, which is slightly slower but is
very resistant to attack. For many applications triple-DES is a very good
encryption algorithm; it has a reasonably lengthy key (112 bits), no patent
issues, and a very long history of withstanding attacks (it's withstood
attacks far longer than any other encryption algorithm with reasonable key
length in the public literature, so it's probably the safest
publicly-available symmetric encryption algorithm when properly implemented).
However, triple-DES is very slow when implemented in software, so triple-DES
can be considered ``safest but slowest.'' Twofish appears to be a good
encryption algorithm, but there are some lingering questions - Sean Murphy
and Fauzan Mirza showed that Twofish has properties that cause many academics
to be concerned (though as of yet no one has managed to exploit these
properties). MARS is highly resistent to ``new and novel'' attacks, but it's
more complex and is impractical on small-ability smartcards. For the moment I
would avoid Twofish - it's quite likely that this will never be exploitable,
but it's hard to be sure and there are alternative algorithms which don't
have these concerns. Don't use IDEA - it's subject to U.S. and European
patents. Don't use stupid algorithms such as XOR with a constant or constant
string, the ROT (rotation) scheme, a Vinegere ciphers, and so on - these can
be trivially broken with today's computers. Don't use ``double DES'' (using
DES twice) - that's subject to a ``man in the middle'' attack that triple-DES
avoids. Your protocol should support multiple encryption algorithms, anyway;
that way, when an encryption algorithm is broken, users can switch to another

For symmetric-key encryption (e.g., for bulk encryption), don't use a key
length less than 90 bits if you want the information to stay secret through
2016 (add another bit for every additional 18 months of security) [Blaze
1996]. For encrypting worthless data, the old DES algorithm has some value,
but with modern hardware it's too easy to break DES's 56-bit key using brute
force. If you're using DES, don't just use the ASCII text key as the key -
parity is in the least (not most) significant bit, so most DES algorithms
will encrypt using a key value well-known to adversaries; instead, create a
hash of the key and set the parity bits correctly (and pay attention to error
reports from your encryption routine). So-called ``exportable'' encryption
algorithms only have effective key lengths of 40 bits, and are essentially
worthless; in 1996 an attacker could spend $10,000 to break such keys in
twelve minutes or use idle computer time to break them in a few days, with
the time-to-break halving every 18 months in either case.

Block encryption algorithms can be used in a number of different modes, such
as ``electronic code book'' (ECB) and ``cipher block chaining'' (CBC). In
nearly all cases, use CBC, and do not use ECB mode - in ECB mode, the same
block of data always returns the same result inside a stream, and this is
often enough to reveal what's encrypted. Many modes, including CBC mode,
require an ``initialization vector'' (IV). The IV doesn't need to be secret,
but it does need to be unpredictable by an attacker. Don't reuse IV's across
sessions - use a new IV each time you start a session.

There are a number of different streaming encryption algorithms, but many of
them have patent restrictions. I know of no patent or technical issues with
WAKE. RC4 was a trade secret of RSA Data Security Inc; it's been leaked
since, and I know of no real legal impediment to its use, but RSA Data
Security has often threatened court action against users of it (it's not at
all clear what RSA Data Security could do, but no doubt they could tie up
users in worthless court cases). If you use RC4, use it as intended - in
particular, always discard the first 256 bytes it generates, or you'll be
vulnerable to attack. SEAL is patented by IBM - so don't use it. SOBER is
patented; the patent owner has claimed that it will allow many uses for free
if permission is requested, but this creates an impediment for later use.
Even more interestingly, block encryption algorithms can be used in modes
that turn them into stream ciphers, and users who want stream ciphers should
consider this approach (you'll be able to choose between far more
publicly-available algorithms).

11.5.3. Public Key Algorithms

For public key cryptography (used, among other things, for signing and
sending secret keys), there are only a few widely-deployed algorithms. One of
the most widely-used algorithms is RSA; RSA's algorithm was patented, but
only in the U.S., and that patent expired in September 2000, so RSA can be
freely used. Never decrypt or sign a raw value that an attacker gives you
directly using RSA and expose the result, because that could expose the
private key (this isn't a problem in practice, because most protocols involve
signing a hash computed by the user - not the raw value - or don't expose the
result). Never decrypt or sign the exact same raw value multiple times (the
original can be exposed). Both of these can be solved by always adding random
padding (PGP does this) - the usual approach is called Optimal Asymmetric
Encryption Padding (OAEP).

The Diffie-Hellman key exchange algorithm is widely used to permit two
parties to agree on a session key. By itself it doesn't guarantee that the
parties are who they say they are, or that there is no middleman, but it does
strongly help defend against passive listeners; its patent expired in 1997.
If you use Diffie-Hellman to create a shared secret, be sure to hash it first
(there's an attack if you use its shared value directly).

NIST developed the digital signature standard (DSS) (it's a modification of
the ElGamal cryptosystem) for digital signature generation and verification;
one of the conditions for its development was for it to be patent-free.

RSA, Diffie-Hellman, and El Gamal's techniques require more bits for the keys
for equivalent security compared to typical symmetric keys; a 1024-bit key in
these systems is supposed to be roughly equivalent to an 80-bit symmetric
key. A 512-bit RSA key is considered completely unsafe; Nicko van Someren has
demonstrated that such small RSA keys can be factored in 6 weeks using only
already-available office hardware (never mind equipment designed for the
job). In the past, a 1024-bit RSA key was considered reasonably secure, but
recent advancements in factorization algorithms (e.g., by D. J. Bernstein)
have raised concerns that perhaps even 1024 bits is not enough for an RSA
key. Certainly, if your application needs to be highly secure or last beyond
2015, you should use a 2048 bit keys.

If you need a public key that requires far fewer bits (e.g., for a
smartcard), then you might use elliptic curve cryptography (IEEE P1363 has
some suggested curves; finding curves is hard). However, be careful -
elliptic curve cryptography isn't patented, but certain speedup techniques
are patented. Elliptic curve cryptography is fast enough that it really
doesn't need these speedups anyway for its usual use of encrypting session /
bulk encryption keys. In general, you shouldn't try to do bulk encryption
with elliptic keys; symmetric algorithms are much faster and are
better-tested for the job.

11.5.4. Cryptographic Hash Algorithms

Some programs need a one-way cryptographic hash algorithm, that is, a
function that takes an ``arbitrary'' amount of data and generates a
fixed-length number that hard for an attacker to invert (e.g., it's difficult
for an attacker to create a different set of data to generate that same
value). For a number of years MD5 has been a favorite, but recent efforts
have shown that its 128-bit length may not be enough [van Oorschot 1994] and
that certain attacks weaken MD5's protection [Dobbertin 1996]. Indeed, there
are rumors that a top industry cryptographer has broken MD5, but is bound by
employee agreement to keep silent (see the Bugtraq 22 August 2000 posting by
John Viega). Anyone can create a rumor, but enough weaknesses have been found
that the idea of completing the break is plausible. If you're writing new
code, use SHA-1 instead of MD5. Don't use the original SHA (now called
``SHA-0''); SHA-0 had the same weakness that MD5 does. If you need more bits
in your hash algorithm, use SHA-256, SHA-384, or SHA-512; you can get the
specifications in NIST FIPS PUB 180-2.

11.5.5. Integrity Checking

When communicating, you need some sort of integrity check (don't depend just
on encryption, since an attacker can then induce changes of information to
``random'' values). This can be done with hash algorithms, but don't just use
a hash function directly (this exposes users to an ``extension'' attack - the
attacker can use the hash value, add data of their choosing, and compute the
new hash). The usual approach is ``HMAC'', which computes the integrity check
  H(k xor opad, H(k xor ipad, data)).                                        
where H is the hash function (typically MD5 or SHA-1) and k is the key. Thus,
integrity checks are often HMAC-MD5 or HMAC-SHA-1. Note that although MD5 has
some weaknesses, as far as I know MD5 isn't vulnerable when used in this
construct, so HMAC-MD5 is (to my knowledge) okay. This is defined in detail
in IETF RFC 2104.

Note that in the HMAC approach, a receiver can forge the same data as a
sender. This isn't usually a problem, but if this must be avoided, then use
public key methods and have the sender ``sign'' the data with the sender
private key - this avoids this forging attack, but it's more expensive and
for most environments isn't necessary.

11.5.6. Randomized Message Authentication Mode (RMAC)

NIST has developed and proposed a new mode for using cryptographic algorithms
called [] Randomized Message
Authentication Code (RMAC). RMAC is intended for use as a message
authentication code technique.

Although there's a formal proof showing that RMAC is secure, the proof
depends on the highly questionable assumption that the underlying
cryptographic algorithm meets the "ideal cipher model" - in particular, that
the algorithm is secure against a variety of specialized attacks, including
related-key attacks. Unfortunately, related-key attacks are poorly studied
for many algorithms; this is not the kind of property or attack that most
people worry about when analyzing with cryptographic algorithms. It's known
triple-DES doesn't have this properly, and it's unclear if other
widely-accepted algorithms like AES have this property (it appears that AES
is at least weaker against related key attacks than usual attacks).

The best advice right now is "don't use RMAC". There are other ways to do
message authentication, such as HMAC combined with a cryptographic hash
algorithm (e.g., HMAC-SHA1). HMAC isn't the same thing (e.g., technically it
doesn't include a nonce, so you should rekey sooner), but the theoretical
weaknesses of HMAC are merely theoretical, while the problems in RMAC seem
far more important in the real world.

11.5.7. Other Cryptographic Issues

You should both encrypt and include integrity checks of data that's
important. Don't depend on the encryption also providing integrity - an
attacker may be able to change the bits into a different value, and although
the attacker may not be able to change it to a specific value, merely
changing the value may be enough. In general, you should use different keys
for integrity and secrecy, to avoid certain subtle attacks.

One issue not discussed often enough is the problem of ``traffic analysis.''
That is, even if messages are encrypted and the encryption is not broken, an
adversary may learn a great deal just from the encrypted messages. For
example, if the presidents of two companies start exchanging many encrypted
email messages, it may suggest that the two comparies are considering a
merger. For another example, many SSH implementations have been found to have
a weakness in exchanging passwords: observers could look at packets and
determine the length (or length range) of the password, even if they couldn't
determine the password itself. They could also also determine other
information about the password that significantly aided in breaking it.

Be sure to not make it possible to solve a problem in parts, and use
different keys when the trust environment (who is trusted) changes. Don't use
the same key for too long - after a while, change the session key or password
so an adversary will have to start over.

Generally you should compress something you'll encrypt - this does add a
fixed header, which isn't so good, but it eliminates many patterns in the
rest of the message as well as making the result smaller, so it's usually
viewed as a ``win'' if compression is likely to make the result smaller.

In a related note, if you must create your own communication protocol,
examine the problems of what's gone on before. Classics such as Bellovin
[1989]'s review of security problems in the TCP/IP protocol suite might help
you, as well as Bruce Schneier [1998] and Mudge's breaking of Microsoft's
PPTP implementation and their follow-on work. Again, be sure to give any new
protocol widespread public review, and reuse what you can.

11.6. Using PAM

Pluggable Authentication Modules (PAM) is a flexible mechanism for
authenticating users. Many Unix-like systems support PAM, including Solaris,
nearly all Linux distributions (e.g., Red Hat Linux, Caldera, and Debian as
of version 2.2), and FreeBSD as of version 3.1. By using PAM, your program
can be independent of the authentication scheme (passwords, SmartCards,
etc.). Basically, your program calls PAM, which at run-time determines which
``authentication modules'' are required by checking the configuration set by
the local system administrator. If you're writing a program that requires
authentication (e.g., entering a password), you should include support for
PAM. You can find out more about the Linux-PAM project at [http://]

11.7. Tools

Some tools may help you detect security problems before you field the result.
They can't find all such problems, of course, but they can help catch
problems that would overwise slip by. Here are a few tools, emphasizing open
source / free software tools.

One obvious type of tool is a program to examine the source code to search
for patterns of known potential security problems (e.g., calls to library
functions in ways are often the source of security vulnerabilities). These
kinds of programs are called ``source code scanners''. Here are a few such

  * Flawfinder, which I've developed; it's available at [http://] This is
    also a program that scans C/C++ source code for common problems, and is
    also licensed under the GPL. Unlike RATS, flawfinder is implemented in
    Python. The developers of RATS and Flawfinder have agreed to find a way
    to work together to create a single ``best of breed'' open source
  * RATS (Rough Auditing Tool for Security) from Secure Software Solutions is
    available at []
    This program scans C/C++ source code for common problems, and is licensed
    under the GPL.
  * ITS4 from Cigital (formerly Reliable Software Technologies, RST) also
    statically checks C/C++ code. It is available free for non-commercial
    use, including its source code and with certain modification and
    redistribution rights. Note that this isn't released as ``open source''
    as defined by the Open Source Definition (OSD) - In particular, OSD point
    6 forbids ``non-commercial use only'' clauses in open source licenses.
    ITS4 is available at []
  * Splint (formerly named LCLint) is a tool for statically checking C
    programs. With minimal effort, splint can be used as a better lint. If
    additional effort is invested adding annotations to programs, splint can
    perform stronger checking than can be done by any standard lint. For
    example, it can be used to statically detect likely buffer overflows. The
    software is licensed under the GPL and is available at [http://]
  * cqual is a type-based analysis tool for finding bugs in C programs. cqual
    extends the type system of C with extra user-defined type qualifiers,
    e.g., it can note that values are ``tainted'' or ``untainted'' (similar
    to Perl's taint checking). The programmer annotates their program in a
    few places, and cqual performs qualifier inference to check whether the
    annotations are correct. cqual presents the analysis results using
    Program Analysis Mode, an emacs-based interface. The current version of
    cqual can detect potential format-string vulnerabilities in C programs. A
    previous incarnation of cqual, Carillon, has been used to find Y2K bugs
    in C programs. The software is licensed under the GPL and is available
    from [] http://
  * Cyclone is a C-like language intended to remove C's security weaknesses.
    In theory, you can always switch to a language that is ``more secure,''
    but this doesn't always help (a language can help you avoid common
    mistakes but it can't read your mind). John Viega has reviewed Cyclone,
    and in December 2001 he said: ``Cyclone is definitely a neat language.
    It's a C dialect that doesn't feel like it's taking away any power, yet
    adds strong safety guarantees, along with numerous features that can be a
    real boon to programmers. Unfortunately, Cyclone isn't yet ready for
    prime time. Even with crippling limitations aside, it doesn't yet offer
    enough advantages over Java (or even C with a good set of tools) to make
    it worth the risk of using what is still a very young technology. Perhaps
    in a few years, Cyclone will mature into a robust, widely supported
    language that comes dangerously close to C in terms of efficiency. If
    that day comes, you'll certainly see me abandoning C for good.'' The
    Cyclone compiler has been released under the GPL and LGPL. You can get
    more information from the []
    Cyclone web site.

Some tools try to detect potential security flaws at run-time, either to
counter them or at least to warn the developer about them. Much of Crispen
Cowan's work, such as StackGuard, fits here.

There are several tools that try to detect various C/C++ memory-management
problems; these are really general-purpose software quality improvement
tools, and not specific to security, but memory management problems can
definitely cause security problems. An especially capable tool is [http://] Valgrind, which detects various memory-management
problems (such as use of uninitialized memory, reading/writing memory after
it's been free'd, reading/writing off the end of malloc'ed blocks, and memory
leaks). Another such tool is Electric Fence (efence) by Bruce Perens, which
can detect certain memory management errors. [
sourcecode.html] Memwatch (public domain) and [
~neldredge/yamd/] YAMD (GPL) can detect memory allocation problems for C and
C++. You can even use the built-in capabilities of the GNU C library's malloc
library, which has the MALLOC_CHECK_ environment variable (see its manual
page for more information). There are many others.

Another approach is to create test patterns and run the program, in attempt
to find weaknesses in the program. Here are a few such tools:

  * BFBTester, the Brute Force Binary Tester, is licensed under the GPL. This
    program does quick security checks of binary programs. BFBTester performs
    checks of single and multiple argument command line overflows and
    environment variable overflows. Version 2.0 and higher can also watch for
    tempfile creation activity (to check for using unsafe tempfile names). At
    one time BFBTester didn't run on Linux (due to a technical issue in
    Linux's POSIX threads implementation), but this has been fixed as of
    version 2.0.1. More information is available at [http://]
  * The [] fuzz program is a tool for testing
    other software. It tests programs by bombarding the program being
    evaluated with random data. This tool isn't really specific to security.
  * [] SPIKE is a "fuzzer creation kit",
    i.e., it's a toolkit designed to create "random" tests to find security
    problems. The SPIKE toolkit is particularly designed for protocol
    analysis by simulating network protocol clients, and SPIKE proXy is a
    tool built on SPIKE to test web applications. SPIKE includes a few
    pre-canned tests. SPIKE is licensed under the GPL.

There are a number tools that try to give you insight into running programs
that can also be useful when trying to find security problems in your code.
This includes symbolic debuggers (such as gdb) and trace programs (such as
strace and ltrace). One interesting program to support analysis of running
code is [] Fenris (GPL license). Its
documentation describes Fenris as a ``multipurpose tracer, stateful analyzer
and partial decompiler intended to simplify bug tracking, security audits,
code, algorithm or protocol analysis - providing a structural program trace,
general information about internal constructions, execution path, memory
operations, I/O, conditional expressions and much more.'' Fenris actually
supplies a whole suite of tools, including extensive forensics capabilities
and a nice debugging GUI for Linux. A list of other promising open source
tools that can be suitable for debugging or code analysis is available at
[] http:// Another interesting program
along these lines is Subterfugue, which allows you to control what happens in
every system call made by a program.

If you're building a common kind of product where many standard potential
flaws exist (like an ftp server or firewall), you might find standard
security scanning tools useful. One good one is []
Nessus; there are many others. These kinds of tools are very useful for doing
regression testing, but since they essentially use a list of past specific
vulnerabilities and common configuration errors, they may not be very helpful
in finding problems in new programs.

Often, you'll need to call on other tools to implement your secure
infrastructure. The [] Open-Source PKI Book
describes a number of open source programs for implmenting a public key
infrastructure (PKI).

Of course, running a ``secure'' program on an insecure platform configuration
makes little sense. You may want to examine hardening systems, which attempt
to configure or modify systems to be more resistant to attacks. For Linux,
one hardening system is Bastille Linux, available at [http://]

11.8. Windows CE

If you're securing a Windows CE Device, you should read Maricia Alforque's
"Creating a Secure Windows CE Device" at [

11.9. Write Audit Records

Write audit logs for program startup, session startup, and for suspicious
activity. Possible information of value includes date, time, uid, euid, gid,
egid, terminal information, process id, and command line values. You may find
the function syslog(3) helpful for implementing audit logs. One awkward
problem is that any logging system should be able to record a lot of
information (since this information could be very helpful), yet if the
information isn't handled carefully the information itself could be used to
create an attack. After all, the attacker controls some of the input being
sent to the program. When recording data sent by a possible attacker,
identify a list of ``expected'' characters and escape any ``unexpected''
characters so that the log isn't corrupted. Not doing this can be a real
problem; users may include characters such as control characters (especially
NIL or end-of-line) that can cause real problems. For example, if an attacker
embeds a newline, they can then forge log entries by following the newline
with the desired log entry. Sadly, there doesn't seem to be a standard
convention for escaping these characters. I'm partial to the URL escaping
mechanism (%hh where hh is the hexadecimal value of the escaped byte) but
there are others including the C convention (\ooo for the octal value and \X
where X is a special symbol, e.g., \n for newline). There's also the
caret-system (^I is control-I), though that doesn't handle byte values over
127 gracefully.

There is the danger that a user could create a denial-of-service attack (or
at least stop auditing) by performing a very large number of events that cut
an audit record until the system runs out of resources to store the records.
One approach to counter to this threat is to rate-limit audit record
recording; intentionally slow down the response rate if ``too many'' audit
records are being cut. You could try to slow the response rate only to the
suspected attacker, but in many situations a single attacker can masquerade
as potentially many users.

Selecting what is ``suspicious activity'' is, of course, dependent on what
the program does and its anticipated use. Any input that fails the filtering
checks discussed earlier is certainly a candidate (e.g., containing NIL).
Inputs that could not result from normal use should probably be logged, e.g.,
a CGI program where certain required fields are missing in suspicious ways.
Any input with phrases like /etc/passwd or /etc/shadow or the like is very
suspicious in many cases. Similarly, trying to access Windows ``registry''
files or .pwl files is very suspicious.

Do not record passwords in an audit record. Often people accidentally enter
passwords for a different system, so recording a password may allow a system
administrator to break into a different computer outside the administrator's

11.10. Physical Emissions

Although it's really outside the scope of this book, it's important to
remember that computing and communications equipment leaks a lot information
that makes them hard to really secure. Many people are aware of TEMPEST
requirements which deal with radio frequency emissions of computers,
displays, keyboards, and other components which can be eavesdropped. The
light from displays can also be eavesdropped, even if it's bounced off an
office wall at great distance [Kuhn 2002]. Modem lights are also enough to
determine the underlying communication.

11.11. Miscellaneous

The following are miscellaneous security guidelines that I couldn't seem to
fit anywhere else:

Have your program check at least some of its assumptions before it uses them
(e.g., at the beginning of the program). For example, if you depend on the
``sticky'' bit being set on a given directory, test it; such tests take
little time and could prevent a serious problem. If you worry about the
execution time of some tests on each call, at least perform the test at
installation time, or even better at least perform the test on application

If you have a built-in scripting language, it may be possible for the
language to set an environment variable which adversely affects the program
invoking the script. Defend against this.

If you need a complex configuration language, make sure the language has a
comment character and include a number of commented-out secure examples.
Often '#' is used for commenting, meaning ``the rest of this line is a

If possible, don't create setuid or setgid root programs; make the user log
in as root instead.

Sign your code. That way, others can check to see if what's available was
what was sent.

In some applications you may need to worry about timing attacks, where the
variation in timing or CPU utilitization is enough to give away important
information. This kind of attack has been used to obtain keying information
from Smartcards, for example. Mauro Lacy has published a paper titled [http:/
/] Remote Timing Techniques, showing that you
can (in some cases) determine over an Internet whether or not a given user id
exists, simply from the effort expended by the CPU (which can be detected
remotely using techniques described in the paper). The only way to deal with
these sorts of problems is to make sure that the same effort is performed
even when it isn't necessary. The problem is that in some cases this may make
the system more vulnerable to a denial of service attack, since it can't
optimize away unnecessary work.

Consider statically linking secure programs. This counters attacks on the
dynamic link library mechanism by making sure that the secure programs don't
use it. There are several downsides to this however. This is likely to
increase disk and memory use (from multiple copies of the same routines).
Even worse, it makes updating of libraries (e.g., for security
vulnerabilities) more difficult - in most systems they won't be automatically
updated and have to be tracked and implemented separately.

When reading over code, consider all the cases where a match is not made. For
example, if there is a switch statement, what happens when none of the cases
match? If there is an ``if'' statement, what happens when the condition is

Merely ``removing'' a file doesn't eliminate the file's data from a disk; on
most systems this simply marks the content as ``deleted'' and makes it
eligible for later reuse, and often data is at least temporarily stored in
other places (such as memory, swap files, and temporary files). Indeed,
against a determined attacker, writing over the data isn't enough. A classic
paper on the problems of erasing magnetic media is Peter Gutmann's paper
[] ``Secure
Deletion of Data from Magnetic and Solid-State Memory''. A determined
adversary can use other means, too, such as monitoring electromagnetic
emissions from computers (military systems have to obey TEMPEST rules to
overcome this) and/or surreptitious attacks (such as monitors hidden in

When fixing a security vulnerability, consider adding a ``warning'' to detect
and log an attempt to exploit the (now fixed) vulnerability. This will reduce
the likelihood of an attack, especially if there's no way for an attacker to
predetermine if the attack will work, since it exposes an attack in progress.
In short, it turns a vulnerability into an intrusion detection system. This
also suggests that exposing the version of a server program before
authentication is usually a bad idea for security, since doing so makes it
easy for an attacker to only use attacks that would work. Some programs make
it possible for users to intentionally ``lie'' about their version, so that
attackers will use the ``wrong attacks'' and be detected. Also, if the
vulnerability can be triggered over a network, please make sure that security
scanners can detect the vulnerability. I suggest contacting Nessus ([http://] and make sure that their open source
security scanner can detect the problem. That way, users who don't check
their software for upgrades will at least learn about the problem during
their security vulnerability scans (if they do them as they should).

Always include in your documentation contact information for where to report
security problems. You should also support at least one of the common email
addresses for reporting security problems (security-alert@SITE, secure@SITE,
or security@SITE); it's often good to have support@SITE and info@SITE working
as well. Be prepared to support industry practices by those who have a
security flaw to report, such as the [
policy.html] Full Disclosure Policy (RFPolicy) and the IETF Internet draft,
``Responsible Vulnerability Disclosure Process''. It's important to quickly
work with anyone who is reporting a security flaw; remember that they are
doing you a favor by reporting the problem to you, and that they are under no
obligation to do so. It's especially important, once the problem is fixed, to
give proper credit to the reporter of the flaw (unless they ask otherwise).
Many reporters provide the information solely to gain the credit, and it's
generally accepted that credit is owed to the reporter. Some vendors argue
that people should never report vulnerabilities to the public; the problem
with this argument is that this was once common, and the result was vendors
who denied vulnerabilities while their customers were getting constantly
subverted for years at a time.

Follow best practices and common conventions when leading a software
development project. If you are leading an open source software / free
software project, some useful guidelines can be found in [
/HOWTO/Software-Proj-Mgmt-HOWTO/index.html] Free Software Project Management
HOWTO and [
index.html] Software Release Practice HOWTO; you should also read [http://] The Cathedral and the Bazaar.

Every once in a while, review security guidelines like this one. At least
re-read the conclusions in Chapter 12, and feel free to go back to the
introduction (Chapter 1) and start again!

Chapter 12. Conclusion

                                       The end of a matter is better than its
                                       beginning, and patience is better than
                                                       Ecclesiastes 7:8 (NIV)

Designing and implementing a truly secure program is actually a difficult
task on Unix-like systems such as Linux and Unix. The difficulty is that a
truly secure program must respond appropriately to all possible inputs and
environments controlled by a potentially hostile user. Developers of secure
programs must deeply understand their platform, seek and use guidelines (such
as these), and then use assurance processes (such as inspections and other
peer review techniques) to reduce their programs' vulnerabilities.

In conclusion, here are some of the key guidelines in this book:

  * Validate all your inputs, including command line inputs, environment
    variables, CGI inputs, and so on. Don't just reject ``bad'' input; define
    what is an ``acceptable'' input and reject anything that doesn't match.
  * Avoid buffer overflow. Make sure that long inputs (and long intermediate
    data values) can't be used to take over your program. This is the primary
    programmatic error at this time.
  * Structure program internals. Secure the interface, minimize privileges,
    make the initial configuration and defaults safe, and fail safe. Avoid
    race conditions (e.g., by safely opening any files in a shared directory
    like /tmp). Trust only trustworthy channels (e.g., most servers must not
    trust their clients for security checks or other sensitive data such as
    an item's price in a purchase).
  * Carefully call out to other resources. Limit their values to valid values
    (in particular be concerned about metacharacters), and check all system
    call return values.
  * Reply information judiciously. In particular, minimize feedback, and
    handle full or unresponsive output to an untrusted user.


Chapter 13. Bibliography

                                       The words of the wise are like goads, 
                                       their collected sayings like firmly   
                                       embedded nails--given by one Shepherd.
                                       Be warned, my son, of anything in     
                                       addition to them. Of making many books
                                       there is no end, and much study       
                                       wearies the body.                     
                                                  Ecclesiastes 12:11-12 (NIV)

Note that there is a heavy emphasis on technical articles available on the
web, since this is where most of this kind of technical information is

[Advosys 2000] Advosys Consulting (formerly named Webber Technical Services).
Writing Secure Web Applications. []

[Al-Herbish 1999] Al-Herbish, Thamer. 1999. Secure Unix Programming FAQ.

[Aleph1 1996] Aleph1. November 8, 1996. ``Smashing The Stack For Fun And
Profit''. Phrack Magazine. Issue 49, Article 14. [
article=p49-14 or alternatively []

[Anonymous 1999] Anonymous. October 1999. Maximum Linux Security: A Hacker's
Guide to Protecting Your Linux Server and Workstation Sams. ISBN: 0672316706.

[Anonymous 1998] Anonymous. September 1998. Maximum Security : A Hacker's
Guide to Protecting Your Internet Site and Network. Sams. Second Edition.
ISBN: 0672313413.

[Anonymous Phrack 2001] Anonymous. August 11, 2001. Once upon a free().
Phrack, Volume 0x0b, Issue 0x39, Phile #0x09 of 0x12. [

[AUSCERT 1996] Australian Computer Emergency Response Team (AUSCERT) and
O'Reilly. May 23, 1996 (rev 3C). A Lab Engineers Check List for Writing
Secure Unix Code. [

[Bach 1986] Bach, Maurice J. 1986. The Design of the Unix Operating System.
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[Beattie 2002] Beattie, Steve, Seth Arnold, Crispin Cowan, Perry Wagle, Chris
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[Bellovin 1989] Bellovin, Steven M. April 1989. "Security Problems in the TCP
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[Bellovin 1994] Bellovin, Steven M. December 1994. Shifting the Odds --
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[Bishop 1996] Bishop, Matt. May 1996. ``UNIX Security: Security in
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[Bishop 1997] Bishop, Matt. October 1997. ``Writing Safe Privileged
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/mab/keylength.txt and []

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  * ISO 13335-2: Managing and Planning IT Security
  * ISO 13335-3: Techniques for the Management of IT Security
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Appendix A. History

Here are a few key events in the development of this book, starting from most
recent events:

2002-10-29 David A. Wheeler
    Version 3.000 released, adding a new section on determining security
    requirements and a discussion of the Common Criteria, broadening the
    document. Many smaller improvements were incorporated as well.
2001-01-01 David A. Wheeler
    Version 2.70 released, adding a significant amount of additional
    material, such as a significant expansion of the discussion of cross-site
    malicious content, HTML/URI filtering, and handling temporary files.
2000-05-24 David A. Wheeler
    Switched to GNU's GFDL license, added more content.
2000-04-21 David A. Wheeler
    Version 2.00 released, dated 21 April 2000, which switched the document's
    internal format from the Linuxdoc DTD to the DocBook DTD. Thanks to Jorge
    Godoy for helping me perform the transition.
2000-04-04 David A. Wheeler
    Version 1.60 released; changed so that it now covers both Linux and Unix.
    Since most of the guidelines covered both, and many/most app developers
    want their apps to run on both, it made sense to cover both.
2000-02-09 David A. Wheeler
    Noted that the document is now part of the Linux Documentation Project
1999-11-29 David A. Wheeler
    Initial version (1.0) completed and released to the public.

Note that a more detailed description of changes is available on-line in the
``ChangeLog'' file.

Appendix B. Acknowledgements

                                       As iron sharpens iron, so one man     
                                       sharpens another.                     
                                                         Proverbs 27:17 (NIV)

My thanks to the following people who kept me honest by sending me emails
noting errors, suggesting areas to cover, asking questions, and so on. Where
email addresses are included, they've been shrouded by prepending my
``thanks.'' so bulk emailers won't easily get these addresses; inclusion of
people in this list is not an authorization to send unsolicited bulk email to

  * Neil Brown (
  * Martin Douda (
  * Jorge Godoy
  * Scott Ingram (
  * Michael Kerrisk
  * Doug Kilpatrick
  * John Levon (
  * Ryan McCabe (
  * Paul Millar (
  * Chuck Phillips (
  * Martin Pool (
  * Eric S. Raymond (
  * Marc Welz
  * Eric Werme (


If you want to be on this list, please send me a constructive suggestion at
[] If you send me a
constructive suggestion, but do not want credit, please let me know that when
you send your suggestion, comment, or criticism; normally I expect that
people want credit, and I want to give them that credit. My current process
is to add contributor names to this list in the document, with more detailed
explanation of their comment in the ChangeLog for this document (available
on-line). Note that although these people have sent in ideas, the actual text
is my own, so don't blame them for any errors that may remain. Instead,
please send me another constructive suggestion.

Appendix C. About the Documentation License

                                       A copy of the text of the edict was to
                                       be issued as law in every province and
                                       made known to the people of every     
                                       nationality so they would be ready for
                                       that day.                             
                                                            Esther 3:14 (NIV)

This document is Copyright (C) 1999-2000 David A. Wheeler. Permission is
granted to copy, distribute and/or modify this document under the terms of
the GNU Free Documentation License (FDL), Version 1.1 or any later version
published by the Free Software Foundation; with the invariant sections being
``About the Author'', with no Front-Cover Texts, and no Back-Cover texts. A
copy of the license is included below in Appendix D.

These terms do permit mirroring by other web sites, but be sure to do the

  * make sure your mirrors automatically get upgrades from the master site,
  * clearly show the location of the master site ([
    secure-programs], with a
    hypertext link to the master site, and
  * give me (David A. Wheeler) credit as the author.


The first two points primarily protect me from repeatedly hearing about
obsolete bugs. I do not want to hear about bugs I fixed a year ago, just
because you are not properly mirroring the document. By linking to the master
site, users can check and see if your mirror is up-to-date. I'm sensitive to
the problems of sites which have very strong security requirements and
therefore cannot risk normal connections to the Internet; if that describes
your situation, at least try to meet the other points and try to occasionally
sneakernet updates into your environment.

By this license, you may modify the document, but you can't claim that what
you didn't write is yours (i.e., plagiarism) nor can you pretend that a
modified version is identical to the original work. Modifying the work does
not transfer copyright of the entire work to you; this is not a ``public
domain'' work in terms of copyright law. See the license in Appendix D for
details. If you have questions about what the license allows, please contact
me. In most cases, it's better if you send your changes to the master
integrator (currently David A. Wheeler), so that your changes will be
integrated with everyone else's changes into the master copy.

I am not a lawyer, nevertheless, it's my position as an author and software
developer that any code fragments not explicitly marked otherwise are so
small that their use fits under the ``fair use'' doctrine in copyright law.
In other words, unless marked otherwise, you can use the code fragments
without any restriction at all. Copyright law does not permit copyrighting
absurdly small components of a work (e.g., ``I own all rights to B-flat and
B-flat minor chords''), and the fragments not marked otherwise are of the
same kind of minuscule size when compared to real programs. I've done my best
to give credit for specific pieces of code written by others. Some of you may
still be concerned about the legal status of this code, and I want make sure
that it's clear that you can use this code in your software. Therefore, code
fragments included directly in this document not otherwise marked have also
been released by me under the terms of the ``MIT license'', to ensure you
that there's no serious legal encumbrance:
  Source code in this book not otherwise identified is                       
  Copyright (c) 1999-2001 David A. Wheeler.                                  
  Permission is hereby granted, free of charge, to any person                
  obtaining a copy of the source code in this book not                       
  otherwise identified (the "Software"), to deal in the                      
  Software without restriction, including without limitation                 
  the rights to use, copy, modify, merge, publish, distribute,               
  sublicense, and/or sell copies of the Software, and to                     
  permit persons to whom the Software is furnished to do so,                 
  subject to the following conditions:                                       
  The above copyright notice and this permission notice shall be             
  included in all copies or substantial portions of the Software.            
  PURPOSE AND NONINFRINGEMENT.                                               
  OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                              

Appendix D. GNU Free Documentation License

Version 1.1, March 2000

Copyright © 2000

      Free Software Foundation, Inc. 
      59 Temple Place, Suite 330, 
Everyone is permitted to copy and distribute verbatim copies of this license
document, but changing it is not allowed.

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Appendix E. Endorsements

This version of the document is endorsed by the original author, David A.
Wheeler, as a document that should improve the security of programs, when
applied correctly. Note that no book, including this one, can guarantee that
a developer who follows its guidelines will produce perfectly secure
software. Modifications (including translations) must remove this appendix
per the license agreement included above.

Appendix F. About the Author


David A. Wheeler

David A. Wheeler is an expert in computer security and has long specialized
in development techniques for large and high-risk software systems. He has
been involved in software development since the mid-1970s, and been involved
with Unix and computer security since the early 1980s. His areas of knowledge
include computer security, software safety, vulnerability analysis,
inspections, Internet technologies, software-related standards (including
POSIX), real-time software development techniques, and numerous computer
languages (including Ada, C, C++, Perl, Python, and Java).

Mr. Wheeler is co-author and lead editor of the IEEE book Software
Inspection: An Industry Best Practice, author of the book Ada95: The Lovelace
Tutorial, and co-author of the GNOME User's Guide. He is also the author of
many smaller papers and articles, including the Linux Program Library HOWTO.

Mr. Wheeler hopes that, by making this document available, other developers
will make their software more secure. You can reach him by email at (no spam please), and you can also see his web site at


[1]  Technically, a hypertext link can be any ``uniform resource identifier''
     (URI). The term "Uniform Resource Locator" (URL) refers to the subset of
     URIs that identify resources via a representation of their primary      
     access mechanism (e.g., their network "location"), rather than          
     identifying the resource by name or by some other attribute(s) of that  
     resource. Many people use the term ``URL'' as synonymous with ``URI'',  
     since URLs are the most common kind of URI. For example, the encoding   
     used in URIs is actually called ``URL encoding''.                       

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