External Control of Critical State Data
Weakness ID: 642 (Weakness Class)Status: Draft
+ Description

Description Summary

The software stores security-critical state information about its users, or the software itself, in a location that is accessible to unauthorized actors.

Extended Description

If an attacker can modify the state information without detection, then it could be used to perform unauthorized actions or access unexpected resources, since the application programmer does not expect that the state can be changed.

State information can be stored in various locations such as a cookie, in a hidden web form field, input parameter or argument, an environment variable, a database record, within a settings file, etc. All of these locations have the potential to be modified by an attacker. When this state information is used to control security or determine resource usage, then it may create a vulnerability. For example, an application may perform authentication, then save the state in an "authenticated=true" cookie. An attacker may simply create this cookie in order to bypass the authentication.

+ Time of Introduction
  • Architecture and Design
  • Implementation
+ Applicable Platforms



Technology Classes

Web-Server: (Often)

+ Common Consequences

An attacker could potentially modify the state in malicious ways. If the state is related to the privileges or level of authentication that the user has, then state modification might allow the user to bypass authentication or elevate privileges.


The state variables may contain sensitive information that should not be known by the client.


By modifying state variables, the attacker could violate the application's expectations for the contents of the state, leading to a denial of service due to an unexpected error condition.

+ Likelihood of Exploit


+ Enabling Factors for Exploitation

An application maintains its own state and/or user state (i.e. application is stateful).

State information can be affected by the user of an application through some means other than the legitimate state transitions (e.g. logging into the system, purchasing an item, making a payment, etc.)

An application does not have means to detect state tampering and behave in a fail safe manner.

+ Demonstrative Examples

Example 1

In the following example, an authentication flag is read from a browser cookie, thus allowing for external control of user state data.

(Bad Code)
Example Language: Java 
Cookie[] cookies = request.getCookies();
for (int i =0; i< cookies.length; i++) {
Cookie c = cookies[i];
if (c.getName().equals("authenticated") && Boolean.TRUE.equals(c.getValue())) {
authenticated = true;

Example 2

The following code segment implements a basic server that uses the "ls" program to perform a directory listing of the directory that is listed in the "HOMEDIR" environment variable. The code intends to allow the user to specify an alternate "LANG" environment variable. This causes "ls" to customize its output based on a given language, which is an important capability when supporting internationalization.

(Bad Code)
Example Language: Perl 
$ENV{"HOMEDIR"} = "/home/mydir/public/";
my $stream = AcceptUntrustedInputStream();
while (<$stream>) {
if (/^ENV ([\w\_]+) (.*)/) {
$ENV{$1} = $2;
elsif (/^QUIT/) { ... }
elsif (/^LIST/) {
open($fh, "/bin/ls -l $ENV{HOMEDIR}|");
while (<$fh>) {
SendOutput($stream, "FILEINFO: $_");

The programmer takes care to call a specific "ls" program and sets the HOMEDIR to a fixed value. However, an attacker can use a command such as "ENV HOMEDIR /secret/directory" to specify an alternate directory, enabling a path traversal attack (CWE-22). At the same time, other attacks are enabled as well, such as OS command injection (CWE-78) by setting HOMEDIR to a value such as "/tmp; rm -rf /". In this case, the programmer never intends for HOMEDIR to be modified, so input validation for HOMEDIR is not the solution. A partial solution would be a whitelist that only allows the LANG variable to be specified in the ENV command. Alternately, assuming this is an authenticated user, the language could be stored in a local file so that no ENV command at all would be needed.

While this example may not appear realistic, this type of problem shows up in code fairly frequently. See CVE-1999-0073 in the observed examples for a real-world example with similar behaviors.

+ Observed Examples
CVE-2005-2428Mail client stores password hashes for unrelated accounts in a hidden form field.
CVE-2008-0306Privileged program trusts user-specified environment variable to modify critical configuration settings.
CVE-1999-0073Telnet daemon allows remote clients to specify critical environment variables for the server, leading to code execution.
CVE-2007-4432Untrusted search path vulnerability through modified LD LIBRARY PATH environment variable.
CVE-2006-7191Untrusted search path vulnerability through modified LD LIBRARY PATH environment variable.
CVE-2008-5738Calendar application allows bypass of authentication by setting a certain cookie value to 1.
CVE-2008-5642Setting of a language preference in a cookie enables path traversal attack.
CVE-2008-5125Application allows admin privileges by setting a cookie value to "admin."
CVE-2008-5065Application allows admin privileges by setting a cookie value to "admin."
CVE-2008-4752Application allows admin privileges by setting a cookie value to "admin."
CVE-2000-0102Shopping cart allows price modification via hidden form field.
CVE-2000-0253Shopping cart allows price modification via hidden form field.
CVE-2008-1319Server allows client to specify the search path, which can be modified to point to a program that the client has uploaded.
+ Potential Mitigations

Phase: Architecture and Design

Understand all the potential locations that are accessible to attackers. For example, some programmers assume that cookies and hidden form fields cannot be modified by an attacker, or they may not consider that environment variables can be modified before a privileged program is invoked.

Phase: Architecture and Design

Do not keep state information on the client without using encryption and integrity checking, or otherwise having a mechanism on the server side to catch state tampering. Use a message authentication code (MAC) algorithm, such as Hash Message Authentication Code (HMAC). Apply this against the state data that you have to expose, which can guarantee the integrity of the data - i.e., that the data has not been modified. Ensure that you use an algorithm with a strong hash function (CWE-328).

Phase: Architecture and Design

Store state information on the server side only. Ensure that the system definitively and unambiguously keeps track of its own state and user state and has rules defined for legitimate state transitions. Do not allow any application user to affect state directly in any way other than through legitimate actions leading to state transitions.

Phase: Architecture and Design

Strategy: Libraries or Frameworks

Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.

With a stateless protocol such as HTTP, use some frameworks can maintain the state for you.

Examples include ASP.NET View State and the OWASP ESAPI Session Management feature.

Be careful of language features that provide state support, since these might be provided as a convenience to the programmer and may not be considering security.

Phase: Architecture and Design

For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.

Phases: Operation; Implementation

If you are using PHP, configure your application so that it does not use register_globals. During implementation, develop your application so that it does not rely on this feature, but be wary of implementing a register_globals emulation that is subject to weaknesses such as CWE-95, CWE-621, and similar issues.

Phase: Testing

Use automated static analysis tools that target this type of weakness. Many modern techniques use data flow analysis to minimize the number of false positives. This is not a perfect solution, since 100% accuracy and coverage are not feasible.

Phase: Testing

Use dynamic tools and techniques that interact with the software using large test suites with many diverse inputs, such as fuzz testing (fuzzing), robustness testing, and fault injection. The software's operation may slow down, but it should not become unstable, crash, or generate incorrect results.

Phase: Testing

Use tools and techniques that require manual (human) analysis, such as penetration testing, threat modeling, and interactive tools that allow the tester to record and modify an active session. These may be more effective than strictly automated techniques. This is especially the case with weaknesses that are related to design and business rules.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfCategoryCategory371State Issues
Development Concepts (primary)699
ChildOfWeakness ClassWeakness Class668Exposure of Resource to Wrong Sphere
Research Concepts (primary)1000
ChildOfCategoryCategory7522009 Top 25 - Risky Resource Management
Weaknesses in the 2009 CWE/SANS Top 25 Most Dangerous Programming Errors (primary)750
RequiredByCompound Element: CompositeCompound Element: Composite352Cross-Site Request Forgery (CSRF)
Research Concepts1000
ParentOfWeakness BaseWeakness Base15External Control of System or Configuration Setting
Research Concepts (primary)1000
ParentOfWeakness ClassWeakness Class73External Control of File Name or Path
Research Concepts (primary)1000
ParentOfCompound Element: CompositeCompound Element: Composite426Untrusted Search Path
Research Concepts (primary)1000
ParentOfWeakness BaseWeakness Base472External Control of Assumed-Immutable Web Parameter
Research Concepts (primary)1000
ParentOfWeakness BaseWeakness Base565Reliance on Cookies without Validation and Integrity Checking
Research Concepts (primary)1000
+ Relevant Properties
  • Accessibility
  • Mutability
  • Trustability
+ Related Attack Patterns
CAPEC-IDAttack Pattern Name
(CAPEC Version: 1.4)
21Exploitation of Session Variables, Resource IDs and other Trusted Credentials
31Accessing/Intercepting/Modifying HTTP Cookies
167Lifting Sensitive Data from the Client
+ References
OWASP. "Top 10 2007-Insecure Direct Object Reference". 2007. <http://www.owasp.org/index.php/Top_10_2007-A4>.
+ Content History
Submission DateSubmitterOrganizationSource
2008-01-30Evgeny LebanidzeCigitalExternal Submission
Modification DateModifierOrganizationSource
2008-07-01Sean EidemillerCigitalExternal
added/updated demonstrative examples
2008-09-08CWE Content TeamMITREInternal
updated Common Consequences, Relationships
2008-10-14CWE Content TeamMITREInternal
updated Description
2009-01-12CWE Content TeamMITREInternal
updated Applicable Platforms, Common Consequences, Demonstrative Examples, Description, Name, Observed Examples, Potential Mitigations, References, Relationships, Relevant Properties, Type
2009-03-10CWE Content TeamMITREInternal
updated Potential Mitigations
2009-07-27CWE Content TeamMITREInternal
updated Related Attack Patterns
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Insufficient Management of User State
2009-01-12External Control of User State Data