CWE 642

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

Languages

All

Technology Classes

Web-Server: (Often)

Common Consequences

Scope	Effect
Integrity	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.
Confidentiality	The state variables may contain sensitive information that should not be known by the client.
Availability	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

High

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>) {

chomp;

if (/^ENV ([\w\_]+) (.*)/) {

$ENV{$1} = $2;

}

elsif (/^QUIT/) { ... }

elsif (/^LIST/) {

open($fh, "/bin/ls -l $ENV{HOMEDIR}|");

while (<$fh>) {

SendOutput($stream, "FILEINFO: $_");

}

close($fh);

}

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

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

Potential Mitigations