Common Weakness Enumeration

A Community-Developed List of Software Weakness Types

CWE/SANS Top 25 Most Dangerous Software Errors
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CWE-621: Variable Extraction Error

Weakness ID: 621
Abstraction: Base
Structure: Simple
Status: Incomplete
Presentation Filter:
+ Description
The product uses external input to determine the names of variables into which information is extracted, without verifying that the names of the specified variables are valid. This could cause the program to overwrite unintended variables.
+ Extended Description

For example, in PHP, extraction can be used to provide functionality similar to register_globals, a dangerous functionality that is frequently disabled in production systems. Calling extract() or import_request_variables() without the proper arguments could allow arbitrary global variables to be overwritten, including superglobals.

Similar functionality is possible in other interpreted languages, including custom languages.

+ Alternate Terms
Variable overwrite
+ Relationships

The table(s) below shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.

+ Relevant to the view "Research Concepts" (CWE-1000)
+ Relevant to the view "Development Concepts" (CWE-699)
+ Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the software life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

+ Applicable Platforms
The listings below show possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.


PHP (Undetermined Prevalence)

+ Common Consequences

The table below specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.


Technical Impact: Modify Application Data

An attacker could modify sensitive data or program variables.
+ Demonstrative Examples

Example 1

This code uses the credentials sent in a POST request to login a user.

(bad code)
Example Language: PHP 
//Log user in, and set $isAdmin to true if user is an administrator
function login($user,$pass){
$query = buildQuery($user,$pass);
if(getUserRole($user) == "Admin"){
$isAdmin = true;



$isAdmin = false;

The call to extract() will overwrite the existing values of any variables defined previously, in this case $isAdmin. An attacker can send a POST request with an unexpected third value "isAdmin" equal to "true", thus gaining Admin privileges.

+ Observed Examples
extract issue enables file inclusion
extract used for register_globals compatibility layer, enables path traversal
extract() buried in include files makes post-disclosure analysis confusing; original report had seemed incorrect.
extract() enables static code injection
import_request_variables() buried in include files makes post-disclosure analysis confusing
+ Potential Mitigations

Phase: Implementation

Strategy: Input Validation

Use whitelists of variable names that can be extracted.

Phase: Implementation

Consider refactoring your code to avoid extraction routines altogether.

Phase: Implementation

In PHP, call extract() with options such as EXTR_SKIP and EXTR_PREFIX_ALL; call import_request_variables() with a prefix argument. Note that these capabilities are not present in all PHP versions.
+ Weakness Ordinalities
(where the weakness exists independent of other weaknesses)
+ Memberships
This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
MemberOfViewView884CWE Cross-section
MemberOfCategoryCategory990SFP Secondary Cluster: Tainted Input to Command
+ Notes

Research Gap

Probably under-reported for PHP. Under-studied for other interpreted languages.
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
Software Fault PatternsSFP24Tainted input to command
+ Content History
Modification DateModifierOrganization
2008-07-01Eric DalciCigital
updated Time_of_Introduction
2008-09-08CWE Content TeamMITRE
updated Description, Relationships, Observed_Example, Other_Notes, Weakness_Ordinalities
2008-10-14CWE Content TeamMITRE
updated Description
2011-06-01CWE Content TeamMITRE
updated Common_Consequences
2012-05-11CWE Content TeamMITRE
updated Common_Consequences, Demonstrative_Examples, Relationships
2012-10-30CWE Content TeamMITRE
updated Potential_Mitigations
2013-02-21CWE Content TeamMITRE
updated Demonstrative_Examples, Relationships
2014-06-23CWE Content TeamMITRE
updated Description, Other_Notes
2014-07-30CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings

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Page Last Updated: January 18, 2018