CWE - CWE-621: Variable Extraction Error (4.19.1)
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    CWE-621: Variable Extraction Error

    Weakness ID: 621
    Vulnerability Mapping: ALLOWED This CWE ID may be used to map to real-world vulnerabilities
    Abstraction: Variant Variant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.
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    + 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
    + Common Consequences
    Section HelpThis table 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.
    Impact Details

    Modify Application Data

    		Scope: Integrity

    An attacker could modify sensitive data or program variables.
    + Potential Mitigations
    Phase(s) Mitigation

    Implementation

    Strategy: Input Validation

    Use allowlists of variable names that can be extracted.

    Implementation
    Consider refactoring your code to avoid extraction routines altogether.

    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.
    + Relationships
    Section Help This table 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" (View-1000)
    Nature Type ID Name
    ChildOf Base Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. 914 Improper Control of Dynamically-Identified Variables
    CanPrecede Base Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. 471 Modification of Assumed-Immutable Data (MAID)
    + Modes Of Introduction
    Section HelpThe different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.
    Phase Note
    Implementation
    + Applicable Platforms
    Section HelpThis listing shows 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.
    Languages

    PHP (Undetermined Prevalence)

    + 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,$pass);
    mysql_query($query);
    if(getUserRole($user) == "Admin"){
    $isAdmin = true;
    }
    }

    $isAdmin = false;
    extract($_POST);
    login(mysql_real_escape_string($user),mysql_real_escape_string($pass));

    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.



    + Selected Observed Examples

    Note: this is a curated list of examples for users to understand the variety of ways in which this weakness can be introduced. It is not a complete list of all CVEs that are related to this CWE entry.

    Reference Description
    extract issue enables file inclusion
    Chain: PHP app uses extract for register_globals compatibility layer (CWE-621), enabling path traversal (CWE-22)
    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
    + Weakness Ordinalities
    Ordinality Description
    Primary
    (where the weakness exists independent of other weaknesses)
    + Memberships
    Section HelpThis 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.
    Nature Type ID Name
    MemberOf ViewView - a subset of CWE entries that provides a way of examining CWE content. The two main view structures are Slices (flat lists) and Graphs (containing relationships between entries). 884 CWE Cross-section
    MemberOf CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic. 990 SFP Secondary Cluster: Tainted Input to Command
    MemberOf CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic. 1409 Comprehensive Categorization: Injection
    + Vulnerability Mapping Notes
    Usage ALLOWED
    (this CWE ID may be used to map to real-world vulnerabilities)
    Reason Acceptable-Use

    Rationale

    		This CWE entry is at the Variant level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.

    Comments

    		Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.
    + Notes

    Research Gap

    Probably under-reported for PHP. Seems under-studied for other interpreted languages.
    + Taxonomy Mappings
    Mapped Taxonomy Name Node ID Fit Mapped Node Name
    Software Fault Patterns SFP24 Tainted input to command
    + Content History
    + Submissions
    Submission Date Submitter Organization
    2007-05-07
    (CWE Draft 6, 2007-05-07)     		CWE Content Team MITRE
    + Modifications
    Modification Date Modifier Organization
    2023-10-26 CWE Content Team MITRE
    updated Observed_Examples
    2023-06-29 CWE Content Team MITRE
    updated Mapping_Notes
    2023-04-27 CWE Content Team MITRE
    updated Relationships, Type
    2022-04-28 CWE Content Team MITRE
    updated Research_Gaps
    2020-06-25 CWE Content Team MITRE
    updated Potential_Mitigations
    2020-02-24 CWE Content Team MITRE
    updated Relationships
    2014-07-30 CWE Content Team MITRE
    updated Relationships, Taxonomy_Mappings
    2014-06-23 CWE Content Team MITRE
    updated Description, Other_Notes
    2013-02-21 CWE Content Team MITRE
    updated Demonstrative_Examples, Relationships
    2012-10-30 CWE Content Team MITRE
    updated Potential_Mitigations
    2012-05-11 CWE Content Team MITRE
    updated Common_Consequences, Demonstrative_Examples, Relationships
    2011-06-01 CWE Content Team MITRE
    updated Common_Consequences
    2008-10-14 CWE Content Team MITRE
    updated Description
    2008-09-08 CWE Content Team MITRE
    updated Description, Relationships, Observed_Example, Other_Notes, Weakness_Ordinalities
    2008-07-01 Eric Dalci Cigital
    updated Time_of_Introduction
    Page Last Updated: January 21, 2026