CWE-621: Variable Extraction Error

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.

Code Examples & CVEs

Demonstrative Examples

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

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.

BadPHP

//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));

Observed CVE Examples (5)

CVE-2006-7135

extract issue enables file inclusion

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CVE-2006-7079

Chain: PHP app uses extract for register_globals compatibility layer (CWE-621), enabling path traversal (CWE-22)

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CVE-2007-0649

extract() buried in include files makes post-disclosure analysis confusing; original report had seemed incorrect.

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CVE-2006-6661

extract() enables static code injection

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CVE-2006-2828

import_request_variables() buried in include files makes post-disclosure analysis confusing

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Source-backed guidance

Additional facts reviewed against primary or authoritative security sources.

Verify controls for CWE-621 with SSDF evidence

Use NIST SSDF verification and vulnerability-response practices to detect CWE-621, Variable Extraction Error, throughout the product lifecycle. Derive review questions, static or dynamic checks, and negative tests from the CWE's causal behavior; define the components and lifecycle stages each check covers; and retain findings with enough evidence to distinguish the root cause from symptoms and impacts. Track escapes and false negatives, then improve the verification plan after every confirmed occurrence.

NIST SP 800-218 Secure Software Development Framework — National Institute of Standards and Technology

Apply Input Validation controls for Variable Extraction Error

MITRE associates mitigation with Implementation; the listed strategies include Input Validation, and Refactoring; recorded impacts include Modify Application Data. Use these source-defined anchors to turn CWE-621 into implementation, review, and verification checks for the affected component.

CWE-621: Variable Extraction Error — MITRE CWE

Triage CWE-621 against known exploitation evidence

Use CISA's Known Exploited Vulnerabilities catalog to test whether a vulnerability mapped to CWE-621, Variable Extraction Error, has evidence of exploitation in the wild. Confirm the CVE-to-CWE root-cause mapping independently before attaching the example, then capture the affected product, required action, and remediation deadline. A missing KEV match is not evidence that the weakness is unexploited, and a KEV entry must not be generalized to every occurrence of this CWE.

Known Exploited Vulnerabilities Catalog — Cybersecurity and Infrastructure Security Agency

Apply precise root-cause mapping to CWE-621

Apply MITRE's full root-cause mapping guidance when using CWE-621, Variable Extraction Error. Separate weakness language from attacker prerequisites and technical impact, check the entry's abstraction and vulnerability-mapping notes, and prefer the most specific Base or Variant supported by the evidence. Record the rejected alternatives and require an independent review before the mapping is used for remediation trends or program metrics.

CVE to CWE Root Cause Mapping Guidance — MITRE CWE

Validate CWE-621 with root-cause mapping checks

Apply MITRE's root-cause mapping quick tips to CWE-621, Variable Extraction Error. Confirm the finding describes the causal weakness rather than an impact or attack pattern, compare the abstraction and mapping notes with plausible alternatives, and have a second reviewer challenge the selection. Preserve the evidence and reasoning so recurring defects can be measured against one consistent identifier.

CVE to CWE Root Cause Mapping Quick Tips — MITRE CWE

Frequently Asked Questions

What is CWE-621: Variable Extraction Error?+

CWE-621: Variable Extraction Error is a Common Weakness Enumeration (CWE) entry maintained by MITRE. 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. 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.

What are the security consequences of Variable Extraction Error?+

If exploited, CWE-621 (Variable Extraction Error) it can compromise Integrity, leading to outcomes such as Modify Application Data.

How do you prevent or mitigate Variable Extraction Error?+

Recommended mitigations for CWE-621 include: Use allowlists of variable names that can be extracted. Consider refactoring your code to avoid extraction routines altogether. 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.

Which programming languages are affected by Variable Extraction Error?+

CWE-621 commonly affects PHP. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.

What are real-world examples of Variable Extraction Error?+

MITRE documents real CVEs mapped to CWE-621, including CVE-2006-7135, CVE-2006-7079, CVE-2007-0649, CVE-2006-6661 and CVE-2006-2828. You can look up the full details of each CVE, including CVSS scores and remediation guidance, on our CVE Lookup tool.

What is the difference between a CWE and a CVE?+

A CWE (Common Weakness Enumeration) like CWE-621 describes a category of software weakness — the underlying flaw type. A CVE (Common Vulnerabilities and Exposures) identifies a specific, real-world vulnerability in a particular product. In short, a CWE is the kind of mistake, and a CVE is an instance of that mistake being found in software.

CWE-621: Variable Extraction Error

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Detection Methods

Code Examples & CVEs

Demonstrative Examples

Observed CVE Examples (5)

CWE Relationships

Parent Weaknesses

Source-backed guidance

Verify controls for CWE-621 with SSDF evidence

Apply Input Validation controls for Variable Extraction Error

Triage CWE-621 against known exploitation evidence

Apply precise root-cause mapping to CWE-621

Validate CWE-621 with root-cause mapping checks

Frequently Asked Questions

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