The product uses a regular expression that either (1) contains an executable component with user-controlled inputs, or (2) allows a user to enable execution by inserting pattern modifiers.
Case (2) is possible in the PHP preg_replace() function, and possibly in other languages when a user-controlled input is inserted into a string that is later parsed as a regular expression.

Primary

Implementation

Confidentiality
Integrity
Availability
Execute Unauthorized Code or Commands

Automated Static Analysis
Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)

Implementation
The regular expression feature in some languages allows inputs to be quoted or escaped before insertion, such as \Q and \E in Perl.

CVE-2006-2059
Executable regexp in PHP by inserting "e" modifier into first argument to preg_replace
https://www.cve.org/CVERecord?id=CVE-2006-2059

CVE-2005-3420
Executable regexp in PHP by inserting "e" modifier into first argument to preg_replace
https://www.cve.org/CVERecord?id=CVE-2005-3420

CVE-2006-2878
Complex curly syntax inserted into the replacement argument to PHP preg_replace(), which uses the "/e" modifier
https://www.cve.org/CVERecord?id=CVE-2006-2878

CVE-2006-2908
Function allows remote attackers to execute arbitrary PHP code via the username field, which is used in a preg_replace function call with a /e (executable) modifier.
https://www.cve.org/CVERecord?id=CVE-2006-2908

SFP24
Tainted input to command

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

Under-studied. The existing PHP reports are limited to highly skilled researchers, but there are few examples for other languages. It is suspected that this is under-reported for all languages. Usability factors might make it more prevalent in PHP, but this theory has not been investigated.

CWE Content Team
MITRE
2007-05-07
Draft 6
2007-05-07

Eric Dalci
Cigital
2008-07-01
updated Time_of_Introduction

CWE Content Team
MITRE
2008-09-08
updated Applicable_Platforms, Relationships, Observed_Example

CWE Content Team
MITRE
2008-10-14
updated Description

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences

CWE Content Team
MITRE
2012-05-11
updated Relationships

CWE Content Team
MITRE
2012-10-30
updated Potential_Mitigations

CWE Content Team
MITRE
2014-06-23
updated Observed_Examples

CWE Content Team
MITRE
2014-07-30
updated Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2017-11-08
updated Observed_Examples

CWE Content Team
MITRE
2020-02-24
updated Relationships

CWE Content Team
MITRE
2020-08-20
updated Relationships

CWE Content Team
MITRE
2020-12-10
updated Relationships

CWE Content Team
MITRE
2023-04-27
updated Relationships

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

CWE Content Team
MITRE
2025-12-11
4.19
2025-12-11
updated Detection_Factors, Weakness_Ordinalities