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Incorrect Regular Expression

Draft Class
Structure: Simple
Description

This vulnerability occurs when a regular expression is written incorrectly, causing it to match or validate data in unintended and potentially dangerous ways.

Extended Description

Incorrect regular expressions are a common source of security flaws because they often fail to properly filter or validate user input. When used in security controls—like input sanitization, access rules, or data validation—a flawed regex pattern can allow malicious data to slip through, bypassing critical protections. This can lead directly to injection attacks, authorization bypasses, or data corruption. Finding and fixing these subtle regex errors manually is time-consuming, especially in large codebases. While SAST tools can flag the problematic pattern, an ASPM platform like Plexicus uses AI to analyze the context and provide specific, automated remediation advice, helping developers apply the correct fix quickly and consistently across the entire application portfolio.
Common Consequences 2
Scope: Other

Impact: Unexpected StateVaries by Context

When the regular expression is not correctly specified, data might have a different format or type than the rest of the program expects, producing resultant weaknesses or errors.

Scope: Access Control

Impact: Bypass Protection Mechanism

In PHP, regular expression checks can sometimes be bypassed with a null byte, leading to any number of weaknesses.
Detection Methods 1
Automated Static AnalysisHigh
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.)
Potential Mitigations 1
Phase: Architecture and Design

Strategy: Refactoring

Regular expressions can become error prone when defining a complex language even for those experienced in writing grammars. Determine if several smaller regular expressions simplify one large regular expression. Also, subject the regular expression to thorough testing techniques such as equivalence partitioning, boundary value analysis, and robustness. After testing and a reasonable confidence level is achieved, a regular expression may not be foolproof. If an exploit is allowed to slip through, then record the exploit and refactor the regular expression.
Demonstrative Examples 2

ID : DX-37

The following code takes phone numbers as input, and uses a regular expression to reject invalid phone numbers.

Code Example:

Bad
Perl
perl

looks like it only has hyphens and digits*

perl
An attacker could provide an argument such as: "; ls -l ; echo 123-456" This would pass the check, since "123-456" is sufficient to match the "\d+-\d+" portion of the regular expression.
ID : DX-154
This code uses a regular expression to validate an IP string prior to using it in a call to the "ping" command.
Code Example:
Bad
Python
python


The ping command treats zero-prepended IP addresses as octal*


python
Since the regular expression does not have anchors (Regular Expression without Anchors), i.e. is unbounded without ^ or $ characters, then prepending a 0 or 0x to the beginning of the IP address will still result in a matched regex pattern. Since the ping command supports octal and hex prepended IP addresses, it will use the unexpectedly valid IP address (Incorrect Parsing of Numbers with Different Radices). For example, "0x63.63.63.63" would be considered equivalent to "99.63.63.63". As a result, the attacker could potentially ping systems that the attacker cannot reach directly.
  
Observed Examples 11
CVE-2002-2109Regexp isn't "anchored" to the beginning or end, which allows spoofed values that have trusted values as substrings.
CVE-2005-1949Regexp for IP address isn't anchored at the end, allowing appending of shell metacharacters.
CVE-2001-1072Bypass access restrictions via multiple leading slash, which causes a regular expression to fail.
CVE-2000-0115Local user DoS via invalid regular expressions.
CVE-2002-1527chain: Malformed input generates a regular expression error that leads to information exposure.
CVE-2005-1061Certain strings are later used in a regexp, leading to a resultant crash.
CVE-2005-2169MFV. Regular expression intended to protect against directory traversal reduces ".../...//" to "../".
CVE-2005-0603Malformed regexp syntax leads to information exposure in error message.
CVE-2005-1820Code injection due to improper quoting of regular expression.
CVE-2005-3153Null byte bypasses PHP regexp check.
CVE-2005-4155Null byte bypasses PHP regexp check.
  
References 1
Writing Secure Code
Michael Howard and David LeBlanc
Microsoft Press
04-12-2002
https://www.microsoftpressstore.com/store/writing-secure-code-9780735617223
ID: REF-7
 
  
    
 
 
Applicable Platforms 
 
 
 
 
 
Languages:
 
 
 Not Language-Specific : Undetermined 
  
  
 
 
 
  
 
 
Modes of Introduction 
 
 
 
 
 
  Implementation  
 
 
 
  
Related Attack Patterns 
        
 
 
Related Weaknesses 
 
 
 
 
  ChildOf:
 Incorrect Comparison (CWE-697) 
  CanPrecede:
 Partial String Comparison (CWE-187) 
  CanPrecede:
 Collapse of Data into Unsafe Value (CWE-182) 
 
 
 
 
Taxonomy Mapping 
  • PLOVER
  
Notes 
RelationshipWhile there is some overlap with allowlist/denylist problems, this entry is intended to deal with incorrectly written regular expressions, regardless of their intended use. Not every regular expression is intended for use as an allowlist or denylist. In addition, allowlists and denylists can be implemented using other mechanisms besides regular expressions.
Research GapRegexp errors are likely a primary factor in many MFVs, especially those that require multiple manipulations to exploit. However, they are rarely diagnosed at this level of detail.