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Incorrect Regular Expression

ID: 185Date: (C)2012-05-14   (M)2018-10-22
Type: weaknessStatus: DRAFT
Abstraction Type: Class


The software specifies a regular expression in a way that causes data to be improperly matched or compared.

Extended Description

When the regular expression is used in protection mechanisms such as filtering or validation, this may allow an attacker to bypass the intended restrictions on the incoming data.

Applicable Platforms
Language Class: All

Time Of Introduction

  • Implementation

Related Attack Patterns

Common Consequences

ScopeTechnical ImpactNotes
Bypass protection mechanism
In PHP, regular expression checks can sometimes be bypassed with a null byte, leading to any number of weaknesses.

Detection Methods

Potential Mitigations

Architecture and Design
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.


Related CWETypeViewChain
CWE-185 ChildOf CWE-896 Category CWE-888  

Demonstrative Examples   (Details)

  1. (Demonstrative Example Id DX-37)

Observed Examples

  1. CVE-2002-2109 : Regexp isn't "anchored" to the beginning or end, which allows spoofed values that have trusted values as substrings.
  2. CVE-2005-1949 : Regexp for IP address isn't anchored at the end, allowing appending of shell metacharacters.
  3. CVE-2001-1072 : Bypass access restrictions via multiple leading slash, which causes a regular expression to fail.
  4. CVE-2000-0115 : Local user DoS via invalid regular expressions.
  5. CVE-2002-1527 : chain: Malformed input generates a regular expression error that leads to information exposure.
  6. CVE-2005-1061 : Certain strings are later used in a regexp, leading to a resultant crash.
  7. CVE-2005-2169 : MFV. Regular expression intended to protect against directory traversal reduces ".../...//" to "../".
  8. CVE-2005-0603 : Malformed regexp syntax leads to information exposure in error message.
  9. CVE-2005-1820 : Code injection due to improper quoting of regular expression.
  10. CVE-2005-3153 : Null byte bypasses PHP regexp check.
  11. CVE-2005-4155 : Null byte bypasses PHP regexp check.

For more examples, refer to CVE relations in the bottom box.

White Box Definitions

Black Box Definitions

Taxynomy Mappings

PLOVER  Regular Expression Error


  1. M. Howard D. LeBlanc .Writing Secure Code 2nd Edition. Microsoft. Section:'Chapter 10, "Using Regular Expressions for Checking Input" Page 350'. Published on 2002.

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