CWE-185 Class Draft

Incorrect Regular Expression

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

Definition

What is CWE-185?

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

Real-world CVEs caused by CWE-185

  • CVE-2002-2109

    Regexp isn't "anchored" to the beginning or end, which allows spoofed values that have trusted values as substrings.

  • CVE-2005-1949

    Regexp for IP address isn't anchored at the end, allowing appending of shell metacharacters.

  • CVE-2001-1072

    Bypass access restrictions via multiple leading slash, which causes a regular expression to fail.

  • CVE-2000-0115

    Local user DoS via invalid regular expressions.

  • CVE-2002-1527

    chain: Malformed input generates a regular expression error that leads to information exposure.

  • CVE-2005-1061

    Certain strings are later used in a regexp, leading to a resultant crash.

  • CVE-2005-2169

    MFV. Regular expression intended to protect against directory traversal reduces ".../...//" to "../".

  • CVE-2005-0603

    Malformed regexp syntax leads to information exposure in error message.

How attackers exploit it

Step-by-step attacker path

  1. 1

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

  2. 2

    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.

  3. 3

    This code uses a regular expression to validate an IP string prior to using it in a call to the "ping" command.

  4. 4

    Since the regular expression does not have anchors (CWE-777), 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 (CWE-1389). 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.

Vulnerable code example

Vulnerable Perl

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

Vulnerable Perl 
$phone = GetPhoneNumber();
  if ($phone =~ /\d+-\d+/) {
```
# looks like it only has hyphens and digits* 
  	system("lookup-phone $phone");} 
  else {
  ```
  	error("malformed number!");
  }
Secure code example

Secure pseudo

Secure pseudo 
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
  const safe = validateAndEscape(input);
  return executeWithGuards(safe);
}
What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist

How to prevent CWE-185

  • 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.
Detection signals

How to detect CWE-185

Automated Static Analysis High

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.)

Plexicus auto-fix

Plexicus auto-detects CWE-185 and opens a fix PR in under 60 seconds.

Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.

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Frequently asked questions

Frequently asked questions

What is CWE-185?

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

How serious is CWE-185?

MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.

What languages or platforms are affected by CWE-185?

MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.

How can I prevent CWE-185?

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…

How does Plexicus detect and fix CWE-185?

Plexicus's SAST engine matches the data-flow signature for CWE-185 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.

Where can I learn more about CWE-185?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/185.html. You can also reference OWASP and NIST documentation for adjacent guidance.

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Weaknesses related to CWE-185

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Resources

Further reading

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