Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.
CWE-186 Base Draft
Overly Restrictive Regular Expression
This weakness occurs when a regular expression is too narrow, failing to detect all the dangerous or unexpected input values it was designed to catch.
Definition
What is CWE-186?
This weakness occurs when a regular expression is too narrow, failing to detect all the dangerous or unexpected input values it was designed to catch.
An overly restrictive regex doesn't match every input it should, creating a gap in your security checks. This isn't about the regex being computationally complex; it's about its logic being incomplete. For example, using /[0-8]/ to validate digits would miss the number '9', potentially allowing an unwanted value to slip through. The result is either false negatives (missing real threats) or false positives (blocking safe input), depending on whether the regex is used to allowlist or blocklist terms.
As a developer, you must carefully review the intended scope of your validation. Test your regular expressions against the full range of both valid and malicious edge cases to ensure they capture everything relevant. This proactive testing helps close the gap between what you think your regex catches and what it actually allows or blocks in practice.
Real-world impact
Real-world CVEs caused by CWE-186
-
MIE. ".php.ns" bypasses ".php$" regexp but is still parsed as PHP by Apache. (manipulates an equivalence property under Apache)
How attackers exploit it
Step-by-step attacker path
- 1
Identify a code path that handles untrusted input without validation.
- 2
Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.
- 3
Deliver the payload through a normal request and observe the application's reaction.
- 4
Iterate until the response leaks data, executes attacker code, or escalates privileges.
Vulnerable code example
Vulnerable pseudo
MITRE has not published a code example for this CWE. The pattern below is illustrative — see Resources for canonical references.
// Example pattern — see MITRE for the canonical references.
function handleRequest(input) {
// Untrusted input flows directly into the sensitive sink.
return executeUnsafe(input);
} Secure code example
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-186
- Implementation 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 your 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 your regular expression.
Detection signals
How to detect CWE-186
Run dynamic application security testing against the live endpoint.
Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.
Code review: flag any new code that handles input from this surface without using the validated framework helpers.
CWE-186
Don't catalog this weakness. Prove it's reachable.
Plexicus turns CWE catalogs into evidence: every CWE-pattern is matched against your real code graph, reach is proven on a sandbox clone, and verified findings ship as reviewed PRs.
Frequently asked questions What is CWE-186?
How serious is CWE-186?
What languages or platforms are affected by CWE-186?
How can I prevent CWE-186?
How does Plexicus detect and fix CWE-186?
Where can I learn more about CWE-186?
Frequently asked questions
What is CWE-186?
This weakness occurs when a regular expression is too narrow, failing to detect all the dangerous or unexpected input values it was designed to catch.
How serious is CWE-186?
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-186?
MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.
How can I prevent CWE-186?
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 your 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-186?
Plexicus's SAST engine matches the data-flow signature for CWE-186 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-186?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/186.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-186
CWE-185 Parent
Incorrect Regular Expression
This vulnerability occurs when a regular expression is written incorrectly, causing it to match or validate data in unintended and…
CWE-625 Sibling
Permissive Regular Expression
This weakness occurs when a regular expression is too permissive, failing to properly validate or sanitize input by allowing unintended…
CWE-184 Peer
Incomplete List of Disallowed Inputs
This vulnerability occurs when a security filter or validation mechanism relies on a 'denylist'—a predefined list of forbidden inputs—but…
CWE-183 Peer
Permissive List of Allowed Inputs
This vulnerability occurs when an application's security filter uses an allowlist that is too broad, mistakenly permitting dangerous…
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