CWE-186: Overly Restrictive Regular Expression

Extended Description

This weakness is not about regular expression complexity. Rather, it is about a regular expression that does not match all values that are intended. Consider the use of a regexp to identify acceptable values or to spot unwanted terms. An overly restrictive regexp misses some potentially security-relevant values leading to either false positives *or* false negatives, depending on how the regexp is being used within the code. Consider the expression /[0-8]/ where the intention was /[0-9]/. This expression is not "complex" but the value "9" is not matched when maybe the programmer planned to check for it.

Source-backed guidance

Additional facts reviewed against primary or authoritative security sources.

Verify controls for CWE-186 with SSDF evidence

Use NIST SSDF verification and vulnerability-response practices to detect CWE-186, Overly Restrictive Regular Expression, throughout the product lifecycle. Derive review questions, static or dynamic checks, and negative tests from the CWE's causal behavior; define the components and lifecycle stages each check covers; and retain findings with enough evidence to distinguish the root cause from symptoms and impacts. Track escapes and false negatives, then improve the verification plan after every confirmed occurrence.

NIST SP 800-218 Secure Software Development Framework — National Institute of Standards and Technology

Apply Refactoring controls for Overly Restrictive Regular Expression

MITRE associates mitigation with Implementation; the listed strategies include Refactoring; recorded impacts include Bypass Protection Mechanism. Use these source-defined anchors to turn CWE-186 into implementation, review, and verification checks for the affected component.

CWE-186: Overly Restrictive Regular Expression — MITRE CWE

Triage CWE-186 against known exploitation evidence

Use CISA's Known Exploited Vulnerabilities catalog to test whether a vulnerability mapped to CWE-186, Overly Restrictive Regular Expression, has evidence of exploitation in the wild. Confirm the CVE-to-CWE root-cause mapping independently before attaching the example, then capture the affected product, required action, and remediation deadline. A missing KEV match is not evidence that the weakness is unexploited, and a KEV entry must not be generalized to every occurrence of this CWE.

Known Exploited Vulnerabilities Catalog — Cybersecurity and Infrastructure Security Agency

Apply precise root-cause mapping to CWE-186

Apply MITRE's full root-cause mapping guidance when using CWE-186, Overly Restrictive Regular Expression. Separate weakness language from attacker prerequisites and technical impact, check the entry's abstraction and vulnerability-mapping notes, and prefer the most specific Base or Variant supported by the evidence. Record the rejected alternatives and require an independent review before the mapping is used for remediation trends or program metrics.

CVE to CWE Root Cause Mapping Guidance — MITRE CWE

Validate CWE-186 with root-cause mapping checks

Apply MITRE's root-cause mapping quick tips to CWE-186, Overly Restrictive Regular Expression. Confirm the finding describes the causal weakness rather than an impact or attack pattern, compare the abstraction and mapping notes with plausible alternatives, and have a second reviewer challenge the selection. Preserve the evidence and reasoning so recurring defects can be measured against one consistent identifier.

CVE to CWE Root Cause Mapping Quick Tips — MITRE CWE

Frequently Asked Questions

What is CWE-186: Overly Restrictive Regular Expression?+

CWE-186: Overly Restrictive Regular Expression is a Common Weakness Enumeration (CWE) entry maintained by MITRE. A regular expression is overly restrictive, which prevents dangerous values from being detected. This weakness is not about regular expression complexity. Rather, it is about a regular expression that does not match all values that are intended. Consider the use of a regexp to identify acceptable values or to spot unwanted terms. An overly restrictive regexp misses some potentially security-relevant values leading to either false positives *or* false negatives, depending on how the regexp is being used within the code. Consider the expression /[0-8]/ where the intention was /[0-9]/. This expression is not "complex" but the value "9" is not matched when maybe the programmer planned to check for it.

What are the security consequences of Overly Restrictive Regular Expression?+

If exploited, CWE-186 (Overly Restrictive Regular Expression) it can compromise Access Control, leading to outcomes such as Bypass Protection Mechanism.

Which programming languages are affected by Overly Restrictive Regular Expression?+

CWE-186 commonly affects Not Language-Specific. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.

What are real-world examples of Overly Restrictive Regular Expression?+

MITRE documents real CVEs mapped to CWE-186, including CVE-2005-1604. You can look up the full details of each CVE, including CVSS scores and remediation guidance, on our CVE Lookup tool.

What is the difference between a CWE and a CVE?+

A CWE (Common Weakness Enumeration) like CWE-186 describes a category of software weakness — the underlying flaw type. A CVE (Common Vulnerabilities and Exposures) identifies a specific, real-world vulnerability in a particular product. In short, a CWE is the kind of mistake, and a CVE is an instance of that mistake being found in software.

CWE-186: Overly Restrictive Regular Expression

Extended Description

Technical Details

Applicable To

Security Consequences

Scope

Impact

Mitigation Strategies

Detection Methods

Code Examples & CVEs

Observed CVE Examples (1)

CWE Relationships

Parent Weaknesses