Description
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.
Potential Impact
Access Control
Bypass Protection Mechanism
Mitigations & Prevention
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
Real-World CVE Examples
| CVE ID | Description |
|---|---|
| CVE-2005-1604 | MIE. ".php.ns" bypasses ".php$" regexp but is still parsed as PHP by Apache. (manipulates an equivalence property under Apache) |
Related Weaknesses
Taxonomy Mappings
- PLOVER: — Overly Restrictive Regular Expression
Frequently Asked Questions
What is CWE-186?
CWE-186 (Overly Restrictive Regular Expression) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Base-level weakness. A regular expression is overly restrictive, which prevents dangerous values from being detected.
How can CWE-186 be exploited?
Attackers can exploit CWE-186 (Overly Restrictive Regular Expression) to bypass protection mechanism. This weakness is typically introduced during the Implementation phase of software development.
How do I prevent CWE-186?
Key mitigations include: 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
What is the severity of CWE-186?
CWE-186 is classified as a Base-level weakness (Medium abstraction). It has been observed in 1 real-world CVEs.