CWE-186 Overly Restrictive Regular Expression

A regular expression is overly restrictive in a way that does not match all intended values.

• Access Control: Bypass Protection Mechanism, Varies by Context — 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.

• [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.

• CVE-2005-1604: MIE. ".php.ns" bypasses ".php$" regexp but is still parsed as PHP by Apache. (manipulates an equivalence property under Apache)