Weakness (CWE)
CWE-1333: Inefficient Regular Expression Complexity
The product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential.
How to prevent it (4 mitigations)
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Architecture and Design
Use regular expressions that do not support backtracking, e.g. by removing nested quantifiers.
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System Configuration
Set backtracking limits in the configuration of the regular expression implementation, such as PHP's pcre.backtrack_limit. Also consider limits on execution time for the process.
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Implementation
Do not use regular expressions with untrusted input. If regular expressions must be used, avoid using backtracking in the expression.
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Implementation
Limit the length of the input that the regular expression will process.
Source: MITRE CWE-1333
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Critical 9.2CVE-2025-25200 Public exploit
Koa has Inefficient Regular Expression Complexity
Koa is expressive middleware for Node.js using ES2017 async functions. Prior to versions 0.21.2, 1.7.1, 2.15.4, and 3.0.0-alpha.3, Koa uses an evil regex to parse the `X-Forwarded-Proto` and `X-Forwarded-Host` HTTP headers. This can be exploited to carry out …