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CVE-2025-25200: Koa has Inefficient Regular Expression Complexity

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.8% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2025-02-12 · updated 2026-06-17
• Weakness: CWE-1333

Description

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 a Denial-of-Service attack. Versions 0.21.2, 1.7.1, 2.15.4, and 3.0.0-alpha.3 fix the issue.

Affected products

• koajs koa: before 0.21.2; from 1.0.0 before 1.7.1; from 2.0.0 before 2.15.4; 3.0.0-alpha0, 3.0.0-alpha1, 3.0.0-alpha2 — fixed in 0.21.2, 1.7.1, 2.15.4

How to fix

Upgrade to a fixed version: koajs koa 0.21.2, 1.7.1, 2.15.4. Check the vendor advisory for mitigations.

References

• https://github.com/koajs/koa/blob/master/lib/request.js#L259
• https://github.com/koajs/koa/blob/master/lib/request.js#L404
• https://github.com/koajs/koa/commit/5054af6e31ffd451a4151a1fe144cef6e5d0d83c
• https://github.com/koajs/koa/commit/5f294bb1c7c8d9c61904378d250439a321bffd32
• https://github.com/koajs/koa/commit/93fe903fc966635a991bcf890cfc3427d33a1a08
• https://github.com/koajs/koa/releases/tag/2.15.4
• https://github.com/koajs/koa/security/advisories/GHSA-593f-38f6-jp5m

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