Koa Vulnerable to Open Redirect via Trailing Double-Slash (//) in back Redirect Logic

Vulnerability Description

Koa is expressive middleware for Node.js using ES2017 async functions. In versions 2.16.2 to before 2.16.3 and 3.0.1 to before 3.0.3, a bypass to CVE-2025-8129 was discovered in the Koa.js framework affecting its back redirect functionality. In certain circumstances, an attacker can manipulate the Referer header to force a user’s browser to navigate to an external, potentially malicious website. This occurs because the implementation incorrectly treats some specially crafted URLs as safe relative paths. Exploiting this vulnerability could allow attackers to perform phishing, social engineering, or other redirect-based attacks on users of affected applications. This issue has been patched in version 3.0.3.

References

• https://github.com/koajs/koa/security/advisories/GHSA-g8mr-fgfg-5qpc
• https://github.com/koajs/koa/commit/769fd75cc6b30d72493b370b5a3ae2332ca03c5b