Alex Brumen

Cache Key Injection Flaw in Nginx Configurations Allows Access Bypass, Data Disclosure and Cached Denial of Service

Security researcher Alex Brumen from YesWeHack has detailed a Cache Key Injection attack vector that exploits poorly constructed cache keys in Nginx. The issue arises when administrators concatenate variable-length values without delimiters, such as $scheme$host$request_uri$http_accept, allowing different requests to produce identical cache keys. Attackers can poison the cache with 404 responses to deliver CPDoS, expose restricted pages like /admin by manipulating path components, or even achieve stored XSS by colliding HTTP and HTTPS responses. The technique can also bypass Cloudflare protection when the Authorization header routes requests directly to the vulnerable Nginx cache. Mitigation requires using explicit delimiters or structured encoding in cache keys, validating the Host header, enforcing HTTPS redirects, and avoiding caching of authenticated requests.