NoorQureshi Web cache poisoning & deception
When it applies
A shared cache (CDN/reverse proxy) sits in front of the app. Poisoning: an unkeyed input influences the response but isn't part of the cache key, so your malicious response gets stored and served to everyone. Deception: the cache is tricked into storing a victim's private page.
Why it works
Caches key on some request parts (URL, maybe some headers) and ignore others ("unkeyed"). If an
unkeyed header/param changes the response (reflected X-Forwarded-Host, a header-driven script
src), you poison the cached copy. Deception exploits path/extension rules that cache things that
shouldn't be cached.
Method
- Map cache behaviour:
X-Cache: hit/miss,Age,Cache-Control, CDN vendor. Send a cache-buster param to test safely without poisoning the real key. - Find unkeyed inputs with Burp Param Miner ("guess headers"):
X-Forwarded-Host,X-Forwarded-Scheme,X-Host, custom headers reflected into the response. - Poison (carefully): make the unkeyed input inject a payload (e.g.
X-Forwarded-Host: evil.com→ a script/link src) that reflects into a cacheable response; confirm it's stored and served to a clean request. - Deception: request
/account.php/nonexistent.css— if the cache stores the private page under the static-looking URL, another user can fetch victims' cached data.
Gotchas
- Test with a unique cache-buster so you don't poison production for real users; then prove one controlled hit.
- Impact = what the poisoned response does (XSS, redirect, defacement) × who receives it.
- CDNs normalize/strip headers differently — the win is a specific input this cache doesn't key on.
Verify success
A clean request (no attacker input) receives your poisoned response, or a victim's private page is served from cache to you.
References
James Kettle "Practical Web Cache Poisoning"; PortSwigger cache labs; Param Miner.