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Application & OT Security · Advanced

HTTP Request Smuggling

Exploiting disagreement between front-end and back-end servers about where an HTTP request ends, to hide a second request.

Definition

Discrepancies in handling Content-Length and Transfer-Encoding headers (CL.TE, TE.CL) or HTTP/2 downgrading let attackers prepend data to other users' requests, bypass controls or poison caches. Mitigations include using HTTP/2 end to end, normalizing ambiguous requests at the front end, rejecting requests with both headers and keeping proxies patched.

Example

A load balancer uses Content-Length while the backend uses chunked encoding, so part of one request is treated as the start of the next.

Exam tip

Root cause is inconsistent request parsing between chained servers.

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