HTTP request smuggling

HTTP
Persistence Compression HTTPS QUIC
Request methods
OPTIONS GET HEAD POST PUT DELETE TRACE CONNECT PATCH
Header fields
Cookie ETag Location HTTP referer DNT X-Forwarded-For
Response status codes
301 Moved Permanently 302 Found 303 See Other 403 Forbidden 404 Not Found 451 Unavailable for Legal Reasons
Security access control methods
Basic access authentication Digest access authentication
Security vulnerabilities
HTTP header injection HTTP request smuggling HTTP response splitting HTTP parameter pollution
v t e

HTTP request smuggling (HRS) is a security exploit on the HTTP protocol that takes advantage of an inconsistency between the interpretation of Content-Length and Transfer-Encoding headers between HTTP server implementations in an HTTP proxy server chain.^[1][2] It was first documented in 2005 by Linhart et al.^[3]

The Transfer-Encoding header works by defining a directive on how to interpret the body of the HTTP request, with the common and necessary directive for this attack being the chunked transfer encoding.^[4] When the Transfer-Encoding header is present, the Content-Length header is supposed to be omitted.^[4] Working similarly but with a different syntax, the Content-Length header works by specifying the size in bytes of the body as a value in the header itself. ^[5] Vulnerabilities arise when both of these headers are included in a malicious HTTP request, bypassing security functions meant to prevent malicious HTTP queries to the server by causing either the front-end or back-end server to incorrectly interpret the request. ^[6] HTTP request smuggling commonly takes the form of CL.TE, TE.CL, or TE.TE, although more complex attacks using HRS do exist. ^[6]