CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination.

HTTP requests or responses ("messages") can be malformed or unexpected in ways that cause web servers or clients to interpret the messages in different ways than intermediary HTTP agents such as load balancers, reverse proxies, web caching proxies, application firewalls, etc. For example, an adversary may be able to add duplicate or different header fields that a client or server might interpret as one set of messages, whereas the intermediary might interpret the same sequence of bytes as a different set of messages. For example, discrepancies can arise in how to handle duplicate headers like two Transfer-encoding (TE) or two Content-length (CL), or the malicious HTTP message will have different headers for TE and CL. The inconsistent parsing and interpretation of messages can allow the adversary to "smuggle" a message to the client/server without the intermediary being aware of it. This weakness is usually the result of the usage of outdated or incompatible HTTP protocol versions in the HTTP agents.

Consequences

• Integrity, Non-Repudiation, Access Control: Unexpected State, Hide Activities, Bypass Protection Mechanism

How to prevent it

• Implementation: Use a web server that employs a strict HTTP parsing procedure, such as Apache [REF-433].
• Implementation: Use only SSL communication.
• Implementation: Terminate the client session after each request.
• System Configuration: Turn all pages to non-cacheable.

Recent advisories

• EEF-CVE-2026-114216 — Chunk extensions accept bare LF and CR, enabling request smuggling in Bandit
• RUSTSEC-2026-0388 — Pingora HTTP/2 :authority and Host header mismatch enables cache poisoning and host-based access control bypass
• GHSA-pqxq-w44r-73xw — AIOHTTP: Pure-Python parser request smuggling
• GHSA-rv87-ccxw-8ggf — Microsoft Security Advisory CVE-2026-88092 - ASP.NET Core Security Feature Bypass Vulnerability
• GHSA-fpq8-4vhr-rcxx — Microsoft Security Advisory CVE-2026-70188 - ASP.NET Core Security Feature Bypass Vulnerability
• GO-2026-6613 — HTTP/1 server connection desynchronization after 2xx CONNECT response in net/http
• GO-2026-6605 — HTTP/1 client connection desynchronization after CONNECT rejection in net/http
• GO-2026-6610 — HTTP/2 transport accepts malformed framing-related headers in net/http
• EEF-CVE-2026-94194 — Mint HTTP/1 client applies chunked framing when chunked is not the final transfer coding, enabling response smuggling through intermediaries
• EEF-CVE-2026-82672 — Unvalidated chunk-size line tail in Mint HTTP/1 client enables response smuggling against strict intermediaries on pooled connections
• GHSA-f52w-8j3h-j724 — Traefik: Rootless HTTP/1 request-target routes as "/" but is forwarded verbatim, bypassing path-scoped routing, middleware guards and access logging
• GHSA-w4v4-9rw7-5326 — Traefik: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling') and Incorrect Authorization
• GHSA-r53p-7pc4-xj5r — undici vulnerable to downstream response splitting via retry interceptor
• GHSA-pf96-p4fj-6566 — HTTPX2: Conflicting Content-Length and Transfer-Encoding headers can be auto-generated
• GHSA-crvj-82cr-hjcx — Hono: Query parser reads parameters after the URL fragment, causing cache-key and proxy interpretation differentials

All advisories for CWE-444