actix-http has HTTP/1.1 CL.TE Request Smuggling

Summary

A vulnerability in `actix-http`'s HTTP/1.1 request parser allows an unauthenticated remote client to smuggle requests in deployments where a front-end HTTP intermediary and the Actix backend disagree about whether `Content-Length` or `Transfer-Encoding: chunked` defines the request body length. ## Severity **Medium**. This is an HTTP request smuggling vulnerability that can be triggered over the network without application-level credentials. Exploitation requires a specific proxy topology: an upstream proxy, WAF, load balancer, or similar intermediary must use `Content-Length` framing while forwarding the conflicting `Transfer-Encoding: chunked` request to an Actix backend over a reused HTTP/1.1 connection. ## Affected Versions - `actix-http`: versions up to and including **3.12.0** ## Description HTTP/1.1 requests that contain both `Content-Length` and `Transfer-Encoding: chunked` are ambiguous and must be rejected by recipients to avoid request smuggling. Affected versions of `actix-http` accepted a request with a syntactically valid `Content-Length` header and `Transfer-Encoding: chunked` on the same HTTP/1.1 message. The parser then selected chunked decoding instead of rejecting the conflicting framing signals. In a CL.TE proxy topology, an intermediary may treat bytes after the declared `Content-Length` body as part of the first request, while the Actix backend stops at the terminating chunk marker and parses the remaining bytes on the backend connection as a second HTTP request. This creates a backend-side request desynchronization primitive. The issue is limited to HTTP/1.1 request parsing. ## Impact **HTTP request smuggling** * **Attack Vector:** Network, unauthenticated. * **Effect:** Backend request desynchronization with low integrity impact to requests processed by the vulnerable Actix service. * **Scope:** Actix services using affected `actix-http` versions behind an HTTP/1.1 intermediary that forwards ambiguous `Content-Length` plus `Transfer-Encoding: chunked` requests and reuses backend connections. No direct confidentiality, availability, or subsequent-system impact is scored for this advisory. ## Fixed Versions This issue is fixed in **actix-http 3.12.1**. The fix rejects HTTP/1.1 requests that contain both `Content-Length` and `Transfer-Encoding: chunked` instead of choosing one framing interpretation. ## Mitigation Users should upgrade to **actix-http 3.12.1** or later. Applications that depend on `actix-http` through `actix-web`, `awc`, or another Actix crate should ensure dependency resolution selects `actix-http` 3.12.1 or later. For example: ```bash cargo update -p actix-http ``` If an immediate upgrade is not possible, configure all upstream HTTP intermediaries to reject HTTP/1.1 requests that contain both `Content-Length` and `Transfer-Encoding`, and avoid forwarding ambiguous request framing to Actix backends. ## Credits Actix thanks [mufeedvh](https://github.com/mufeedvh) who disclosed this issue through coordinated disclosure.