## Summary There is a critical vulnerability in Traefik's default HTTP reverse proxy that leads to unauthenticated cross-user response poisoning. When a client opens an HTTP/2 or HTTP/3 `CONNECT` request, Traefik forwards it — body included — to an HTTP/1.1 upstream over a shared `net/http.Transport`. If the upstream answers the CONNECT with a keep-alive non-2xx response without draining the body, the now-desynchronized backend socket is returned to Traefik's shared connection pool and reused for other clients, letting an attacker make a different client read a response the attacker smuggled — which may be another user's authenticated or private content. The entrypoint's `sanitizePath` option (default `true`) is not a reliable defense: backends that answer `CONNECT /` with a keep-alive non-2xx remain exploitable. The experimental FastProxy implementation was not affected. The issue is fixed by deferring the forwarded CONNECT payload until the backend accepts the tunnel, by not returning CONNECT connections to the shared idle pool, and by discarding the CONNECT body in the ForwardAuth path. ## Patches - https://github.com/traefik/traefik/releases/tag/v2.11.53 - https://github.com/traefik/traefik/releases/tag/v3.6.24 - https://github.com/traefik/traefik/releases/tag/v3.7.9 ## For more information If you have any questions or comments about this advisory, please [open an issue](https://github.com/traefik/traefik/issues). <details> <summary>Original Description</summary> ## Summary Traefik's default reverse proxy forwards a plain HTTP/2 or HTTP/3 `CONNECT` request and its body to an HTTP/1.1 upstream through a shared `net/http.Transport`. When the upstream answers the CONNECT with a keep-alive non-2xx response and does not drain the body, Traefik returns the now desynchronized backend socket to its shared pool and reuses it for other clients. An unauthenticated attacker uses this to make a different client read the attacker's smuggled response. Traefik's default proxy is `net/http/httputil.ReverseProxy` over a shared `http.Transport`, so it inherits the same root cause as the Caddy `reverse_proxy` CONNECT pool poisoning. Traefik ships one partial mitigation Caddy does not. The entrypoint option `sanitizePath` (default `true`) rewrites the forwarded CONNECT's empty path to `/`, so Traefik emits `CONNECT /` instead of authority-form `CONNECT host:port`. This is not a reliable defense. It avoids the smuggle only against backends that reject `CONNECT /` by closing the connection (Apache, nginx). Backends that answer `CONNECT /` with a keep-alive non-2xx and leave the body undrained still cross. That set includes any Go `net/http` server and gunicorn/Flask. Confirmed on the official image `traefik:v3.6.23` (a currently supported release), default configuration, against stock `go-httpbin` (Go) and `kennethreitz/httpbin` (Python gunicorn/Flask), attacker and victim in separate containers, over both HTTP/2 and HTTP/3. ## Affected - `traefik:v3.6.23` (official image) and current v3, default configuration, standard proxy to an HTTP/1.1 upstream. Backend keep-alive pooling is on by default (`MaxIdleConnsPerHost` 200). - Attacker frontend is HTTP/2 or HTTP/3. An HTTP/1.1 frontend is not affected. - The upstream keeps the connection alive after a non-2xx to the forwarded CONNECT and does not drain the body. - The experimental FastProxy implementation is not affected (see Not affected). ## Details Three behaviors compose. 1. Traefik forwards a plain CONNECT as an ordinary proxied request. The default proxy is `httputil.ReverseProxy` with a shared `http.Transport` (`pkg/proxy/httputil/proxy.go`). The director assigns the outbound `URL.Host` directly and does not reject CONNECT, leaving the request body a live stream. The client places a raw HTTP/1.1 request in that body (H2/H3 DATA frames), which is written onto the backend socket after the CONNECT header block. 2. `net/http` writes the CONNECT body unframed and pools the socket. For a CONNECT the transport writes the body with no `Content-Length` and no `Transfer-Encoding`. The upstream answers a keep-alive non-2xx and parses the trailing bytes as a pipelined request. Go reads the non-2xx response and returns the socket to the shared idle pool once the request body reaches EOF (the `wroteRequest` gate), while the smuggled request's response is still pending. 3. Desynchronized reuse. The smuggled request targets a slow endpoint so its response arrives after the socket is pooled. A different client that reuses the socket reads the pending smuggled response as its own. `sanitizePath` (default `true`, `pkg/server/server_entrypoint_tcp.go`) calls `req.URL.JoinPath()`, which turns the CONNECT's empty path into `/`. Traefik emits `CONNECT /`. Whether that stops the smuggle depends only on the backend: Apache and nginx answer `400 Bad Request` with `Connection: close` (socket torn down, no cross); Go `net/http` and gunicorn/Flask

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