Traefik: Cross-user response poisoning via proxied CONNECT on Traefik's shared backend keep-alive pool
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 `4
