CVE-2026-71324: Traefik: Cross-user response poisoning via proxied CONNECT on Traefik's shared backend keep-alive pool

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.

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.