CVE-2026-71324
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
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<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,
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Exploitation outlook
- Low exploitation risk0.55% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 44% of all EPSS-scored CVEs.
Advisory coverage (2)
External references
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