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

Severity: critical (CVSS 9.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-08-06 · updated 2026-09-16
Weakness: CWE-444

Description

Traefik is an open source HTTP reverse proxy and load balancer. Prior to 2.11.53, 3.6.24, and 3.7.9, Traefik's default HTTP 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 desynchronized backend socket to its shared pool and reuses it for other clients. An unauthenticated attacker can use this behavior to make a different client read the attacker's smuggled response, which can include authenticated or private content from another request. The ForwardAuth middleware with forwardBody true and preserveRequestMethod true can re-issue a CONNECT with the buffered body attached, exposing the auth-client pool to the same desynchronization. This issue is fixed in 2.11.53, 3.6.24, and 3.7.9.

Affected products

Traefik: before 2.11.53; from 3.0.0 before 3.6.24; from 3.7.0 before 3.7.9 — fixed in 2.11.53, 3.6.24, 3.7.9

How to fix

Upgrade to a fixed version: Traefik 2.11.53, 3.6.24, 3.7.9. Check the vendor advisory for mitigations.

References

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