Traefik is an open source HTTP reverse proxy and load balancer. From 2.11.26 until 2.11.57 and 3.7.13, Traefik forwards a client-supplied Connection header requesting Upgrade, the Upgrade: h2c token, and HTTP2-Settings to a shared backend. If the backend accepts h2c and returns 101 Switching Protocols, Traefik enters a raw tunnel and no longer applies routers, BasicAuth, ForwardAuth, IPAllowList, RateLimit, access logging, metrics, or tracing to later HTTP/2 requests, allowing an unauthenticated request through an unprotected route to reach protected paths on the same backend. This issue is fixed in 2.11.57 and 3.7.13.

Affected: Go: github.com/traefik/traefik/v3, Go: github.com/traefik/traefik/v2

## Summary

There is a high-severity request-smuggling vulnerability in Traefik's handling of the HTTP/1.1 `Upgrade` mechanism. Since Traefik moved to unencrypted HTTP/2 with prior knowledge (Go 1.24), a client-initiated `Upgrade: h2c` request header and its connection-specific `HTTP2-Settings` header were forwarded to the backend. A backend that honours the h2c upgrade and answers `101 Switching Protocols` puts Traefik into a raw byte tunnel that bypasses the router and the entire middleware chain (authentication, IPAllowList, rate limiting) on a shared backend. The fix stops forwarding the `Upgrade: h2c` token and the `HTTP2-Settings` header; `Upgrade: websocket` is unaffected. Exploitation requires a backend that upgrades h2c without validating the `Connection` listing; common off-the-shelf servers were not exploitable in testing.

Traefik v3.4.2 through v3.6 are end-of-life and are also affected; users on those versions must upgrade to v3.7.13.

## Patches

- https://github.com/traefik/traefik/releases/tag/v2.11.57 - https://github.com/traefik/traefik/releases/tag/v3.7.13

## 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 HTTP reverse proxy forwards arbitrary `Connection: Upgrade` / `Upgrade: <token>` requests to the backend. Upgrade tokens are not restricted to protocols explicitly supported…

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