Traefik is an HTTP reverse proxy and load balancer. Prior to version 2.4.13, there exists a potential header vulnerability in Traefik's handling of the Connection header. Active exploitation of this issue is unlikely, as it requires that a removed header would lead to a privilege escalation, however, the Traefik team has addressed this issue to prevent any potential abuse. If one has a chain of Traefik middlewares, and one of them sets a request header, then sending a request with a certain Connection header will cause it to be removed before the request is sent. In this case, the backend does not see the request header. A patch is available in version 2.4.13. There are no known workarounds aside from upgrading.

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

# Impact

There exists a potential header vulnerability in Traefik's handling of the Connection header. Active exploitation of this issue is unlikely, as it requires that a removed header would lead to a privilege escalation, however, the Traefik team has addressed this issue to prevent any potential abuse.

# Details

If you have a chain of Traefik middlewares, and one of them sets a request header `Important-Security-Header`, then sending a request with the following Connection header will cause it to be removed before the request was sent:

``` curl 'https://example.com' -H "Connection: Important-Security-Header" -0 ```

In this case, the backend does not see the request header `Important-Security-Header`.

# Patches

Traefik v2.4.x: https://github.com/traefik/traefik/releases/tag/v2.4.13

# Workarounds

No.

# For more information

If you have any questions or comments about this advisory, [open an issue](https://github.com/traefik/traefik/issues).