Impact

People that configure mTLS between Traefik and clients.

For a request, the TLS configuration choice can be different than the router choice, which implies the use of a wrong TLS configuration.

• When sending a request using FQDN handled by a router configured with a dedicated TLS configuration, the TLS configuration falls back to the default configuration that might not correspond to the configured one.

• If the CNAME flattening is enabled, the selected TLS configuration is the SNI one and the routing uses the CNAME value, so this can skip the expected TLS configuration.

Patches

Traefik v2.6.x: https://github.com/traefik/traefik/releases/tag/v2.6.1

Workarounds

Add the FDQN to the host rule:

Example:

  whoami:
    image: traefik/whoami:v1.7.1
    labels:
      traefik.http.routers.whoami.rule: Host(`whoami.example.com`, `whoami.example.com.`)
      traefik.http.routers.whoami.tls: true
      traefik.http.routers.whoami.tls.options: mtls@file

There is no workaround if the CNAME flattening is enabled.

For more information

If you have any questions or comments about this advisory, please open an issue.