GHSA-g936-7jqj-mwv8: TSDProxy: Internal proxy auth token forwarded to backend services enables management API escalation

A vulnerability was discovered in TSDProxy where it forwards its internal per-process authentication token to all proxied backend services. When identityHeaders is enabled (the default), tsdproxy injects x-tsdproxy-auth-token into every upstream HTTP request alongside user identity headers. This token is the same secret used by the management HTTP server to trust forwarded Tailscale identity claims. A backend that receives this token can replay it from localhost to the management port with an arbitrary x-tsdproxy-id value, bypassing Tailscale authentication entirely.

The token is forwarded unconditionally: ProviderUserMiddleware always calls WhoisNewContext regardless of whether the user is authenticated. In the ReverseProxy.Rewrite function, WhoisFromContext returns ok=true even for zero-value Whois{} (unauthenticated or Funnel requests). The HeaderAuthToken is set for every request when identityHeaders=true.

The attack requires the backend to reach 127.0.0.1:8080. This holds in: (1) non-Docker deployments where tsdproxy and a backend run on the same host, (2) Docker host-network-mode containers, (3) containers sharing tsdproxy’s network namespace.