GHSA-g936-7jqj-mwv8

## 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`

```go // 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:

```bash # 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 ```

## Fix

Remove `HeaderAuthToken` from the outgoing backend request, and guard identity-header injection on `user.ID != ""`:

```go // port.go: only inject headers for actually authenticated users if identityHeaders { if user, ok := model.WhoisFromContext(r.In.Context()); ok && user.ID != "" { r.Out.Header.Set(consts.HeaderID, user.ID) // HeaderAuthToken should NOT be forwarded to backends } } ```

## Impact

An attacker with code execution in any backend proxied by tsdproxy (on the same host) gains full management API control: restart or pause all proxied services (DoS), enumerate all proxy configurations and backend network topology, and trigger webhook deliveries (SSRF via configured webhook URLs).

## Credits

Reported by Vishal Shukla (@shukla304 / @therawdev).

## Sponsorship

This audit is from an AI-assisted research agent at [sechub.dev](https://sechub.dev). Running it on OSS projects is free for maintainers.