TSDProxy Internal Authentication Token Vulnerability Leading to Management API Escalation

A critical vulnerability (GHSA-g936-7jqj-mwv8) has been identified in TSDProxy, a Tailscale proxy service. This vulnerability arises because TSDProxy, by default, forwards its internal per-process authentication token (x-tsdproxy-auth-token) to all proxied backend services when the identityHeaders feature is enabled. This token is crucial for the management HTTP server to trust forwarded Tailscale identity claims. An attacker who has achieved code execution on a backend service co-located on the same host or within the same network namespace as TSDProxy can capture this token. By replaying the stolen token, along with a crafted x-tsdproxy-id header, to TSDProxy's local management API (typically on 127.0.0.1:8080), the attacker can bypass authentication entirely. This grants them full control over the TSDProxy management API, enabling a range of malicious actions, including service disruption and sensitive information exposure. The vulnerability affects go/github.com/almeidapaulopt/tsdproxy versions prior to 1.4.4-0.20260603142855-434819b4421e.

Attack Chain

1. TSDProxy is deployed with the identityHeaders configuration enabled (this is the default setting).
2. A backend service is deployed on the same host as TSDProxy, within a Docker container in host-network-mode, or in a container sharing TSDProxy's network namespace.
3. An attacker successfully gains code execution on the co-located backend service.
4. As TSDProxy forwards requests to the compromised backend, it unconditionally injects its internal x-tsdproxy-auth-token into the HTTP request headers.
5. The attacker, having control of the backend, captures the x-tsdproxy-auth-token from the incoming HTTP request headers.
6. From the compromised backend, the attacker crafts and executes an HTTP request (e.g., using curl) to the TSDProxy management API at 127.0.0.1:8080. This request includes the stolen x-tsdproxy-auth-token and an arbitrary x-tsdproxy-id header.
7. TSDProxy's management API, designed to trust requests from localhost with a valid x-tsdproxy-auth-token, grants the attacker full administrative control without requiring proper Tailscale authentication.

Impact

Successful exploitation of this vulnerability leads to full compromise of the TSDProxy management API. An attacker can leverage this access to perform various damaging actions, including restarting or pausing all proxied services, effectively causing a Denial of Service (DoS) condition. They can also enumerate all proxy configurations, exposing sensitive details about the backend network topology and service configurations. Furthermore, the attacker can trigger webhook deliveries configured within TSDProxy, potentially leading to Server-Side Request Forgery (SSRF) if webhook URLs are external or sensitive.

Recommendation

• Update go/github.com/almeidapaulopt/tsdproxy to version 1.4.4-0.20260603142855-434819b4421e or newer immediately, as outlined in the GitHub Advisory GHSA-g936-7jqj-mwv8.
• Deploy the Sigma rule "Detect TSDProxy Management API Authentication Bypass Attempt" to your SIEM solution to detect suspicious process creations attempting to exploit this vulnerability.
• Implement network monitoring for outbound HTTP connections originating from backend processes and destined for 127.0.0.1:8080.
• Block connections to 127.0.0.1:8080 from untrusted or non-whitelisted processes if possible, to mitigate unauthorized access to the TSDProxy management API.