Tailscale Fixes SSH Flaws That Let a Trusted Device Reach Root-Only Services

Tailscale has released version 1.104.4 to fix flaws in its built-in SSH server. The flaws could let an attacker reach services and files on other servers that only the root user should be able to touch. They sit in how Tailscale SSH forwards connections to a Unix socket, a job the Tailscale background process carries out with root privileges. A timing trick let an attacker swap in a different path after the safety check, defeating the safeguard meant to limit which sockets could be reached.

The attack is not open to anyone. The attacker must already control a device that the network's access rules permit to open an SSH session to the target, with forwarding allowed in that rule. Tailscale says the flaws do not allow an unauthenticated attacker to connect to arbitrary servers, and the reach is still limited to machines the policy already allows. The server runs only on Linux and the open-source macOS build. The research, called Tailworm, came from Erica Windisch of Cyberdione Labs. In her lab tests, forwarding to the Docker control socket produced a root shell and exposed the system password file. Tailscale's bulletin describes the reach more narrowly, naming root-only sockets such as the Docker socket or the tailscaled local API.

In 1.104.4, Tailscale now carries out forwarding as the logged-in user instead of root, and relies on the operating system's own file permissions rather than its earlier path check. The release covers 4 security bulletins.