Tailscale has released version 1.104.4 to fix flaws in its built-in SSH server that could let an attacker reach services and files on other servers that only the root user should be able to touch.

The flaws are in how Tailscale SSH forwards network connections, a job the Tailscale background process carries out with root privileges. To use them, an 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.

The flaws come from a report called Tailworm, published days before the fix by Erica Windisch of Cyberdione Labs, who tested released versions of the Tailscale client and the self-hosted headscale control server in a lab.

Tailscale SSH decides who can connect using a device's identity and the network's policy file, not a password or key, so an attacker does not need stolen credentials, only a foothold on a device the policy already trusts.

The flaws "don't allow an unauthenticated attacker to connect to arbitrary Tailscale SSH servers," Tailscale said, and an attacker can still reach only the machines the policy file already allows.

Tailscale SSH's server runs only on Linux and the open-source macOS build; the attacker's own device can be any platform.

The problem is in socket forwarding. When a permitted SSH session forwarded a connection to a Unix socket on the server, the Tailscale process checked the path but then operated as root. A timing trick let an attacker swap in a different path after that check, defeating the safeguard meant to limit which sockets forwarding could reach.

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