Non-interactive Tailscale SSH sessions on FreeBSD may use the effective group ID of the tailscaled process

A vulnerability identified in the implementation of Tailscale SSH in FreeBSD allowed commands to be run with a higher privilege group ID than that specified by Tailscale SSH access rules.

Affected platforms: FreeBSD

Patched Tailscale client versions: v1.38.2 or later

What happened?

A difference in the behavior of the FreeBSD setgroups system call from POSIX meant that the Tailscale client running on a FreeBSD-based operating system did not appropriately restrict groups on the host when using Tailscale SSH. When accessing a FreeBSD host over Tailscale SSH, the egid of the tailscaled process was used instead of that of the user specified in Tailscale SSH access rules.

Who is affected?

9 tailnets with 22 FreeBSD nodes running Tailscale SSH since Tailscale v1.34 (released on 2022-12-04) may have had Tailscale SSH sessions with a higher privilege group ID than that specified in Tailscale SSH access rules.

We have notified the affected organizations where we have security contacts.

What is the impact?

Tailscale SSH commands may have been run with a higher privilege group ID than that specified in Tailscale SSH access rules if they met all of the following criteria:

• The destination node was a FreeBSD device with Tailscale SSH enabled;
• Tailscale SSH access rules permitted access for non-root users; and
• A non-interactive SSH session was used.

What do I need to do?

If you are running Tailscale on FreeBSD, upgrade to v1.38.2 or later to remediate the issue. Admins of a tailnet can view FreeBSD nodes with unpatched versions in the admin console.

To update the local ports tree in advance of what's available upstream, you can:

1. cd /usr/ports/security/tailscale
2. edit the Makefile to set PORTVERSION to 1.38.2
3. make makesum
4. make install

Tailscale SSH on other platforms is not affected.

Credits

We would like to thank Ryan Belgrave for reporting this issue.

References

• TS-2023-003