Summary

The runTailscalePing method of the TailscalePing class injects the hostname parameter inside a shell command, leading to a command injection and the possibility to run arbitrary commands on the server.

Details

When adding a new monitor on Uptime Kuma, we can select the "Tailscale Ping" type. Then we can add a hostname and insert a command injection payload into it. The front-end application requires that the field follow a specific pattern, this validation only happens on the front-end and can be removed by removing the attribute pattern on the input element.

We can finally add the new monitor and observe that our command is being executed.

NOTE: When using Uptime Kuma inside a container, the "TailScale Ping" type is not visible. We can fake this information by intercepting WebSocket messages and set the isContainer option to false.

PoC

• Authenticate.
• Create a new monitor.
• Select the TailScale Ping type (if not visible, see the note in the details section).
• Insert the command injection payload inside the hostname field. (for example $(id >&2))
• Remove the pattern requirement on the field.
• Save and start the monitor.

Impact

An authenticated user can execute arbitrary command on the server running Uptime Kuma.

Remediation

There are other command execution in the codebase, they use a method spawn from the child_process module which does not interpret the command as a shell command, the same thing should be done here.

NOTE: The Tailscale CLI seems to support the -- sequence. It should be used between the ping subcommand and the hostname argument to avoid argument injection.