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. https://github.com/louislam/uptime-kuma/blob/dc4242019331e65a79ac16deef97510144e01b12/server/monitor-types/tailscale-ping.js#L40-L46 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…Read More