Beszel before 0.19.0 contains an insecure direct object reference vulnerability in the POST and DELETE /api/beszel/user-alerts handlers that allows any authenticated user to create or delete alerts on systems they cannot access. Attackers can supply arbitrary system IDs in the request body to register alert rules and receive notifications disclosing target system names and metrics.

Beszel is a server monitoring platform. Prior to 0.18.7, some API endpoints in the Beszel hub accept a user-supplied system ID and proceed without further checks that the user should have access to that system. As a result, any authenticated user can access these routes for any system if they know the system's ID. System IDs are random 15 character alphanumeric strings, and are not exposed to all users. However, it is theoretically possible for an authenticated user to enumerate a valid system ID via web API. To use the containers endpoints, the user would also need to enumerate a container ID, which is 12 digit hexadecimal string. This vulnerability is fixed in 0.18.7.