Private room metadata disclosure via unconditional visibility

Information disclosure in Matrix.org Synapse before 1.41.1 allows remote authenticated users to read private room metadata through group APIs. The get_rooms_in_group handler passed allow_private=True to generate_room_entry for every listed room instead of checking whether the requester had joined it. Exploitation requires enable_group_creation=true, the vulnerable homeserver to participate in the room, and knowledge of its room ID; the default setting is false.