Better Messages < 3.0.0 - Missing Authorization to Authenticated (Custom+) Chat-Room Transcript Disclosure via '/thread/<id>' REST Endpoint

Description

The Better Messages – Chat Rooms, Group Chat, Private Messages & AI Chat Bots plugin for WordPress is vulnerable to authorization bypass in all versions up to, and including, 2.15.33. This is due to the plugin not properly verifying that a user is authorized to perform an action. This makes it possible for authenticated attackers, with custom-level access and above, to access the full message transcript, thread metadata, and user data of any chat-room thread without authentication. This is only exploitable when the chat room's only_joined_can_read setting retains its default value of '0'.