The ProfileGrid – User Profiles, Groups and Communities plugin for WordPress is vulnerable to Insecure Direct Object Reference in all versions up to, and including, 5.9.4.2 via the pm_messenger_show_messages function due to missing validation on a user controlled key. This makes it possible for authenticated attackers, with Subscriber-level access and above, to read private conversations of other users.
The system's authorization functionality does not prevent one user from gaining access to another user's data or record by modifying the key value identifying the data.