The User Profile Picture plugin for WordPress is vulnerable to Insecure Direct Object Reference in all versions up to, and including, 2.6.1 via the 'rest_api_change_profile_image' function due to missing validation on a user controlled key. This makes it possible for authenticated attackers, with Author-level access and above, to update the profile picture of any user.
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.