App Builder <= 5.5.10 - Insecure Direct Object Reference to Authenticated (Subscriber+) Arbitrary User Avatar Modification via 'user_id' Parameter

The App Builder – Create Native Android & iOS Apps On The Flight plugin for WordPress is vulnerable to Insecure Direct Object Reference in all versions up to and including 5.6.0. This is due to missing authorization validation in the `upload_avatar()` function, which accepts an attacker-controlled `user_id` parameter from the POST request body and uses it to update user meta without verifying that the authenticated requester owns or has permission to modify the target account. This makes it possible for authenticated attackers, with Subscriber-level access and above, to overwrite the profile avatar of any arbitrary user on the site, including administrators, by supplying a target `user_id` in the request body to the `/wp-json/app-builder/v1/upload-avatar` endpoint.