Ultimate Member <= 2.6.6 - Privilege Escalation via Arbitrary User Meta Updates

The Ultimate Member plugin for WordPress is vulnerable to privilege escalation in versions up to, and including, 2.6.6. This is due to the plugin using a predefined list of user meta keys that are banned which can be bypassed via a few method like adding slashes to the user meta key. This makes it possible for unauthenticated attackers to register on a site as an administrator.