Groups <= 3.7.0 - Authenticated (Subscriber+) Insecure Direct Object Reference to Arbitrary Group Join

The Groups plugin for WordPress is vulnerable to Insecure Direct Object Reference in all versions up to, and including, 3.7.0 via the 'group_id' parameter of the group_join function due to missing validation on a user controlled key. This makes it possible for authenticated attackers, with Subscriber-level access and above, to register for groups other than ones set in the shortcode.