BuddyPress Groupblog <= 1.9.3 - Authenticated (Subscriber+) Privilege Escalation to Administrator via Group Blog IDOR

Summary

The BuddyPress Groupblog plugin for WordPress is vulnerable to Privilege Escalation in all versions up to, and including, 1.9.3. This is due to the group blog settings handler accepting the `groupblog-blogid`, `default-member`, and `groupblog-silent-add` parameters from user input without proper authorization checks. The `groupblog-blogid` parameter allows any group admin (including Subscribers who create their own group) to associate their group with any blog on the Multisite network, including the main site (blog ID 1). The `default-member` parameter accepts any WordPress role, including `administrator`, without validation against a whitelist. When combined with `groupblog-silent-add`, any user who joins the attacker's group is automatically added to the targeted blog with the injected role. This makes it possible for authenticated attackers, with Subscriber-level access and above, to escalate any user (including themselves via a second account) to Administrator on the main site of the Multisite network.