User Registration & Membership < 5.1.6 - Unauthenticated Missing Authorization to Admin Approval Bypass via 'action' Parameter

Description

The User Registration & Membership plugin for WordPress is vulnerable to Missing Authorization in all versions up to, and including, 5.1.5. This is due to the is_admin_creation_process() method relying solely on the presence of action=createuser in the $_REQUEST superglobal without performing any authentication or capability check. This makes it possible for unauthenticated attackers to bypass the admin approval requirement when registering new accounts via the fallback submission path.