User Registration & Membership < 5.2.3 - Unauthenticated Privilege Escalation via Unbound members_data Membership ID

The User Registration & Membership WordPress plugin before 5.2.3 does not validate that the membership tier submitted during public registration is one of the tiers allowed by the registration form before assigning that tier's associated user role, allowing unauthenticated users to register into an arbitrary published membership tier and obtain its role — up to administrator when such a tier exists.