Login as User <= 1.0.3 - Authenticated (Subscriber+) Privilege Escalation via 'oclaup_original_admin' Cookie

The Login as User plugin for WordPress is vulnerable to Privilege Escalation in all versions up to, and including, 1.0.3. This is due to the handle_return_to_admin() function trusting a client-controlled cookie (oclaup_original_admin) to determine which user to authenticate as, without any server-side verification that the cookie value was legitimately set during an admin-initiated user switch. This makes it possible for authenticated attackers, with Subscriber-level access and above, to escalate their privileges to administrator by setting the oclaup_original_admin cookie to an administrator's user ID and triggering the "Return to Admin" functionality.