An authentication bypass vulnerability exists in the WordPress Pie Register plugin ≤ 3.7.1.4 that allows unauthenticated attackers to impersonate arbitrary users by submitting a crafted POST request to the login endpoint. By setting social_site=true and manipulating the user_id_social_site parameter, an attacker can generate a valid WordPress session cookie for any user ID, including administrators. Once authenticated, the attacker may exploit plugin upload functionality to install a malicious plugin containing arbitrary PHP code, resulting in remote code execution on the underlying server.