WPML plugin: a logout race lets attackers log in as other users

The WPML Multilingual CMS plugin for WordPress is vulnerable to Authentication Bypass via Race Condition in versions up to, and including, 4.9.7. This is due to the process_iframe_request function's language-per-domain SSO handler generating the cross-domain sign-in token via wp_create_nonce('wpml_sso') on the logout landing page, where the browser has already discarded its authentication cookies — so wp_get_session_token() returns an empty string and the token collapses to the shared anonymous nonce for the current 12-hour tick rather than a user-bound secret — and then storing it in the wpml_sso_user_<token>_<sha256('wpml_sso'.$domain)> transient mapped to the victim's user_id with no binding to the specific sign-out action or requesting client. Because an unauthenticated attacker can independently derive the identical token by triggering the logout confirmation flow on wp-login.php?action=logout, and because the is_iframe_request() gate conditions (the per-domain SSO started transient and domain-hash derived from the public constant SSO_NONCE) are fully computable within the 60-second SSO_TIMEOUT window, an unauthenticated attacker can submit the forged parameter with a status to a secondary language-domain endpoint before the legitimate iframe fires, causing WordPress authentication cookies to be issued for the victim. This makes it possible for unauthenticated attackers to gain access to accounts of users — including administrators — who are actively in the process of logging out during the race window.