CVE-2026-76877: WPML Multilingual CMS <= 4.9.7 - Unauthenticated Authentication Bypass in Language-per-Domain SSO via 'wpml_sso_token_domain' Parameter

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 processiframerequest function's language-per-domain SSO handler generating the cross-domain sign-in token via wpcreatenonce('wpmlsso') on the logout landing page, where the browser has already discarded its authentication cookies — so wpgetsessiontoken() 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 wpmlssouser<token><sha256('wpmlsso'.$domain)> transient mapped to the victim's userid 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 isiframerequest() gate conditions (the per-domain SSO started transient and domain-hash derived from the public constant SSONONCE) are fully computable within the 60-second SSOTIMEOUT window, an unauthenticated attacker can submit the forged wpmlssotokendomain parameter with a wpmlusersignedin status to a secondary language-domain endpoint before the legitimate wpmlusersignedout 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.