Serendipity Reliance on Cookies without Validation and Integrity Checking Vulnerability - CVE-2026-39963

Serendipity is a PHP-powered weblog engine. In versions 2.6-beta2 and below the serendipity_setCookie() function in include/functions_config.inc.php uses _SERVER39HTTP_HOST39 without validation as the domain parameter of setcookie(). An attacker who can influence the Host header at login time such as via MITM reverse proxy misconfiguration or load balancer manipulation can force authentication cookies including session tokens and auto-login tokens to be scoped to an attacker-controlled domain. This enables session fixation token leakage to attacker-controlled infrastructure and privilege escalation if an admin logs in under a poisoned Host header. This issue has been fixed in version 2.6.0.