CVE-2026-39963: s9y Serendipity

Medium severity, CVSS 6.9. EPSS: 0.3% chance of exploitation in the next 30 days.

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 $_SERVER['HTTP_HOST'] 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.

Affected products

• s9y Serendipity: before 2.6.0 (fixed in 2.6.0); version 2.6.0 only

Published 2026-04-15. Last modified 2026-06-17.