CVE-2026-39963: Serendipity: Host Header Injection enables authentication cookie scoping to an attacker-controlled domain

Severity: medium (CVSS 6.9 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-04-15 · updated 2026-10-26
Weakness: CWE-565

Description

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; 2.6.0-beta1 — fixed in 2.6.0

How to fix

Upgrade to a fixed version: s9y Serendipity 2.6.0. Check the vendor advisory for mitigations.

References

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