Vulnerability · Published Apr 14, 2026 · Assigned by GitHub_M

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

Medium 6.9

In plain English

A medium-severity security flaw in Serendipity from s9y. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 13% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)6.9Medium

Exploit odds (30 days)0.2%top 86.9% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official 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 packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Packagist	s9y/serendipity	2.6.0	GHSA-4m6c-649p-f6gf

Affected products (as reported)

Vendor	Product	Versions
s9y	Serendipity	< 2.6.0

Weakness: CWE-565 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:L/A:N

References

https://github.com/s9y/Serendipity/security/advisories/GHSA-4m6c-649p-f6gf x_refsource_CONFIRM
https://github.com/s9y/Serendipity/releases/tag/2.6.0 x_refsource_MISC