CVE-2026-39963 Serendipity: Host Header Injection enables authentication cookie scoping to an attacker-controlled domain
Attackers can manipulate Host headers to scope authentication cookies to malicious domains, enabling session fixation and privilege escalation.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-39963?
CVE-2026-39963 is a medium-severity vulnerability in S9y Serendipity, published on 14 Apr 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
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.
Weakness: CWE-565
Which versions are affected by CVE-2026-39963?
2 open source packages ship an affected version, including s9y/serendipity. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
s9y/serendipity Packagist | All versions | 2.6.0 |
s9y/serendipity Packagist | All versions | 2.6.0 |
Products include S9y Serendipity.
How do I fix CVE-2026-39963?
Upgrade s9y/serendipity to 2.6.0 or later.
Is CVE-2026-39963 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-39963 exists.
Other vulnerabilities in S9y Serendipity
Sources
Also known as GHSA-4m6c-649p-f6gf.