Vulnerability in S9y Serendipity
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 $_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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 6.9 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:L/A:N.
EPSS exploit prediction
EPSS: 0.003 (20.3th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 20th percentile — 20.3% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.002 · newest: 0.003 · change: +0.001
Affected products
- S9y Serendipity — versions 2.6.0, < 2.6.0
Weakness classification (CWE)
References
- github.com/s9y/Serendipity/security/advisories/GHSA-4m6c-649p-f6gf (Exploit, Mitigation, Vendor Advisory)
- github.com/s9y/Serendipity/releases/tag/2.6.0 (Product)
Frequently asked questions
- What is CVE-2026-39963?
- CVE-2026-39963 is a medium-severity vulnerability in S9y Serendipity, classified under Reliance on Cookies without Validation and Integrity Checking. CVSS score: 6.9/10. Published 2026-04-15.
- How severe is CVE-2026-39963?
- Medium severity. CVSS v3 base score is 6.9 out of 10.