Vulnerability in S9y Serendipity
CVE-2026-39971
Serendipity is a PHP-powered weblog engine. In versions 2.6-beta2 and below, the email sending functionality in include/functions.inc.php inserts $_SERVER['HTTP_HOST'] directly into the Message-ID SMTP header without validation, and the existing sanitization function serendipity_isResponseClean() is not called on HTTP_HOST before embedding it. An attacker who can control the Host header during an email-triggering action such as comment notifications or subscription emails can inject arbitrary SMTP headers into outgoing emails. This enables identity spoofing, reply hijacking via manipulated Message-ID threading, and email reputation abuse through the attacker's domain being embedded in legitimate mail headers. This issue has been fixed in version 2.6.0.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.2 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N.
EPSS exploit prediction
EPSS: 0.003 (20.6th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 21st percentile — 20.6% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.003 · change: +0.000
Affected products
- S9y Serendipity — versions 2.6.0, < 2.6.0
Weakness classification (CWE)
References
- github.com/s9y/Serendipity/security/advisories/GHSA-458g-q4fh-mj6r (Exploit, Mitigation, Vendor Advisory)
- github.com/s9y/Serendipity/releases/tag/2.6.0 (Patch)
Frequently asked questions
- What is CVE-2026-39971?
- CVE-2026-39971 is a high-severity vulnerability in S9y Serendipity, classified under HTTP Response Splitting. CVSS score: 7.2/10. Published 2026-04-15.
- How severe is CVE-2026-39971?
- High severity. CVSS v3 base score is 7.2 out of 10.