Vulnerability Description
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.
CVSS Score
HIGH
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| S9Y | Serendipity | < 2.6.0 |
Related Weaknesses (CWE)
References
- https://github.com/s9y/Serendipity/releases/tag/2.6.0Patch
- https://github.com/s9y/Serendipity/security/advisories/GHSA-458g-q4fh-mj6rExploitVendor AdvisoryMitigation
FAQ
What is CVE-2026-39971?
CVE-2026-39971 is a vulnerability with a CVSS score of 7.2 (HIGH). 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 SMT...
How severe is CVE-2026-39971?
CVE-2026-39971 has been rated HIGH with a CVSS base score of 7.2/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-39971?
Check the references section above for vendor advisories and patch information. Affected products include: S9Y Serendipity.