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…

PriorityP342high7.2CVSS 3.1

AVNACLPRNUINSCCLILAN

EPSS

0.26%

16.7th percentile

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.

Affected

3 ranges

Vendor	Product	Version range	Fixed in
s9y	serendipity	< 2.6.0	2.6.0
s9y	serendipity	—	—
s9y	serendipity	>= 0 < 2.6.0	2.6.0

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