CVE-2026-39971: Serendipity: Host Header Injection leads to SMTP header injection via unvalidated HTTP_HOST

Severity: high (CVSS 7.2 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-04-15 · updated 2026-10-26
Weakness: CWE-113

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.

Affected products

s9y Serendipity: before 2.6.0; 2.6.0-beta1 — fixed in 2.6.0

How to fix

Upgrade to a fixed version: s9y Serendipity 2.6.0. Check the vendor advisory for mitigations.

References

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