# CVE-2026-39971

## Summary

- **CVE ID:** CVE-2026-39971
- **Severity:** HIGH
- **CVSS Score:** 7.2 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N)
- **CWE:** CWE-113
- **Published:** Apr 14, 2026
- **Last Modified:** Apr 15, 2026

## 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 (< 2.6.0)

## References

- [CNA](https://github.com/s9y/Serendipity/security/advisories/GHSA-458g-q4fh-mj6r)
- [CNA](https://github.com/s9y/Serendipity/releases/tag/2.6.0)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.26%
- **EPSS Percentile:** 17.1

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-10._