CVE-2026-39971
HIGHDescription
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.
How to fix
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N
Exploit Intelligence
Low risk: more likely to be exploited than 17% of all known CVEs.
References
Related Vulnerabilities
Other CWE-113 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2022-37436 | Medium | 5.3 | 58% | - | Fix |
| CVE-2024-52875 | High | 8.8 | 29% | - | - |
| CVE-2016-5699 | Medium | 6.1 | 10% | - | Fix |
| CVE-2016-8024 | High | 8.1 | 8.7% | - | - |
| CVE-2017-17742 | Medium | 5.3 | 5.8% | - | Fix |
| CVE-2016-5325 | Medium | 6.1 | 4.1% | - | Fix |
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-04-23.