Vulnerability · Published Apr 14, 2026 · Assigned by GitHub_M

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

High 7.2

In plain English

A high-severity security flaw in Serendipity from s9y. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 17% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)7.2High

Exploit odds (30 days)0.3%top 82.8% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official 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 packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Packagist	s9y/serendipity	2.6.0	GHSA-458g-q4fh-mj6r

Affected products (as reported)

Vendor	Product	Versions
s9y	Serendipity	< 2.6.0

Weakness: CWE-113 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

References

https://github.com/s9y/Serendipity/security/advisories/GHSA-458g-q4fh-mj6r x_refsource_CONFIRM
https://github.com/s9y/Serendipity/releases/tag/2.6.0 x_refsource_MISC