{"id":"CVE-2026-39971","aliases":["GHSA-458g-q4fh-mj6r"],"url":"https://o3.security/vulnerability/CVE-2026-39971","summary":"Serendipity: Host Header Injection leads to SMTP header injection via unvalidated HTTP_HOST","details":"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.","published":"2026-04-14T23:35:49.305Z","modified":"2026-08-07T11:49:53.263896676Z","cvss":{"score":7.2,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Packagist","name":"s9y/serendipity","fixedVersion":"2.6.0"}],"fix":null,"references":[{"type":"WEB","url":"https://github.com/s9y/Serendipity/releases/tag/2.6.0"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/39xxx/CVE-2026-39971.json"},{"type":"ADVISORY","url":"https://github.com/s9y/Serendipity/security/advisories/GHSA-458g-q4fh-mj6r"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-39971"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-07T11:49:53.263896676Z"}}