{"id":"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.","aliases":["GHSA-458g-q4fh-mj6r"],"modified":"2026-08-12T03:51:45.383078601Z","published":"2026-04-14T23:35:49.305Z","database_specific":{"cna_assigner":"GitHub_M","cwe_ids":["CWE-113"],"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/39xxx/CVE-2026-39971.json"},"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"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/s9y/serendipity","events":[{"introduced":"0"},{"fixed":"87696c92730d75785d17b515125be3862cdfb49e"}],"database_specific":{"extracted_events":[{"introduced":"0"},{"fixed":"2.6.0"}],"source":["AFFECTED_FIELD","CPE_RANGE","REFERENCES"],"cpe":"cpe:2.3:a:s9y:serendipity:*:*:*:*:*:*:*:*"}}],"versions":["2.6-beta1","2.5.0","2.5-beta1","2.4.0","2.4-beta1","2.3.0","2.3-rc1","2.3-beta1","2.2.1-alpha1","2.1.1","2.1.0","2.1-rc1","2.1-beta3","2.1-beta2","2.1-beta1"],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-39971.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N"}]}