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

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'HTTPHOST' directly into the Message-ID SMTP header without validation, and the existing sanitization function serendipityisResponseClean is not...