Serendipity has a Host Header Injection allows SMTP header injection via unvalidated HTTP_HOST in Message-ID email header

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.2 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

EPSS

• CVE-2026-39971: 0.26% estimated exploitation activity in the next 30 days; percentile 17.16 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary Serendipity inserts $SERVER['HTTPHOST'] 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: The existing sanitization function only blocks \r\n and URL-encoded variants: Critically, serendipityisResponseClean() is not even called on HTTPHOST 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 Resulting malicious Message-ID header in outgoing email: Impact An attacker can control the domain portion of the Message-ID header in all outgoing emails sent by…

Published by the source

14 Apr 2026, 22:32 UTC

Last modified by the source

27 Apr 2026, 15:02 UTC

Source last checked

26 Oct 2026, 14:39 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-458g-q4fh-mj6r

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# Patch Next advisory brief: CVE-2026-39971

Advisory: Serendipity has a Host Header Injection allows SMTP header injection via unvalidated HTTP_HOST in Message-ID email header
IDs: CVE-2026-39971, GHSA-458g-q4fh-mj6r
Record: https://patchnext.com/advisories/cve-2026-39971 (revision 1, source checked 26 Oct 2026, 14:39 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: s9y/serendipity 2.6.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist s9y/serendipity: affected < 2.6.0; fixed in 2.6.0

## Suggested inspection (read-only)
- Packagist s9y/serendipity: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on s9y/serendipity.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-458g-q4fh-mj6r (GitHub-reviewed advisory GHSA-458g-q4fh-mj6r lists affected ranges: Packagist s9y/serendipity >=0 <2.6.0)

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