# CVE-2026-39971: Serendipity has a Host Header Injection allows SMTP header injection via unvalidated HTTP_HOST in Message-ID email header

Source: https://junglewise.ai/threats/cve-2026-39971-serendipity-has-a-host-header-injection-allows-smtp-header  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-15. Updated: 2026-05-11  
License: CC BY 4.0 with a link back

CVE-2026-39971 · Severity: high · CVSS 7.2 · Published 2026-04-15

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.

Technologies: [S9y Serendipity](https://junglewise.ai/threats/technologies/s9y-serendipity-1). Vendors: [Packagist](https://junglewise.ai/threats/vendors/packagist), [S9y](https://junglewise.ai/threats/vendors/s9y).

## Quick answers

**Is CVE-2026-39971 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-39971 being exploited in the wild.

**How severe is CVE-2026-39971?** High severity, with a CVSS score of 7.2, published 15 April 2026.

**Which products and versions are affected?** s9y/serendipity (Packagist) < 2.6.0, ECOSYSTEM:introduced=0,fixed=2.6.0.

**Is there a fix for CVE-2026-39971?** Yes. s9y/serendipity (Packagist): upgrade to 2.6.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**Am I affected by CVE-2026-39971?** Check whether you run s9y/serendipity (Packagist) or S9y Serendipity at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- s9y/serendipity (Packagist): upgrade to 2.6.0 or later

  ```sh
  composer require s9y/serendipity:^2.6.0
  ```


## Affected products

- s9y/serendipity (Packagist) < 2.6.0, ECOSYSTEM:introduced=0,fixed=2.6.0
- S9y Serendipity

## Identifiers

- CVE-2026-39971
- GHSA-458g-q4fh-mj6r

## References

- [https://api.github.com/users/mabjr33](https://api.github.com/users/mabjr33)
- [https://github.com/mabjr33](https://github.com/mabjr33)
- [https://api.github.com/users/mabjr33/gists%7B/gist_id%7D](https://api.github.com/users/mabjr33/gists%7B/gist_id%7D)
- [https://api.github.com/users/mabjr33/repos](https://api.github.com/users/mabjr33/repos)
- [https://avatars.githubusercontent.com/u/162270335?v=4](https://avatars.githubusercontent.com/u/162270335?v=4)
- [https://api.github.com/users/mabjr33/events%7B/privacy%7D](https://api.github.com/users/mabjr33/events%7B/privacy%7D)

## Related threats

- [Serendipity Host Header Injection in serendipity_setCookie](https://junglewise.ai/threats/cve-2026-39963-serendipity-host-header-injection-in-serendipity-setcookie)
- [Serendipity 2.5.0 contains a remote code execution vulnerability that allows authenticated administrators to upload…](https://junglewise.ai/threats/cve-2024-58282-serendipity-2-5-0-contains-a-remote-code-execution-vulnerability)