# CVE-2026-39963: Serendipity Host Header Injection in serendipity_setCookie

Source: https://junglewise.ai/threats/cve-2026-39963-serendipity-host-header-injection-in-serendipity-setcookie  
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-2026-39963 · Severity: medium · CVSS 6.9 · Published 2026-04-15

Serendipity, a popular PHP-based weblog engine, is vulnerable to a security flaw where an attacker can manipulate the domain to which login cookies are sent. By tricking a user or exploiting network configurations, an attacker can cause the system to send sensitive session tokens to a domain they control. This could allow an attacker to hijack user sessions, including administrative accounts, potentially leading to full site takeover.

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

## Quick answers

**Is CVE-2026-39963 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-39963 being exploited in the wild.

**How severe is CVE-2026-39963?** Medium severity, with a CVSS score of 6.9, published 15 April 2026.

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

**Is there a fix for CVE-2026-39963?** 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.

**How is CVE-2026-39963 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-39963?** Check whether you run s9y/serendipity (Packagist), S9y Serendipity or 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
  ```


## Technical details

The `serendipity_setCookie()` function in `include/functions_config.inc.php` uses the unvalidated `$_SERVER['HTTP_HOST']` variable to define the `domain` attribute for session and auto-login cookies. An attacker can perform Host Header Injection by providing a malicious `Host` header during a login request. If successful, the victim's browser will scope sensitive cookies (like `author_token` and `author_autologintoken`) to the attacker's domain. This facilitates session hijacking, session fixation, and credential leakage. The vulnerability is patched in version 2.6.0 by validating the host against the configured site URL.

## Affected products

- s9y Serendipity < 2.6.0

## Identifiers

- CVE-2026-39963
- GHSA-4m6c-649p-f6gf

## Timeline

- 2026-04-13: disclosed
- 2026-04-14: advisory: GitHub Advisory published
- 2026-04-15: patched: NVD publication and release of version 2.6.0

## 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 open redirect in exit.php](https://junglewise.ai/threats/cve-2026-67350-serendipity-open-redirect-in-exit-php)
- [Serendipity privilege escalation via authentication context confusion](https://junglewise.ai/threats/cve-2026-67351-serendipity-privilege-escalation-via-authentication-context)
- [Serendipity has a Host Header Injection allows SMTP header injection via unvalidated HTTP_HOST in Message-ID email header](https://junglewise.ai/threats/cve-2026-39971-serendipity-has-a-host-header-injection-allows-smtp-header)
- [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)
- [Serendipity SQL injection in include/functions_entries.inc.php](https://junglewise.ai/threats/cve-2017-5609-serendipity-sql-injection-in-include-functions-entries-inc-php)