Serendipity has a Host Header Injection allows authentication cookie scoping to attacker-controlled domain in functions_config.inc.php

Exploitation

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

CVSS

6.9 · CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:L/A:N

EPSS

• CVE-2026-39963: 0.22% estimated exploitation activity in the next 30 days; percentile 13.13 (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 The serendipitysetCookie() function uses $SERVER['HTTPHOST'] without validation as the domain parameter of setcookie(). An attacker can force authentication cookies — including session tokens and auto-login tokens — to be scoped to an attacker-controlled domain, facilitating session hijacking. Details In include/functionsconfig.inc.php:726: This function is called during login with sensitive cookies: If an attacker can influence the Host header at login time (e.g. via MITM, reverse proxy misconfiguration, or load balancer), authentication cookies are issued scoped to the attacker's domain instead of the legitimate one. PoC Expected output: Impact - Session fixation — attacker pre-sets a cookie scoped to their domain, then tricks the victim into authenticating, inheriting the poisoned token - Token leakage — authorautologintoken scoped to wrong domain may be sent to attacker-controlled infrastructure - Privilege escalation — if admin logs in under a poisoned Host header, their…

Published by the source

14 Apr 2026, 22:32 UTC

Last modified by the source

15 Apr 2026, 21:14 UTC

Source last checked

26 Oct 2026, 14:39 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-4m6c-649p-f6gf

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# Patch Next advisory brief: CVE-2026-39963

Advisory: Serendipity has a Host Header Injection allows authentication cookie scoping to attacker-controlled domain in functions_config.inc.php
IDs: CVE-2026-39963, GHSA-4m6c-649p-f6gf
Record: https://patchnext.com/advisories/cve-2026-39963 (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: Medium (CVSS 6.9)
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-4m6c-649p-f6gf (GitHub-reviewed advisory GHSA-4m6c-649p-f6gf lists affected ranges: Packagist s9y/serendipity >=0 <2.6.0)

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