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

> Attackers can manipulate Host headers to scope authentication cookies to malicious domains, enabling session fixation and privilege escalation.

- **Severity:** Medium 6.9 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 14 Apr 2026
- **Updated:** 15 Apr 2026
- **Also known as:** GHSA-4m6c-649p-f6gf

## What is CVE-2026-39963?

CVE-2026-39963 is a medium-severity vulnerability in S9y Serendipity, published on 14 Apr 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official 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.

Weakness: CWE-565 (Reliance on Cookies without Validation and Integrity Checking)

## Which versions are affected by CVE-2026-39963?

2 open source packages ship an affected version, including s9y/serendipity. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `s9y/serendipity` | Packagist | All versions | 2.6.0 |
| `s9y/serendipity` | Packagist | All versions | 2.6.0 |

Products: S9y Serendipity.

## How do I fix CVE-2026-39963?

Upgrade s9y/serendipity to 2.6.0 or later.

## Is CVE-2026-39963 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-39963 exists.

## Other vulnerabilities in S9y Serendipity

- [CVE-2005-1449](https://insights.sbomify.com/CVE-2005-1449/): Other in n/a (critical 10.0)
- [CVE-2005-1452](https://insights.sbomify.com/CVE-2005-1452/): Missing authorization in n/a (critical 10.0)
- [CVE-2020-10964](https://insights.sbomify.com/CVE-2020-10964/): File upload in n/a (critical 9.8)
- [CVE-2011-1134](https://insights.sbomify.com/CVE-2011-1134/): Cross-site scripting in n/a (critical 9.8)
- [CVE-2016-10752](https://insights.sbomify.com/CVE-2016-10752/): File upload in n/a (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-39963 and fix it if it is affected.

What it is: CVE-2026-39963 is a medium-severity vulnerability in S9y Serendipity, published on 14 Apr 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-39963 exists.

Affected packages and fixed versions:
- s9y/serendipity (Packagist): affected all versions; fixed in 2.6.0
- s9y/serendipity (Packagist): affected all versions; fixed in 2.6.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-39963/ (Markdown: https://insights.sbomify.com/CVE-2026-39963/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-39963
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-39963
- OSV: https://osv.dev/vulnerability/CVE-2026-39963

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-39963/
Summaries are generated and may contain mistakes; the official description above is the record.