# CVE-2026-96259: Mattermost server-side request forgery via OAuth endpoints configurable by a System Administrator

> A System Administrator can exploit missing internal connection filters in OAuth endpoints to perform server-side request forgery and read internal network responses.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 22 Sep 2026
- **Updated:** 23 Sep 2026
- **Also known as:** GHSA-j85c-qqpv-57m3

## What is CVE-2026-96259?

CVE-2026-96259 is a medium-severity server-side request forgery vulnerability in Mattermost Mattermost Server, published on 22 Sep 2026. An attacker needs administrator access to exploit it, with no action from the victim.

### Official description

Mattermost versions 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10, 11.10.x <= 11.10.1 fail to apply the internal-connection filter to OAuth endpoint requests, which allows a System Administrator to make the server issue requests to internal network addresses and read the responses via the configured OAuth token and userinfo endpoints.. Mattermost Advisory ID: MMSA-2026-00776

Weakness: CWE-918 (Server-Side Request Forgery)

## Which versions are affected by CVE-2026-96259?

It affects 1 product.


Products: Mattermost Mattermost Server.

## How do I fix CVE-2026-96259?

No fix has been published for CVE-2026-96259 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-96259 being exploited?

There is no known exploitation of CVE-2026-96259 in the wild and no public exploit code.

## Other vulnerabilities in Mattermost Mattermost Server

- [CVE-2025-12419](https://insights.sbomify.com/CVE-2025-12419/): Account takeover on OAuth/OpenID-enabled servers (critical 9.9)
- [CVE-2025-12421](https://insights.sbomify.com/CVE-2025-12421/): Account Takeover via Code Exchange Endpoint (critical 9.9)
- [CVE-2025-4981](https://insights.sbomify.com/CVE-2025-4981/): Path Traversal Leading to RCE by Any Authenticated Mattermost User (critical 9.9)
- [CVE-2025-20051](https://insights.sbomify.com/CVE-2025-20051/): Arbitrary file read via block duplication in Mattermost Boards (critical 9.9)
- [CVE-2025-25279](https://insights.sbomify.com/CVE-2025-25279/): Arbitrary file read in Mattermost Boards via import & export board archive (critical 9.9)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-96259 and fix it if it is affected.

What it is: CVE-2026-96259 is a medium-severity server-side request forgery vulnerability in Mattermost Mattermost Server, published on 22 Sep 2026. An attacker needs administrator access to exploit it, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-96259 in the wild and no public exploit code.

Affected product: Mattermost Mattermost Server. No package-level version data is available, so check the vendor advisory linked from the reference page.

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-2026-96259/ (Markdown: https://insights.sbomify.com/CVE-2026-96259/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-96259
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-96259
- OSV: https://osv.dev/vulnerability/CVE-2026-96259

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-96259/
Summaries are generated and may contain mistakes; the official description above is the record.