# CVE-2026-9859: Missing authorization in Mattermost

> Authenticated board editors can expose boards to arbitrary channels due to missing role-based authorization checks on the batch endpoint.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 17 Aug 2026
- **Updated:** 18 Aug 2026
- **Also known as:** GHSA-27x7-qjjm-4f83, MINI-fqhh-9xh8-qh4v, MINI-x29h-rc66-2p37

## What is CVE-2026-9859?

CVE-2026-9859 is a medium-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 17 Aug 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to enforce PermissionManageBoardRoles on the channelId field of the batch endpoint, which allows an authenticated board editor to relink any board they can edit to an arbitrary channel via a crafted PATCH request. Mattermost Advisory ID: MMSA-2026-00686

Weakness: CWE-863 (Incorrect Authorization)

## Which versions are affected by CVE-2026-9859?

It affects 1 product.


Products: Mattermost Mattermost Server.

## How do I fix CVE-2026-9859?

Apply the vendor's patch.

## Is CVE-2026-9859 being exploited?

There is no known exploitation of CVE-2026-9859 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-2026-9859 and fix it if it is affected.

What it is: CVE-2026-9859 is a medium-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 17 Aug 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-9859 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-2026-9859/ (Markdown: https://insights.sbomify.com/CVE-2026-9859/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-9859
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-9859
- OSV: https://osv.dev/vulnerability/CVE-2026-9859

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-9859/
Summaries are generated and may contain mistakes; the official description above is the record.