# CVE-2026-8821: Playbooks run owner channel membership permission bypass

> Authenticated users can bypass channel membership restrictions to add arbitrary users to restricted channels via playbook run creation.

- **Severity:** High 7.1 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/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:** 14 Sep 2026
- **Updated:** 14 Sep 2026
- **Also known as:** GHSA-wrc8-ccxq-937h, MINI-5vrf-rprg-9hmg, MINI-f9q4-3973-xffg

## What is CVE-2026-8821?

CVE-2026-8821 is a high-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 14 Sep 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Mattermost versions 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 fail to validate channel member-management permission during playbook run creation, allowing an authenticated channel member to add an arbitrary user to a restricted channel via the run owner field.. Mattermost Advisory ID: MMSA-2026-00677

Weakness: CWE-862 (Missing Authorization)

## Which versions are affected by CVE-2026-8821?

It affects 1 product.


Products: Mattermost Mattermost Server.

## How do I fix CVE-2026-8821?

Apply the vendor's patch.

## Is CVE-2026-8821 being exploited?

There is no known exploitation of CVE-2026-8821 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-2026-8821 and fix it if it is affected.

What it is: CVE-2026-8821 is a high-severity missing authorization vulnerability in Mattermost Mattermost Server, published on 14 Sep 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-2026-8821 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-2026-8821/ (Markdown: https://insights.sbomify.com/CVE-2026-8821/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-8821
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-8821
- OSV: https://osv.dev/vulnerability/CVE-2026-8821

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-8821/
Summaries are generated and may contain mistakes; the official description above is the record.