---
title: "CVE-2026-8821: Mattermost - Broken Access Control"
description: "Mattermost 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 contains a broken access control caused by failure to validate channel member-management permission during playbook run creation, letting authenticated channel members add arbitrary users to restricted channels via the run owner field, exploit requires authentication."
url: https://panop.io/threat-heartbeat/CVE-2026-8821/
---

# CVE-2026-8821 Mattermost - Broken Access Control

Mattermost 11.9.x <= 11.9.0, 11.8.x <= 11.8.4, 11.7.x <= 11.7.7, 10.11.x <= 10.11.22 contains a broken access control caused by failure to validate channel member-management permission during playbook run creation, letting authenticated channel members add arbitrary users to restricted channels via the run owner field, exploit requires authentication.

CVSS base score 7.1 /10

High

CISA KEV

Not listed

Public exploit

None known

Authentication

Required

EPSS

0.29% chance of exploitation in 30 days

[View the NVD record ↗](https://nvd.nist.gov/vuln/detail/CVE-2026-8821)

## Details

Type

Broken Access Control

Impact

Broken Access Control

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N

Published

14 September 2026

Updated

15 October 2026

Weakness

[CWE-862](https://cwe.mitre.org/data/definitions/862.html)

## Vendor fix

Update to versions later than 11.9.0, 11.8.4, 11.7.7, or 10.11.22 or the latest available version.

Patching closes this CVE. It does not tell you which of your systems run the affected version, or what else is exposed alongside them.

## References

- [https://nvd.nist.gov/vuln/detail/CVE-2026-8821](https://nvd.nist.gov/vuln/detail/CVE-2026-8821)
- [https://mattermost.com/security-updates](https://mattermost.com/security-updates)

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## What this record can't tell you

- Whether you run the affected software, and where
- Whether an attacker can actually reach and exploit it
- The misconfigurations that never get a CVE number
- What it would cost the business, ranked into what to fix first

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