# CVE-2026-8821

## Summary

- **CVE ID:** CVE-2026-8821
- **Severity:** HIGH
- **CVSS Score:** 7.1 (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N)
- **CWE:** CWE-862
- **Published:** Sep 14, 2026
- **Last Modified:** Sep 14, 2026

## 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

## Affected Products

- Mattermost — Mattermost (11.9.0)
- Mattermost — Mattermost (11.8.0)
- Mattermost — Mattermost (11.7.0)
- Mattermost — Mattermost (10.11.0)
- Mattermost — Mattermost (11.10.0)
- Mattermost — Mattermost (11.9.1)
- Mattermost — Mattermost (11.8.5)
- Mattermost — Mattermost (11.7.8)
- Mattermost — Mattermost (10.11.23)

## References

- [CNA](https://mattermost.com/security-updates)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.17%
- **EPSS Percentile:** 6.8

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-18._