# CVE-2026-28759: Insufficient authorization in shared channel membership sync allows remote cluster to remove users from arbitrary channels

> CVE-2026-28759: Insufficient authorization in shared channel membership sync allows remote cluster to remove users from arbitrary channels. CVSS 4.3 (CVSS 3.1).

Canonical: https://rewindex.com/cve/CVE-2026-28759
Last updated: 2026-10-14T20:10:00Z

- Published: 2026-05-18 06:50 UTC · Updated: 2026-10-14 20:10 UTC · Assigner: Mattermost
- CVSS: 4.3 (CVSS 3.1) · Severity: Medium · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N
- EPSS: 0.15%, 4th percentile (scored 2026-10-25)
- CISA KEV: no
- CWE: CWE-863

## Description

Mattermost versions 11.5.x \<= 11.5.1, 10.11.x \<= 10.11.13, 11.4.x \<= 11.4.3 fail to validate that a remote cluster has access to a channel before processing membership removal requests during shared channel membership sync, which allows a malicious remote cluster to remove any user from any channel, including private channels, via crafted membership sync messages targeting channels the remote cluster is not authorized to access. Mattermost Advisory ID: MMSA-2026-00576

## Affected vendors and products

- Mattermost: Mattermost

## Tracked companies

- [Mattermost](https://rewindex.com/company/mattermost.md)

## References

- [CVE record on cve.org](https://www.cve.org/CVERecord?id=CVE-2026-28759)
- [NVD entry](https://nvd.nist.gov/vuln/detail/CVE-2026-28759)
- [https://mattermost.com/security-updates](https://mattermost.com/security-updates)

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_Sources: CVE records from the CVE Program (cve.org). CVE® is a registered trademark of The MITRE Corporation. (https://www.cve.org/) This product uses data from the NVD API but is not endorsed or certified by the NVD. (https://nvd.nist.gov/) Known Exploited Vulnerabilities catalog from CISA. (https://www.cisa.gov/known-exploited-vulnerabilities-catalog) EPSS scores from FIRST (Exploit Prediction Scoring System). (https://www.first.org/epss/)_


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Canonical: https://rewindex.com/cve/CVE-2026-28759 · Markdown: https://rewindex.com/cve/CVE-2026-28759.md · JSON: https://rewindex.com/api/v1/cves/CVE-2026-28759 · Data licensed CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) · Generated 2026-10-26T02:14:32.851Z