CVE-2026-28759

Insufficient authorization in shared channel membership sync allows remote cluster to remove users from arbitrary channels

State: Published
Severity: 4.3 Medium
Published: May 18, 2026
Updated: May 18, 2026
Reserved: Mar 10, 2026

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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

Scores

CVSS	Score	Vector	Source
3.1	4.3 Medium	CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N	Mattermost

Weaknesses

CWE-863 Incorrect Authorization

Affected Products

Product	Versions
MattermostMattermost	11.5.0 through 11.5.1Affected 10.11.0 through 10.11.13Affected 11.4.0 through 11.4.3Affected 11.6.0Not affected 11.5.2Not affected 10.11.14Not affected 11.4.4Not affected

References

MMSA-2026-00576vendor-advisory

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