CVE-2026-28759
Insufficient authorization in shared channel membership sync allows remote cluster to remove users from arbitrary channels
CVSS Score
4.3
EPSS Score
0.0%
EPSS Percentile
8th
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
| CWE | CWE-863 |
| Vendor | mattermost |
| Product | mattermost |
| Published | May 18, 2026 |
| Last Updated | May 18, 2026 |
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CVSS v3 Breakdown
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N Attack Vector
Network
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
Low
Availability
None
Affected Versions
Mattermost / Mattermost
11.5.0 ≤ 11.5.1 10.11.0 ≤ 10.11.13 11.4.0 ≤ 11.4.3
Credits
daw10