CVE-2026-28759
Medium
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
SeverityMedium
CVSS4.3
Exploit probability—
Product
Mattermost Server
PublishedMay 18, 2026
First seenMay 18, 2026
Weakness typeCWE-863Incorrect authorization
How to fix
Update
- mattermost mattermost_server10.11.14, 11.4.4, 11.5.2 or later
- github.com/mattermost/mattermost/server/v811.5.2, 10.11.14, 11.4.4, 8.0.0-20260216150504-8738f8c4b3d4 or later
- github.com/mattermost/mattermost-server5.3.2-0.20260216150504-8738f8c4b3d4, 10.11.14+incompatible, 11.4.4+incompatible, 11.5.2+incompatible or later
Vendor advisories
Affected products
| Product | Affected versions | Fixed in |
|---|---|---|
| mattermost mattermost_server | 10.11.0 ≤ v < 10.11.14, 11.4.0 ≤ v < 11.4.4, 11.5.0 ≤ v < 11.5.2 | 10.11.14, 11.4.4, 11.5.2 |
| github.com/mattermost/mattermost/server/v8Go | 11.5.0 ≤ v < 11.5.2, 10.11.0 ≤ v < 10.11.14, 11.4.0 ≤ v < 11.4.4, v < 8.0.0-20260216150504-8738f8c4b3d4 | 11.5.2, 10.11.14, 11.4.4, 8.0.0-20260216150504-8738f8c4b3d4 |
| github.com/mattermost/mattermost-serverGo | v < 5.3.2-0.20260216150504-8738f8c4b3d4, 10.11.0+incompatible ≤ v < 10.11.14+incompatible, 11.4.0+incompatible ≤ v < 11.4.4+incompatible, 11.5.0+incompatible ≤ v < 11.5.2+incompatible | 5.3.2-0.20260216150504-8738f8c4b3d4, 10.11.14+incompatible, 11.4.4+incompatible, 11.5.2+incompatible |
| github.com/mattermost/mattermost-server/v5Go | No range specified | — |
| github.com/mattermost/mattermost-server/v6Go | No range specified | — |
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