Vulnerability · Published May 18, 2026 · Assigned by Mattermost
CVE-2026-28759Insufficient authorization in shared channel membership sync allows remote cluster to remove users from arbitrary channels
Medium 4.3
In plain English
A medium-severity incorrect authorization in Mattermost from Mattermost. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 4% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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 packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Go | github.com/mattermost/mattermost-server | 10.11.14+incompatible, 11.4.4+incompatible, 11.5.2+incompatible | GO-2026-5256 |
| Go | github.com/mattermost/mattermost-server | 5.3.2-0.20260216150504-8738f8c4b3d4 | GHSA-8h9w-w78c-vvr3 |
| Go | github.com/mattermost/mattermost-server/v5 | no fix listed | GO-2026-5256 |
| Go | github.com/mattermost/mattermost-server/v6 | no fix listed | GO-2026-5256 |
| Go | github.com/mattermost/mattermost/server/v8 | 11.5.2 | GHSA-8h9w-w78c-vvr3 |
| Go | github.com/mattermost/mattermost/server/v8 | 8.0.0-20260216150504-8738f8c4b3d4 | GO-2026-5256 |
| Go | github.com/mattermost/mattermost/server/v8 | 10.11.14 | GHSA-8h9w-w78c-vvr3 |
| Go | github.com/mattermost/mattermost/server/v8 | 11.4.4 | GHSA-8h9w-w78c-vvr3 |
| Go | github.com/mattermost/mattermost/server/v8 | 8.0.0-20260216150504-8738f8c4b3d4 | GHSA-8h9w-w78c-vvr3 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Mattermost | Mattermost | 11.5.0 – through 11.5.110.11.0 – through 10.11.1311.4.0 – through 11.4.3 |
Weakness: CWE-863 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N
References
- MMSA-2026-00576 vendor-advisory