CVE-2026-28759
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
- CVSS 3.1
- 4.3
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N - EPSS
- 0.2% chance of exploitation in the next 30 days; higher than 4% of CVEs (scores of 2026-10-25)
- CWE
- CWE-863
- Published
- Modified
- Record from
- the NVD, synced
Affected
- mattermost:mattermost_server
Packages (OSV)
| Advisory | Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|---|
| GHSA-8h9w-w78c-vvr3 | Go | github.com/mattermost/mattermost/server/v8 | >=11.5.0 <11.5.2 | 11.5.2 |
| GHSA-8h9w-w78c-vvr3 | Go | github.com/mattermost/mattermost/server/v8 | >=10.11.0 <10.11.14 | 10.11.14 |
| GHSA-8h9w-w78c-vvr3 | Go | github.com/mattermost/mattermost/server/v8 | >=11.4.0 <11.4.4 | 11.4.4 |
| GHSA-8h9w-w78c-vvr3 | Go | github.com/mattermost/mattermost/server/v8 | <8.0.0-20260216150504-8738f8c4b3d4 | 8.0.0-20260216150504-8738f8c4b3d4 |
| GHSA-8h9w-w78c-vvr3 | Go | github.com/mattermost/mattermost-server | <5.3.2-0.20260216150504-8738f8c4b3d4 | 5.3.2-0.20260216150504-8738f8c4b3d4 |
| GO-2026-5256 | Go | github.com/mattermost/mattermost-server | >=10.11.0+incompatible <10.11.14+incompatible >=11.4.0+incompatible <11.4.4+incompatible >=11.5.0+incompatible <11.5.2+incompatible | 10.11.14+incompatible, 11.4.4+incompatible, 11.5.2+incompatible |
| GO-2026-5256 | Go | github.com/mattermost/mattermost-server/v5 | all versions | no fix published |
| GO-2026-5256 | Go | github.com/mattermost/mattermost-server/v6 | all versions | no fix published |
| GO-2026-5256 | Go | github.com/mattermost/mattermost/server/v8 | <8.0.0-20260216150504-8738f8c4b3d4 | 8.0.0-20260216150504-8738f8c4b3d4 |