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 products
| Vendor | Product | Affected | Fixed |
|---|---|---|---|
| Mattermost | Mattermost | 11.5.0 – ≤ 11.5.1, 10.11.0 – ≤ 10.11.13, 11.4.0 – ≤ 11.4.3 | 11.6.0 |
Fixed in Linux distributions
Distributions backport fixes, so the version that fixes this vulnerability differs per release — upgrade to at least the version shown. From the distributions' security trackers and advisories.
openSUSE
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| openSUSE Leap 16.0 | govulncheck-vulndb | 0.0.20260723T184607-160000.1.1 | openSUSE-SU-2026:21483-1 |
Affected packages
| Ecosystem | Package | Affected | Fixed | Record |
|---|---|---|---|---|
| Go | github.com/mattermost/mattermost-server | < 5.3.2-0.20260216150504-8738f8c4b3d4 | 5.3.2-0.20260216150504-8738f8c4b3d4 | GHSA-8h9w-w78c-vvr3 |
| 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 | — | GO-2026-5256 |
| Go | github.com/mattermost/mattermost-server/v6 | all versions | — | GO-2026-5256 |
| 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 |
Advisories 1
- openSUSE-SU-2026:21483-1 Security update for govulncheck-vulndb SUSE / openSUSE Security Updates · Jul 31, 2026
References
- MMSA-2026-00576 vendor-advisory