GHSA-8h9w-w78c-vvr3
Mattermost does not verify remote cluster channel access when processing shared channel membership removals
Packages
| Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|
| Go | github.com/mattermost/mattermost/server/v8 | >= 11.5.0, < 11.5.2 | 11.5.2 |
| Go | github.com/mattermost/mattermost/server/v8 | >= 10.11.0, < 10.11.14 | 10.11.14 |
| Go | github.com/mattermost/mattermost/server/v8 | >= 11.4.0, < 11.4.4 | 11.4.4 |
| Go | github.com/mattermost/mattermost/server/v8 | < 8.0.0-20260216150504-8738f8c4b3d4 | 8.0.0-20260216150504-8738f8c4b3d4 |
| Go | github.com/mattermost/mattermost-server | < 5.3.2-0.20260216150504-8738f8c4b3d4 | 5.3.2-0.20260216150504-8738f8c4b3d4 |
Details
As written in the advisory (Markdown source, shown as plain text).
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.
References
External links from the advisory. They lead to third-party sites.