GHSA-8h9w-w78c-vvr3 Mattermost does not verify remote cluster channel access when processing shared channel membership removals

• Go github.com/mattermost/mattermost/server/v8: >= 11.5.0, < 11.5.2 → fixed in 11.5.2 • Go github.com/mattermost/mattermost/server/v8: >= 10.11.0, < 10.11.14 → fixed in 10.11.14 • Go github.com/mattermost/mattermost/server/v8: >= 11.4.0, < 11.4.4 → fixed in 11.4.4 • Go github.com/mattermost/mattermost/server/v8: < 8.0.0-20260216150504-8738f8c4b3d4 → fixed in 8.0.0-20260216150504-8738f8c4b3d4 • Go github.com/mattermost/mattermost-server: < 5.3.2-0.20260216150504-8738f8c4b3d4 → fixed in 5.3.2-0.20260216150504-8738f8c4b3d4

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.