Insufficient authorization in shared channel membership sync allows remote cluster to remove users from arbitrary channels

Summary

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

Timeline

1. May 18, 2026, 6:50 AM UTCCVE published
2. May 18, 2026, 2:35 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: no · Technical impact: partial

3. May 18, 2026, 2:36 PM UTCRecord last updated

Affected versions

• Mattermost Mattermost

  From 11.5.0 · through 11.5.1; From 10.11.0 · through 10.11.13; From 11.4.0 · through 11.4.3

• github.com/mattermost/mattermost/server/v8Go

  From 11.5.0 · fixed in 11.5.2

• github.com/mattermost/mattermost/server/v8Go

  From 10.11.0 · fixed in 10.11.14

• github.com/mattermost/mattermost/server/v8Go

  From 11.4.0 · fixed in 11.4.4

• github.com/mattermost/mattermost/server/v8Go

  fixed in 8.0.0-20260216150504-8738f8c4b3d4

• github.com/mattermost/mattermost-serverGo

  fixed in 5.3.2-0.20260216150504-8738f8c4b3d4

• github.com/mattermost/mattermost-serverGo

  From 10.11.0+incompatible · fixed in 10.11.14+incompatible; From 11.4.0+incompatible · fixed in 11.4.4+incompatible; From 11.5.0+incompatible · fixed in 11.5.2+incompatible

• github.com/mattermost/mattermost-server/v5Go

  All versions

• github.com/mattermost/mattermost-server/v6Go

  All versions

References 1