Insufficient authorization in shared channel membership sync grants team-level access instead of channel-level access

Summary

Mattermost versions 11.2.x <= 11.2.2, 10.11.x <= 10.11.10, 11.4.x <= 11.4.0, 11.3.x <= 11.3.1 fail to restrict team-level access when processing membership sync from a remote cluster, which allows a malicious remote cluster to grant a user access to an entire private team instead of only the shared channel via sending crafted membership sync messages that trigger team membership assignment. Mattermost Advisory ID: MMSA-2026-00574

Timeline

1. Mar 26, 2026, 10:43 AM UTCCVE published
2. Mar 26, 2026, 1:58 PM UTCCISA SSVC assessment

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

3. Mar 26, 2026, 1:58 PM UTCRecord last updated

Affected versions

• Mattermost Mattermost

  From 11.2.0 · through 11.2.2; From 10.11.0 · through 10.11.10; From 11.4.0 · through 11.4.0; From 11.3.0 · through 11.3.1

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

  From 11.4.0-rc1 · fixed in 11.4.1

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

  From 11.3.0-rc1 · fixed in 11.3.2

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

  From 11.2.0-rc1 · fixed in 11.2.3

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

  From 10.11.0-rc1 · fixed in 10.11.11

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

  From 8.0.0-20260105080200-d27a2195068d · fixed in 8.0.0-20260217110922-b7d4a1f1f59b

• github.com/mattermost/mattermost-serverGo

  From 10.11.0-rc1+incompatible · fixed in 10.11.11+incompatible; From 11.2.0-rc1+incompatible · fixed in 11.2.3+incompatible; From 11.3.0-rc1+incompatible · fixed in 11.3.2+incompatible; From 11.4.0-rc1+incompatible · fixed in 11.4.1+incompatible

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

  All versions

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

  All versions

References 1