Guest users can view group member IDs without respecting view restrictions

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 apply view restrictions when retrieving group member IDs, which allows authenticated guest users to enumerate user IDs outside their allowed visibility scope via the group retrieval endpoint.. Mattermost Advisory ID: MMSA-2026-00594

Timeline

1. Mar 26, 2026, 4:23 PM UTCCVE published
2. Mar 26, 2026, 5:37 PM UTCCISA SSVC assessment

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

3. Mar 26, 2026, 5:51 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 · 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+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