Mattermost group syncable endpoints allow privilege escalation via scheme_admin

Summary

Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, 10.11.x <= 10.11.16 Mattermost fails to require role-management authorization when setting the scheme_admin flag on group syncable link and patch endpoints, which allows a user with group-link permissions to escalate themselves and group members to team or channel admin via crafted API requests.. Mattermost Advisory ID: MMSA-2026-00665

Timeline

1. Jun 12, 2026, 12:00 AM UTCCISA SSVC assessment

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

2. Jun 12, 2026, 3:54 PM UTCCVE published
3. Jun 13, 2026, 3:56 AM UTCRecord last updated

Affected versions

• Mattermost Mattermost

  From 11.6.0 · through 11.6.1; From 11.5.0 · through 11.5.4; From 10.11.0 · through 10.11.15; From 10.11.0 · through 10.11.16

• github.com/mattermost/mattermost-serverGo

  From 11.6.0 · fixed in 11.6.1

• github.com/mattermost/mattermost-serverGo

  From 11.5.0 · fixed in 11.5.5

• github.com/mattermost/mattermost-serverGo

  From 10.11.0 · fixed in 10.11.17

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

  From 8.0.0-20250731163400-5b955468ea1e · fixed in 8.0.0-20260506065351-202d125afa87

• github.com/mattermost/mattermost-serverGo

  From 10.11.0+incompatible · fixed in 10.11.17+incompatible; From 11.5.0+incompatible · fixed in 11.5.5+incompatible; From 11.6.0+incompatible · fixed in 11.6.1+incompatible

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

  All versions

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

  All versions

References 1