Race Condition in Guest Magic Link Authentication Allows Token Reuse

Summary

Mattermost versions 10.11.x <= 10.11.12, 11.5.x <= 11.5.0, 11.4.x <= 11.4.2, 11.3.x <= 11.3.2 fail to enforce atomic single-use consumption of guest magic link tokens, which allows an attacker with access to a valid magic link to establish multiple independent authenticated sessions via concurrent requests.. Mattermost Advisory ID: MMSA-2026-00624

Timeline

1. Apr 15, 2026, 11:00 AM UTCCVE published
2. Apr 15, 2026, 2:00 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: yes · Technical impact: partial

3. Apr 15, 2026, 2:00 PM UTCRecord last updated

Affected versions

• Mattermost Mattermost

  From 10.11.0 · through 10.11.12; From 11.5.0 · through 11.5.0; From 11.4.0 · through 11.4.2; From 11.3.0 · through 11.3.2

• github.com/mattermost/mattermost-serverGo

  From 10.11.0-rc1+incompatible · fixed in 10.11.13+incompatible; From 11.3.0-rc1+incompatible · fixed in 11.3.3+incompatible; From 11.4.0-rc1+incompatible · fixed in 11.4.3+incompatible; From 11.5.0-rc1+incompatible · fixed in 11.5.0+incompatible

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

  All versions

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

  All versions

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

  From 8.0.0-20250721062209-4952acea88ce · fixed in 8.0.0-20250723052842-4cb8d8940332

• github.com/mattermost/mattermost-serverGo

  From 10.11.0-rc1 · fixed in 10.11.13

• github.com/mattermost/mattermost-serverGo

  From 11.5.0-rc1 · fixed in 11.5.0

• github.com/mattermost/mattermost-serverGo

  From 11.4.0-rc1 · fixed in 11.4.3

• github.com/mattermost/mattermost-serverGo

  From 11.3.0-rc1 · fixed in 11.3.3

References 1