Account Takeover via Substring Matching in OpenID Connect Authentication

Summary

Mattermost versions 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to properly validate user identity in the OpenID {{IsSameUser()}} comparison logic, which allows an attacker to take over arbitrary user accounts via an overly permissive substring matching flaw in the user discovery flow.. Mattermost Advisory ID: MMSA-2026-00590

Timeline

1. Mar 25, 2026, 4:28 PM UTCCVE published
2. Mar 26, 2026, 3:55 AM UTCCISA SSVC assessment

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

3. Mar 26, 2026, 1:19 PM UTCRecord last updated

Affected versions

• Mattermost Mattermost

  From 11.4.0 · through 11.4.0; From 11.3.0 · through 11.3.1; From 11.2.0 · through 11.2.3; From 10.11.0 · through 10.11.11

• github.com/mattermost/mattermost-serverGo

  From 11.4.0-rc1 · fixed in 11.4.1

• github.com/mattermost/mattermost-serverGo

  From 11.3.0-rc1 · fixed in 11.3.2

• github.com/mattermost/mattermost-serverGo

  From 11.2.0-rc1 · fixed in 11.2.4

• github.com/mattermost/mattermost-serverGo

  From 10.11.0-rc1 · fixed in 10.11.12

• 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 10.11.12+incompatible; From 11.2.0-rc1+incompatible · fixed in 11.2.4+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