GHSA-fg35-5rf6-qg3gMediumCVSS 5.7

Mattermost allows attackers to take over arbitrary user accounts via overly permissive substring matching flaw

Published
March 25, 2026
Last Modified
March 31, 2026

🔗 CVE IDs covered (1)

CVE-2026-27656 →

📋 Description

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

🎯 Affected products5

  • go/github.com/mattermost/mattermost-server:>= 11.4.0-rc1, < 11.4.1
  • go/github.com/mattermost/mattermost-server:>= 11.3.0-rc1, < 11.3.2
  • go/github.com/mattermost/mattermost-server:>= 11.2.0-rc1, < 11.2.4
  • go/github.com/mattermost/mattermost-server:>= 10.11.0-rc1, < 10.11.2
  • go/github.com/mattermost/mattermost/server/v8:>= 8.0.0-20260105080200-d27a2195068d, < 8.0.0-20260217110922-b7d4a1f1f59b

🔗 References (3)