GHSA-fg35-5rf6-qg3g Mattermost allows attackers to take over arbitrary user accounts via overly permissive substring matching flaw

• Go github.com/mattermost/mattermost-server: >= 11.4.0-rc1, < 11.4.1 → fixed in 11.4.1 • Go github.com/mattermost/mattermost-server: >= 11.3.0-rc1, < 11.3.2 → fixed in 11.3.2 • Go github.com/mattermost/mattermost-server: >= 11.2.0-rc1, < 11.2.4 → fixed in 11.2.4 • Go github.com/mattermost/mattermost-server: >= 10.11.0-rc1, < 10.11.12 → fixed in 10.11.12 • Go github.com/mattermost/mattermost/server/v8: >= 8.0.0-20260105080200-d27a2195068d, < 8.0.0-20260217110922-b7d4a1f1f59b → fixed in 8.0.0-20260217110922-b7d4a1f1f59b

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