CVE-2026-27656 Account Takeover via Substring Matching in OpenID Connect Authentication

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...