CVE-2026-27656: Account Takeover via Substring Matching in OpenID Connect Authentication
Published Mar 25, 2026
·Updated
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 Software
5 affected components
Mattermost Mattermost<=11.4.0, <=11.3.1, <=11.2.3, <=10.11.11
Mattermost Mattermost Server>=10.11.0<10.11.12
Mattermost Mattermost Server>=11.2.0<11.2.4
Mattermost Mattermost Server>=11.3.0<11.3.2
Mattermost Mattermost Server>=11.4.0<11.4.1
Remediation
Information
Update Mattermost to versions 11.5.0, 11.4.1, 11.3.2, 11.2.4, 10.11.12 or higher.
Event History
Mar 25, 2026
CVE Published
via MITRE·04:28 PM
Data Sourced
via MITRE·04:28 PM
RemedyDescriptionSeverityWeakness
Data Sourced
via NVD·05:16 PM
DescriptionSeverityWeaknessAffected Software