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
History

Thu, 26 Mar 2026 19:00:00 +0000

Type Values Removed Values Added
First Time appeared Mattermost mattermost Server
CPEs cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*
Vendors & Products Mattermost mattermost Server

Thu, 26 Mar 2026 14:15:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'total'}, 'version': '2.0.3'}


Thu, 26 Mar 2026 12:00:00 +0000

Type Values Removed Values Added
First Time appeared Mattermost
Mattermost mattermost
Vendors & Products Mattermost
Mattermost mattermost

Wed, 25 Mar 2026 16:45:00 +0000

Type Values Removed Values Added
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
Title Account Takeover via Substring Matching in OpenID Connect Authentication
Weaknesses CWE-303
References
Metrics cvssV3_1

{'score': 5.7, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N'}


cve-icon MITRE

Status: PUBLISHED

Assigner: Mattermost

Published:

Updated: 2026-03-26T13:19:52.338Z

Reserved: 2026-02-23T22:07:32.808Z

Link: CVE-2026-27656

cve-icon Vulnrichment

Updated: 2026-03-26T13:19:49.531Z

cve-icon NVD

Status : Analyzed

Published: 2026-03-25T17:16:56.797

Modified: 2026-03-26T18:51:38.050

Link: CVE-2026-27656

cve-icon Redhat

No data.