CVE-2026-27656

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

Scoring

Severity

MEDIUM

CVSS base score

5.7

CVSS vector

CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N

EPSS probability

0.18%

CWE

CWE-303

Published

2026-03-25

Last modified

2026-03-26

Affected products

• Mattermost Mattermost
• Mattermost Mattermost
• Mattermost Mattermost
• Mattermost Mattermost
• Mattermost Mattermost
• Mattermost Mattermost
• Mattermost Mattermost
• Mattermost Mattermost

Weakness type

• CWE-303

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