EUVD-2026-15805
GHSA-fg35-5rf6-qg3g
CVSS Vector
CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N
Lifecycle Timeline
4Description
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
Analysis
Mattermost fails to properly validate user identity in OpenID Connect authentication logic due to an overly permissive substring matching flaw in the IsSameUser() comparison function, allowing attackers with high privileges to take over arbitrary user accounts through the user discovery flow. This affects Mattermost versions 10.11.0-10.11.11, 11.2.0-11.2.3, 11.3.0-11.3.1, and 11.4.0. …
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Remediation
Within 30 days: Identify affected systems and apply vendor patches as part of regular patch cycle. Monitor vendor channels for patch availability.
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External POC / Exploit Code
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