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

CVSS v3 5.7 MEDIUM

5.7^/10

CVSS v3 : MEDIUM

Vector :

Exploitability : 0.5 / Impact : 5.2

Attack Vector NETWORK

Attack Complexity HIGH

Privileges Required HIGH

User Interaction REQUIRED

Confidentiality Impact HIGH

Integrity Impact HIGH

Availability Impact NONE

Scope UNCHANGED

References

Link	Resource
https://mattermost.com/security-updates	Vendor Advisory

Configurations

Configuration 1 (hide)

OR	cpe:2.3:a:mattermost:mattermost_server::::::::
	cpe:2.3:a:mattermost:mattermost_server::::::::
	cpe:2.3:a:mattermost:mattermost_server::::::::
	cpe:2.3:a:mattermost:mattermost_server::::::::

History

26 Mar 2026, 18:51

Type	Values Removed	Values Added
Summary		(es) Las versiones de Mattermost 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 no validan correctamente la identidad del usuario en la lógica de comparación OpenID {{IsSameUser()}}, lo que permite a un atacante tomar el control de cuentas de usuario arbitrarias a través de un fallo de coincidencia de subcadenas excesivamente permisivo en el flujo de descubrimiento de usuarios. ID de aviso de Mattermost: MMSA-2026-00590
First Time		Mattermost Mattermost mattermost Server
References	~~() https://mattermost.com/security-updates -~~	() https://mattermost.com/security-updates - Vendor Advisory
CPE		cpe:2.3:a:mattermost:mattermost_server::::::::

25 Mar 2026, 17:16

Type	Values Removed	Values Added
New CVE

Information

Published : 2026-03-25 17:16

Updated : 2026-03-26 18:51

NVD link : CVE-2026-27656

Mitre link : CVE-2026-27656

CVE.ORG link : CVE-2026-27656

JSON object : View

Products Affected

mattermost

mattermost_server

CWE

CWE-303

Incorrect Implementation of Authentication Algorithm

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