CVE-2026-27656 MEDIUM

CVE-2026-27656: Account Takeover via Substring Matching in OpenID Connect Authentication

Vendor Mattermost
Product Mattermost
Weakness CWE-303
Published March 25, 2026
Last update March 26, 2026
View on NVD All CVEs

CVSS base score

5.7/10
Attack vector Network
Attack complexity High
Privileges required High
User interaction Required
Confidentiality High
Integrity High

CVSS vector

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

What the vulnerability does

01Description

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

Key dates

02Disclosure timeline

March 25, 2026 CVE published
March 26, 2026 Record updated