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

Severity: medium (CVSS 6.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-03-25 · updated 2026-06-17
Weakness: CWE-303

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

Affected products

Mattermost: from 10.11.0 before 10.11.12; from 11.2.0 before 11.2.4; from 11.3.0 before 11.3.2; from 11.4.0 before 11.4.1 — fixed in 10.11.12, 11.2.4, 11.3.2, 11.4.1

How to fix

Upgrade to a fixed version: Mattermost 10.11.12, 11.2.4, 11.3.2, 11.4.1. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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