CVE-2026-27656

low-risk
Published 2026-03-25

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

Do I need to act?

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0.03% chance of exploitation
EPSS score — low exploit probability
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Not on CISA KEV list
No confirmed active exploitation reported to CISA
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Patch status unknown
Check vendor advisories for fix availability and mitigation guidance
5
CVSS 5.7/10 Medium
NETWORK / HIGH complexity

Affected Products (1)

Affected Vendors

Mattermost

References (1)

Vendor Advisory https://mattermost.com/security-updates
23
/ 100
low-risk
Severity 18/34 · Moderate
Exploitability 0/34 · Minimal
Exposure 5/34 · Minimal