CVE-2026-27656: Mattermost allows attackers to take over arbitrary user accounts via overly permissive substring matching flaw

March 25, 2026 (updated March 31, 2026)

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

References

github.com/advisories/GHSA-fg35-5rf6-qg3g
github.com/mattermost/mattermost
mattermost.com/security-updates
nvd.nist.gov/vuln/detail/CVE-2026-27656

Code Behaviors & Features

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