CVE-2026-27656

Account Takeover via Substring Matching in OpenID Connect Authentication

CVSS 5.7 MEDIUMEPSS 0.2%CWE-303

Vexday Risk Score

13Low

SSVC decision (CISA)

Track

No exploitation signal → monitor

CVSS 5.7EPSS 0.2%KEV nãoPoC —Nuclei —Metasploit —Patch referenciado

Lifecycle

25 Mar 2026Published on NVD

Recommendation: Monitor — no exploitation signal at the moment.

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

Affected products

Mattermost · Mattermost

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References

https://mattermost.com/security-updates