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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

PUBLISHED Reserved 2026-02-23 | Published 2026-03-25 | Updated 2026-03-26 | Assigner Mattermost




MEDIUM: 5.7CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N

Problem types

CWE-303: Incorrect Implementation of Authentication Algorithm

Product status

Default status
unaffected

11.4.0 (semver)
affected

11.3.0 (semver)
affected

11.2.0 (semver)
affected

10.11.0 (semver)
affected

11.5.0
unaffected

11.4.1
unaffected

11.3.2
unaffected

11.2.4
unaffected

10.11.12
unaffected

Credits

Christopher Poile finder

References

mattermost.com/security-updates (MMSA-2026-00590) vendor-advisory

cve.org (CVE-2026-27656)

nvd.nist.gov (CVE-2026-27656)

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