CVE-2026-27656

published 2026-03-25

CVE-2026-27656: 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…

Priority P4 33 medium 6.1 CVSS 3.1

AV N AC L PR H UI R S U C H I H A N

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

13 ranges

Vendor	Product	Version range	Fixed in
github.com	mattermost_mattermost-server	>= 10.11.0-rc1 < 10.11.12	10.11.12
github.com	mattermost_mattermost-server	>= 11.2.0-rc1 < 11.2.4	11.2.4
github.com	mattermost_mattermost-server	>= 11.3.0-rc1 < 11.3.2	11.3.2
github.com	mattermost_mattermost-server	>= 11.4.0-rc1 < 11.4.1	11.4.1
github.com	mattermost_mattermost_server_v8	>= 8.0.0-20260105080200-d27a2195068d < 8.0.0-20260217110922-b7d4a1f1f59b	8.0.0-20260217110922-b7d4a1f1f59b
mattermost	mattermost	10.11.0 – 10.11.11	—
mattermost	mattermost	11.2.0 – 11.2.3	—
mattermost	mattermost	11.3.0 – 11.3.1	—
mattermost	mattermost	11.4.0 – 11.4.0	—
mattermost	mattermost_server	>= 10.11.0 < 10.11.12	10.11.12
mattermost	mattermost_server	>= 11.2.0 < 11.2.4	11.2.4
mattermost	mattermost_server	>= 11.3.0 < 11.3.2	11.3.2
mattermost	mattermost_server	>= 11.4.0 < 11.4.1	11.4.1

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CWE-303