CVE-2026-27656
Medium
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
SeverityMedium
CVSS5.7
Exploit probability—
Product
Mattermost Server
PublishedMar 25, 2026
First seenMar 25, 2026
Weakness typeCWE-303CWE-303
How to fix
Update
- mattermost mattermost_server10.11.12, 11.2.4, 11.3.2, 11.4.1 or later
- github.com/mattermost/mattermost-server11.4.1, 11.3.2, 11.2.4, 10.11.12, 10.11.12+incompatible, 11.2.4+incompatible, 11.3.2+incompatible, 11.4.1+incompatible or later
- github.com/mattermost/mattermost/server/v88.0.0-20260217110922-b7d4a1f1f59b or later
Vendor advisories
Affected products
| Product | Affected versions | Fixed in |
|---|---|---|
| mattermost mattermost_server | 10.11.0 ≤ v < 10.11.12, 11.2.0 ≤ v < 11.2.4, 11.3.0 ≤ v < 11.3.2, 11.4.0 ≤ v < 11.4.1 | 10.11.12, 11.2.4, 11.3.2, 11.4.1 |
| github.com/mattermost/mattermost-serverGo | 11.4.0-rc1 ≤ v < 11.4.1, 11.3.0-rc1 ≤ v < 11.3.2, 11.2.0-rc1 ≤ v < 11.2.4, 10.11.0-rc1 ≤ v < 10.11.12, 10.11.0-rc1+incompatible ≤ v < 10.11.12+incompatible, 11.2.0-rc1+incompatible ≤ v < 11.2.4+incompatible, 11.3.0-rc1+incompatible ≤ v < 11.3.2+incompatible, 11.4.0-rc1+incompatible ≤ v < 11.4.1+incompatible | 11.4.1, 11.3.2, 11.2.4, 10.11.12, 10.11.12+incompatible, 11.2.4+incompatible, 11.3.2+incompatible, 11.4.1+incompatible |
| github.com/mattermost/mattermost/server/v8Go | 8.0.0-20260105080200-d27a2195068d ≤ v < 8.0.0-20260217110922-b7d4a1f1f59b | 8.0.0-20260217110922-b7d4a1f1f59b |
| github.com/mattermost/mattermost-server/v5Go | No range specified | — |
| github.com/mattermost/mattermost-server/v6Go | No range specified | — |
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