CVE-2026-27656
published 2026-03-25CVE-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…
medium6.1CVSS 3.1
AVNACLPRHUIRSUCHIHAN
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 |