CVE-2026-27656
MEDIUM5.7EPSS 0.04%Mattermost allows attackers to take over arbitrary user accounts via overly permissive substring matching flaw
Published: 3/25/2026Modified: 3/31/2026
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
Affected packages (2)
- Go/github.com/mattermost/mattermost-server>= 11.4.0-rc1, < 11.4.1
- Go/github.com/mattermost/mattermost/server/v8>= 8.0.0-20260105080200-d27a2195068d, < 8.0.0-20260217110922-b7d4a1f1f59b
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 3.1 | MEDIUM5.7 | CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N |