CVE-2026-27656
Mattermost allows attackers to take over arbitrary user accounts via overly permissive substring matching flaw
5.7
MEDIUM
CVSS 3.1
EPSS 0.04%
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
How to fix CVE-2026-27656
To remediate CVE-2026-27656, upgrade the affected package to a fixed version below.
- —upgrade to 11.4.1 or later
- —upgrade to 8.0.0-20260217110922-b7d4a1f1f59b or later
Is CVE-2026-27656 being exploited?
Low — EPSS is 0.0%, meaning exploitation activity has not been observed at scale.
Affected packages (2)
- >= 11.4.0-rc1, < 11.4.1
- >= 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 |