CVE-2026-27656

EUVD-2026-15805
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
ProviderTypeBase ScoreAtk. VectorAtk. ComplexityPriv. RequiredVector
NISTPrimary
5.7 MEDIUM
NETWORK
HIGH
HIGH
CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N
MattermostCNA
5.7 MEDIUM
NETWORK
HIGH
HIGH
CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N
Base Score
CVSS 3.x
EPSS Score
Percentile: Unknown
Affected Products (NVD)
VendorProductVersion
mattermostmattermost_server
10.11.0 ≤
𝑥
< 10.11.12
mattermostmattermost_server
11.2.0 ≤
𝑥
< 11.2.4
mattermostmattermost_server
11.3.0 ≤
𝑥
< 11.3.2
mattermostmattermost_server
11.4.0 ≤
𝑥
< 11.4.1
𝑥
= Vulnerable software versions
Common Weakness Enumeration
References
https://mattermost.com/security-updates