CVE-2026-27656
Account Takeover via Substring Matching in OpenID Connect Authentication
CVSS Score
5.7
EPSS Score
0.0%
EPSS Percentile
7th
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
| CWE | CWE-303 |
| Vendor | mattermost |
| Product | mattermost |
| Published | Mar 25, 2026 |
| Last Updated | Mar 26, 2026 |
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CVSS v3 Breakdown
CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N Attack Vector
Network
Attack Complexity
High
Privileges Required
High
User Interaction
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
None
Affected Versions
Mattermost / Mattermost
11.4.0 ≤ 11.4.0 11.3.0 ≤ 11.3.1 11.2.0 ≤ 11.2.3 10.11.0 ≤ 10.11.11
Credits
Christopher Poile