Medium severity · published Mar 25, 2026
CVE-2026-27656
Account Takeover via Substring Matching in OpenID Connect Authentication
6.1
CVSS
0.2%
EPSS
No
On KEV
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
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:U/C:H/I:H/A:N
Attack vector
Network
Complexity
Low
Privileges
High
User interaction
Required
Confidentiality
High
Integrity
High
Availability
None
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.2% · higher than 8% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
MA
Mattermost · Mattermost
11.4.0 through 11.4.0; 11.3.0 through 11.3.1; 11.2.0 through 11.2.3; 10.11.0 through 10.11.11 · source: vendor
MS
Mattermost Server · Mattermost
10.11.0 before 10.11.12; 11.2.0 before 11.2.4; 11.3.0 before 11.3.2; 11.4.0 before 11.4.1 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-27656
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-27656
Vendor Advisory
https://mattermost.com/security-updates
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