High severity · published Aug 17, 2026
CVE-2026-9816
Insufficient server-side validation of board member role fields permits privilege escalation
8.3
CVSS
0.4%
EPSS
No
On KEV
Description
Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to validate BoardMember.Scheme* fields server-side on insert and archive-import paths which allows a board editor or non-guest team member to grant board admin to arbitrary users via POST /api/v2/boards/{boardID}/members and POST /api/v2/teams/{teamID}/archive/import.. Mattermost Advisory ID: MMSA-2026-00685
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:H/A:H
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
None
Confidentiality
Low
Integrity
High
Availability
High
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.4% · higher than 28% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
MA
Mattermost · Mattermost
11.7.0 through 11.7.6; 10.11.0 through 10.11.21; 11.8.0 through 11.8.3 · source: vendor
MS
Mattermost Server · Mattermost
10.11.0 before 10.11.22; 11.7.0 before 11.7.7; 11.8.0 before 11.8.4 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-9816
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-9816
Vendor Advisory
https://mattermost.com/security-updates
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