CVE-2026-9816
High
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
SeverityHigh
CVSS8.3
Exploit probability29. percentile0.4%
Product
Mattermost Server
PublishedAug 17, 2026
First seenAug 17, 2026
Weakness typeCWE-863Incorrect authorization
How to fix
Update
- mattermost mattermost_server10.11.22, 11.7.7, 11.8.4 or later
Vendor advisories
Affected products
| Product | Affected versions | Fixed in |
|---|---|---|
| mattermost mattermost_server | 10.11.0 ≤ v < 10.11.22, 11.7.0 ≤ v < 11.7.7, 11.8.0 ≤ v < 11.8.4 | 10.11.22, 11.7.7, 11.8.4 |
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