CVE-2026-9816: Insufficient server-side validation of board member role fields permits privilege escalation

Severity: high (CVSS 8.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-08-17 · updated 2026-08-19
Weakness: CWE-863

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

Affected products

Mattermost: from 10.11.0 before 10.11.22; from 11.7.0 before 11.7.7; from 11.8.0 before 11.8.4 — fixed in 10.11.22, 11.7.7, 11.8.4

How to fix

Upgrade to a fixed version: Mattermost 10.11.22, 11.7.7, 11.8.4. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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