CVE-2026-9816

Insufficient server-side validation of board member role fields permits privilege escalation

State: Published
Severity: 8.3 High
Published: Aug 17, 2026
Updated: Aug 18, 2026
Reserved: May 28, 2026

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

Scores

CVSS	Score	Vector	Source
3.1	8.3 High	CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:H/A:H	Mattermost

Weaknesses

CWE-863 Incorrect Authorization

Affected Products

Product	Versions
MattermostMattermost	11.7.0 through 11.7.6Affected 10.11.0 through 10.11.21Affected 11.8.0 through 11.8.3Affected 11.9.0Not affected 11.7.7Not affected 10.11.22Not affected 11.8.4Not affected

References

MMSA-2026-00685vendor-advisory

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