CVE-2026-9816

Received Received - Intake

Privilege Escalation in Mattermost via Board Admin Grant

Publication date: 2026-08-17

Last updated on: 2026-08-17

Assigner: Mattermost, Inc.

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 Scores

EPSS Scores

Probability:
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Meta Information

Published

2026-08-17

Last Modified

2026-08-17

Generated

2026-08-18

AI Q&A

2026-08-18

EPSS Evaluated

N/A

NVD

CVE-2026-9816

EUVD

EUVD-2026-60540

Affected Vendors & Products

Vendor	Product	Version / Range
mattermost	mattermost	to 11.7.6 (inc)
mattermost	mattermost	to 10.11.21 (inc)
mattermost	mattermost	to 11.8.3 (inc)

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-863	The product performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check.

Attack-Flow Graph

Executive Summary

This vulnerability affects Mattermost versions 11.7.x through 11.7.6, 10.11.x through 10.11.21, and 11.8.x through 11.8.3. It allows a board editor or non-guest team member to grant board admin privileges to arbitrary users by exploiting improper server-side validation of BoardMember.Scheme* fields during insert and archive-import operations.

Detection Guidance

This vulnerability affects Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, and 11.8.x <= 11.8.3. To detect it, check your Mattermost server version using the command: mattermost version. If your version falls within the affected ranges, the system is vulnerable.

Impact Analysis

An attacker with board editor or team member access could escalate their privileges to board admin, potentially gaining control over sensitive team data, modifying board configurations, or manipulating user permissions. This could lead to unauthorized access to confidential information or disruption of team workflows.

Compliance Impact

This vulnerability could lead to unauthorized access to sensitive data, violating GDPR's data protection principles or HIPAA's security requirements for protected health information. Organizations using affected Mattermost versions may face compliance violations, legal penalties, or reputational damage due to potential data breaches.

Mitigation Strategies

Update Mattermost to a patched version (11.7.7 or later, 10.11.22 or later, or 11.8.4 or later) to address the server-side validation failure in BoardMember.Scheme* fields.

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