ReviewHigh

CVE-2026-9816

Mattermost Mattermost, mattermost server

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: 8.3
EPSS: -
CISA KEV: Not listed
Published: 2026.08.18

PRIORITY ASSESSMENT

Review

The CVSS severity warrants an early asset and exposure review.

Known exploitationNot established by KEV

Exploit probability-

Technical severityCVSS 8.3

Vulnerability overview

Affected product and versions

Product: Mattermost Mattermost, mattermost server
Affected versions: >= 11.7.0 <= 11.7.6, >= 10.11.0 <= 10.11.21, >= 11.8.0 <= 11.8.3, >= 10.11.0 < 10.11.22, >= 11.7.0 < 11.7.7, >= 11.8.0 < 11.8.4
Fixed versions: 10.11.22, 11.7.7, 11.8.4

Recommended response sequence

Confirm exposure before applying a vendor-supported change.

Full remediation guide

1
Identify
Confirm that Mattermost Mattermost, mattermost server and an affected version are present.
2
Prioritize
Combine exploitation signals with asset exposure and business criticality.
3
Remediate
Follow the vendor advisory or supported update path and preserve rollback options.
4
Verify
Recheck the version, service health, access paths, and relevant logs.

Technical data

CVSS vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:H/A:H
CWE: CWE-863