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… (CVE-2026-9816)
Mattermost versions 11.7.x up to 11.7.6, 10.11.x up to 10.11.21, and 11.8.x up to 11.8.3 contain a vulnerability where the BoardMember.Scheme fields are not properly validated server-side during insert and archive-import operations. This flaw allows a board editor or any non-guest team member to escalate privileges by granting board admin rights to arbitrary users via specific API endpoints.
AI Analysis
Technical Summary
The vulnerability identified as CVE-2026-9816 affects Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, and 11.8.x <= 11.8.3. It arises from a failure to validate the BoardMember.Scheme fields on the server side during insert and archive-import API paths. This improper validation enables a board editor or non-guest team member to escalate privileges by assigning board admin rights to arbitrary users through POST requests to /api/v2/boards/{boardID}/members and /api/v2/teams/{teamID}/archive/import. The vulnerability is classified under CWE-863 (Incorrect Authorization).
Potential Impact
An attacker with board editor or non-guest team member privileges can exploit this vulnerability to grant themselves or others board admin rights, potentially leading to unauthorized administrative control over boards. This can impact confidentiality, integrity, and availability of board data as indicated by the CVSS vector (Confidentiality Low, Integrity High, Availability High).
Mitigation Recommendations
Patch status is not yet confirmed — check the vendor advisory (Mattermost Advisory ID: MMSA-2026-00685) for current remediation guidance. Until a patch is applied, restrict board editor and team member privileges to trusted users only to minimize risk.
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… (CVE-2026-9816)
Description
Mattermost versions 11.7.x up to 11.7.6, 10.11.x up to 10.11.21, and 11.8.x up to 11.8.3 contain a vulnerability where the BoardMember.Scheme fields are not properly validated server-side during insert and archive-import operations. This flaw allows a board editor or any non-guest team member to escalate privileges by granting board admin rights to arbitrary users via specific API endpoints.
CVSS v3.1
Score 8.3high
Affected software
pkg:github/mattermost/mattermost-serverRun on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The vulnerability identified as CVE-2026-9816 affects Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, and 11.8.x <= 11.8.3. It arises from a failure to validate the BoardMember.Scheme fields on the server side during insert and archive-import API paths. This improper validation enables a board editor or non-guest team member to escalate privileges by assigning board admin rights to arbitrary users through POST requests to /api/v2/boards/{boardID}/members and /api/v2/teams/{teamID}/archive/import. The vulnerability is classified under CWE-863 (Incorrect Authorization).
Potential Impact
An attacker with board editor or non-guest team member privileges can exploit this vulnerability to grant themselves or others board admin rights, potentially leading to unauthorized administrative control over boards. This can impact confidentiality, integrity, and availability of board data as indicated by the CVSS vector (Confidentiality Low, Integrity High, Availability High).
Mitigation Recommendations
Patch status is not yet confirmed — check the vendor advisory (Mattermost Advisory ID: MMSA-2026-00685) for current remediation guidance. Until a patch is applied, restrict board editor and team member privileges to trusted users only to minimize risk.
Technical Details
- Gcve Source
- db.gcve.eu
- Osv Id
- GHSA-h2c9-x337-f543
- Osv Schema Version
- 1.4.0
- Aliases
- ["CVE-2026-9816"]
- Ecosystems
- []
- Database Specific Severity
- HIGH
- Cvss Version
- 3.1
Threat ID: 6a83aa65bf8831d539e30d6a
Added to database: 08/18/2026, 00:42:13 UTC
Last enriched: 08/18/2026, 01:26:10 UTC
Last updated: 08/18/2026, 02:01:20 UTC
Views: 3
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