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CVE-2026-9816

8.3 · HIGH

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…

Published Aug 17, 2026CWE-863EPSS 0.35%

Overview

CVE-2026-9816 is a high-severity vulnerability. It was published on August 17, 2026 and has a CVSS 3.1 base score of 8.3 (HIGH).

This vulnerability has a CVSS 3.1 base score of 8.3, rated HIGH. It can be exploited remotely over the network. Some level of privileges is required for exploitation.

Technical 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

Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

References

Refmattermost.com/security-updates

How CyberStrike helps

Frequently asked questions

What is CVE-2026-9816?

CVE-2026-9816 is a high-severity vulnerability. It was published on August 17, 2026 and has a CVSS 3.1 base score of 8.3 (HIGH).

How severe is CVE-2026-9816?

This vulnerability has a CVSS 3.1 base score of 8.3, rated HIGH. It can be exploited remotely over the network. Some level of privileges is required for exploitation.

How do I fix or remediate CVE-2026-9816?

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

How can CyberStrike help with CVE-2026-9816?

CyberStrike is an open-source, AI-augmented pentesting engine. Rather than flag a version number, it tests whether an issue like CVE-2026-9816 is actually reachable in your own applications and APIs, then verifies each finding before reporting it. That tells you if the weakness is a real, exploitable risk in your environment or a false positive you can safely deprioritize.

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