Mattermost Team Object Exposure via Scheme Teams API

Executive Summary

CVE-2026-9820 is a vulnerability in Mattermost versions 11.7.x (up to and including 11.7.2) and 10.11.x (up to and including 10.11.19). The issue involves insufficient sanitization of team objects returned by the 'scheme teams' endpoint. Specifically, a user with the 'User Manager' role can exploit this flaw to obtain invite links for private teams.

These invite links can then be used to join or share access to private teams via the 'scheme teams' API endpoint, bypassing intended access controls.

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Detection Guidance

Detecting CVE-2026-9820 on your network or system involves checking for vulnerable Mattermost versions and monitoring suspicious activity related to the scheme teams API endpoint. Below are some approaches:

• Verify the installed Mattermost version to confirm if it falls within the vulnerable ranges (11.7.0 to 11.7.2 or 10.11.0 to 10.11.19). You can check the version via the Mattermost web interface (System Console > About) or by querying the server directly if you have access. Example command for self-hosted instances:
• ```bash curl -s http://<mattermost-server>/api/v4/system/version | jq .version ```
• Monitor network traffic or logs for unusual access to the `/api/v4/teams/scheme_teams` endpoint, particularly by users with the User Manager role. Look for repeated or unauthorized requests to this endpoint, which may indicate exploitation attempts.
• Review Mattermost logs for unexpected team membership changes or invite link sharing, especially for private teams. Logs are typically located in `/var/log/mattermost/` or configured log directories.
• If you have access to the Mattermost database, you can query for team objects that may have been improperly accessed or modified. Example SQL query (adjust based on your database):
• ```sql SELECT * FROM Teams WHERE SchemeID IS NOT NULL AND AllowOpenInvite = true; ```

Since this vulnerability involves improper sanitization of team objects, ensure that no unauthorized users have gained access to private teams via invite links shared through the scheme teams endpoint.

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Impact Analysis

If you are using an affected version of Mattermost, this vulnerability could have the following impacts:

• Unauthorized access to private teams: A user with the User Manager role could gain access to private teams they are not supposed to join, compromising the confidentiality of team discussions and data.
• Data leakage: Sensitive information shared within private teams could be exposed to unauthorized individuals.
• Privilege escalation: The vulnerability could allow a user with limited privileges (User Manager role) to escalate their access, potentially leading to further security breaches.

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Compliance Impact

This vulnerability could impact compliance with several common standards and regulations, depending on the nature of the data handled by your Mattermost instance:

• GDPR (General Data Protection Regulation): If private teams contain personal data of EU citizens, unauthorized access could lead to a data breach, violating GDPR requirements for data protection and potentially resulting in significant fines.
• HIPAA (Health Insurance Portability and Accountability Act): For organizations handling protected health information (PHI), this vulnerability could lead to unauthorized disclosure of PHI, violating HIPAA's privacy and security rules.
• Other standards (e.g., ISO 27001, SOC 2): The vulnerability could indicate a failure in access control mechanisms, which are critical requirements for these standards. Non-compliance could affect certifications or audits.

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Mitigation Strategies

To mitigate CVE-2026-9820 (MMSA-2026-00671), follow these immediate steps:

• Upgrade Mattermost to a patched version. The vulnerability affects versions 11.7.x <= 11.7.2 and 10.11.x <= 10.11.19. Apply the latest security updates provided by Mattermost to resolve the issue.
• Restrict User Manager role permissions temporarily if an upgrade cannot be performed immediately. Limit access to the scheme teams API endpoint to prevent unauthorized use of invite links for private teams.
• Monitor and audit access to private teams, particularly focusing on users with the User Manager role, to detect any suspicious activity or unauthorized access attempts.

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