Mattermost Remote Cluster Metadata Enumeration via Autocomplete

Executive Summary

CVE-2026-9824 is a vulnerability in Mattermost server versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, and 10.11.x <= 10.11.19. The issue occurs because the software fails to check the 'manage_shared_channels' permission in the /share-channel autocomplete handler.

This allows an authenticated user who does not have the 'manage_shared_channels' permission to enumerate configured remote cluster connection metadata. The enumeration can happen via slash command autocomplete functionality.

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Detection Guidance

Detecting this vulnerability requires checking if your Mattermost instance is running a vulnerable version and verifying whether unauthorized users can enumerate remote cluster connection metadata via the /share-channel autocomplete handler.

• Check the Mattermost server version: Run the following command on the server hosting Mattermost to verify the installed version: 'mattermost version'.
• Compare the installed version against the vulnerable versions: 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, or 10.11.x <= 10.11.19. If your version falls within these ranges, it is vulnerable.
• Test for unauthorized enumeration: As an authenticated user without the 'manage_shared_channels' permission, attempt to use the slash command autocomplete for '/share-channel'. If metadata about remote cluster connections is returned, the vulnerability is present.

No specific network-based detection commands are provided, as this is an application-level vulnerability requiring access to the Mattermost interface or server logs.

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Impact Analysis

This vulnerability can impact you in the following ways:

• Unauthorized users may gain access to sensitive metadata about remote cluster connections in your Mattermost deployment.
• While the CVSS score (4.3) indicates a low severity, it could still expose information that might be used in further attacks or reconnaissance.
• If your organization relies on Mattermost for secure communications, this could undermine trust in the platform's permission controls.

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Compliance Impact

The impact on compliance depends on the nature of the data exposed and your organization's regulatory obligations:

• GDPR: If the exposed metadata includes personal data or could lead to further exposure of personal data, this may constitute a breach of confidentiality under GDPR. Organizations may need to assess whether this triggers reporting obligations.
• HIPAA: If the Mattermost instance is used to handle protected health information (PHI) and the exposed metadata relates to PHI-sharing configurations, this could be considered a security incident under HIPAA. Covered entities would need to evaluate whether this constitutes a breach requiring notification.
• Other standards (e.g., ISO 27001, NIST): This vulnerability may indicate a failure in access control mechanisms, which could be flagged during audits or assessments.

The specific compliance impact depends on how Mattermost is used in your environment and the sensitivity of the data involved.

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Mitigation Strategies

To mitigate this vulnerability, follow these immediate steps:

• Upgrade Mattermost to the latest patched version. Refer to the Mattermost security updates page for the fixed versions (e.g., versions later than 11.7.2, 11.6.4, or 10.11.19).
• If upgrading is not immediately possible, restrict access to the Mattermost instance to trusted users only and monitor for unusual activity related to the /share-channel autocomplete handler.
• Review and audit user permissions to ensure that only authorized users have the 'manage_shared_channels' permission, reducing the risk of exploitation.
• Monitor Mattermost logs for any attempts to access or enumerate remote cluster connection metadata via the vulnerable endpoint.

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