CVE-2026-2299: Mattermost Google Drive Plugin Authorization Bypass – Private Channel File Disclosure

Technical summary

CVE-2026-2299 is a broken access control vulnerability (CWE-862) in the Mattermost Google Drive plugin's file creation endpoint. The plugin fails to verify whether an authenticated user has authorization to share files to specific private channels. An attacker with valid Mattermost credentials and an active Google Drive connection can exploit this by directly invoking the file creation endpoint with a target channel ID they do not belong to, bypassing membership validation checks. This results in unauthorized file placement in restricted channels and disclosure of private channel membership information through error messages or successful operations. The CVSS 3.1 score of 4.2 (MEDIUM) reflects low attack complexity but limited scope and partial impact to confidentiality and integrity.

Business impact

This vulnerability poses a targeted insider threat risk rather than a perimeter-level concern. An attacker with legitimate Mattermost access—whether an employee, contractor, or compromised account—can exfiltrate knowledge about private team structures and expose confidential file-sharing relationships. In regulated environments handling sensitive data, unauthorized placement of files into unintended channels could violate data governance policies and audit trails. The reputational and compliance risk depends on channel sensitivity; exposure of executive or legal channels presents higher stakes than general team collaboration areas.

Affected systems

The Mattermost Google Drive plugin is affected in all versions prior to 1.1.0. Severity varies by deployment: on-premises Mattermost instances with the plugin installed and Google Drive integration enabled face direct risk. Cloud-hosted Mattermost workspaces depend on whether the instance has the Google Drive plugin active. Organizations using only native Mattermost file uploads or alternative integrations (e.g., OneDrive, AWS S3) are not affected. The attack surface is limited to users with both Mattermost authentication and an active Google account connection configured in their profile.

Exploitability

Exploitation requires two prerequisites: valid Mattermost user credentials and an authenticated Google Drive connection linked to that account. An attacker cannot exploit this remotely without these credentials, placing the attack in the 'authenticated' threat model. Attack complexity is rated as high because the attacker must know or enumerate valid private channel IDs to exploit the vulnerability effectively; however, in small to medium organizations, channel identifiers are often discoverable through UI inspection or prior access. No user interaction is required once the attacker has credentials. The vulnerability is not listed in CISA's Known Exploited Vulnerabilities (KEV) catalog as of the publication date, suggesting active exploitation in the wild is not yet documented.

Remediation

Organizations using the Mattermost Google Drive plugin must upgrade to version 1.1.0 or later. This patched version implements proper channel membership validation before allowing file operations. Interim mitigations for unpatched instances include: restricting Google Drive plugin access via admin controls to trusted users only, disabling the plugin entirely if not essential, implementing role-based access controls to limit user permissions for sensitive channels, and monitoring audit logs for anomalous file-sharing activity. Post-remediation, review file-sharing history in affected private channels to identify any unauthorized uploads or metadata leakage.

Patch guidance

Apply the Mattermost Google Drive plugin update to version 1.1.0 or later. Verify the patch through the Mattermost plugin marketplace or your organization's package management system. Test the update in a non-production Mattermost workspace first to ensure compatibility with your specific deployment configuration, particularly if you have custom channel structures or role definitions. After deployment, confirm that the plugin correctly rejects file-sharing attempts to unauthorized channels. Document the patch date and version applied for compliance and audit purposes.

Detection guidance

Monitor Mattermost audit logs for file-creation operations targeting private channels by users who do not appear in the channel member roster. Track API calls to the Google Drive plugin endpoint with unusual channel ID parameters, particularly repeated attempts with different channel IDs from a single user account. Implement logging of Google Drive integration failures, as blocked or rejected authorization events may indicate patch testing or exploitation attempts. Inspect the Google Drive audit log for files shared to Mattermost integrations originating from accounts with unexpected or one-time access patterns. Alert on any successful file uploads to private channels followed immediately by deletion or modification, a common cover-up tactic.

Why prioritize this

Despite the MEDIUM CVSS score, prioritize this vulnerability if your organization uses the Mattermost Google Drive plugin in environments with sensitive private channels (legal, finance, executive, or HR). The vulnerability is primarily an insider threat and data governance concern rather than an external attack vector. Deprioritization is justified only if the plugin is disabled, unused, or confined to non-sensitive channels. For teams managing confidential projects, intellectual property, or regulated data, upgrade within 30 days. For general communication workspaces, the standard patch cycle (60–90 days) is acceptable given the low active exploitation risk.

Risk score, explained

The CVSS 3.1 score of 4.2 reflects a vulnerability with limited scope and impact but clear exploitability among authenticated users. The vector CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N indicates network accessibility, high attack complexity (requires knowledge of channel IDs and valid credentials), low privilege requirement (any authenticated user), no user interaction, unchanged scope, and partial impact to confidentiality and integrity with no availability impact. The score appropriately downgrades the severity from HIGH to MEDIUM because the attack requires valid credentials, limiting the attacker pool. In organizational contexts where insider threats are weighted heavily or private channels contain highly sensitive data, risk tolerance may justify treating this as MEDIUM-HIGH operationally despite the baseline CVSS.

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