CVE-2026-3113: Mattermost Information Disclosure Flaw

CVE-2026-3113 Overview

CVE-2026-3113 is an insecure permissions vulnerability affecting multiple versions of Mattermost, a widely-used open-source team collaboration platform. The vulnerability exists in the bulk export functionality, where downloaded bulk exports are created without proper file permissions. This allows other local users on the server to read the contents of the bulk export, potentially exposing sensitive organizational data, private messages, and user information.

Critical Impact

Local users on the server can access bulk export data containing sensitive team communications, user credentials, and organizational information due to improper file permission settings.

Affected Products

• Mattermost 11.4.x versions <= 11.4.0
• Mattermost 11.3.x versions <= 11.3.1
• Mattermost 11.2.x versions <= 11.2.3
• Mattermost 10.11.x versions <= 10.11.11

Discovery Timeline

• 2026-03-26 - CVE-2026-3113 published to NVD
• 2026-03-26 - Last updated in NVD database

Technical Details for CVE-2026-3113

Vulnerability Analysis

This vulnerability is classified under CWE-732 (Incorrect Permission Assignment for Critical Resource). The flaw occurs when Mattermost generates bulk export files, which are intended to contain comprehensive data exports of the platform's content. These exports typically include sensitive information such as user messages, channel data, team configurations, and potentially authentication-related information.

The core issue is that when these export files are written to the file system, the application fails to restrict the file permissions appropriately. Instead of setting restrictive permissions that would limit access to only the Mattermost service account or administrative users, the files are created with overly permissive settings that allow any local user on the server to read the contents.

The attack requires local access to the server and some user interaction (such as an administrator initiating a bulk export), but once exploited, it can lead to significant confidentiality breaches.

Root Cause

The root cause of CVE-2026-3113 is the failure to properly set file permissions during the bulk export process. When the export file is created on the file system, the application does not explicitly set restrictive file mode bits, allowing the default system umask or overly permissive settings to take effect. This results in the export file being world-readable or readable by users who should not have access to the sensitive export data.

Attack Vector

The attack vector for this vulnerability requires local access to the Mattermost server. An attacker with a low-privileged user account on the same system can monitor for bulk export operations and then read the exported files to gain access to sensitive data. The attack scenario involves:

1. An attacker gains or already has local user access to the Mattermost server
2. A legitimate administrator initiates a bulk export operation through the Mattermost interface
3. The bulk export file is created with insecure permissions
4. The attacker reads the export file, gaining access to private messages, user data, and organizational information

The vulnerability exploitation does not require any special privileges beyond basic local user access, making it accessible to insider threats, compromised service accounts, or users who have gained limited access through other means.

Detection Methods for CVE-2026-3113

Indicators of Compromise

• Unexpected access to bulk export directories by non-administrative users
• File access logs showing reads of export files by unauthorized local accounts
• Presence of copied or exfiltrated bulk export data in user directories
• Unusual file permission changes on Mattermost data directories

Detection Strategies

• Monitor file access events on the Mattermost export directory for unauthorized read operations
• Implement file integrity monitoring on directories where bulk exports are stored
• Review audit logs for bulk export operations and correlate with file access events
• Deploy endpoint detection capabilities to identify suspicious file access patterns by local users

Monitoring Recommendations

• Enable detailed file system auditing on the Mattermost server for export-related directories
• Configure SIEM rules to alert on file access by non-service accounts to export directories
• Regularly review user account access and permissions on Mattermost server systems
• Implement real-time alerting for any bulk export operations to ensure proper oversight

How to Mitigate CVE-2026-3113

Immediate Actions Required

• Upgrade Mattermost to the latest patched version immediately
• Audit existing bulk export files and set restrictive permissions (chmod 600) on any exports
• Review server user accounts for any unauthorized or suspicious local accounts
• Implement strict access controls limiting local server access to essential personnel only

Patch Information

Mattermost has released security updates to address this vulnerability. Organizations should upgrade to versions beyond the affected ranges (11.4.0, 11.3.1, 11.2.3, and 10.11.11). Refer to the Mattermost Security Updates page for the latest patch information and download links. The advisory reference is MMSA-2026-00593.

Workarounds

• Manually set restrictive file permissions on bulk export files immediately after creation using chmod 600
• Restrict local server access to only essential administrative personnel
• Move bulk exports to a secure directory with restricted parent permissions
• Consider using encrypted storage for export files as an additional layer of protection

# Configuration example - Set restrictive permissions on export directory
chmod 700 /opt/mattermost/data/export
chown mattermost:mattermost /opt/mattermost/data/export

# Set restrictive permissions on existing export files
find /opt/mattermost/data/export -type f -name "*.zip" -exec chmod 600 {} \;
find /opt/mattermost/data/export -type f -name "*.zip" -exec chown mattermost:mattermost {} \;