CVE-2024-39839
📋 TL;DR
This vulnerability allows users on remote Mattermost servers to set arbitrary usernames that sync to local servers when shared channels are enabled. It affects Mattermost installations with shared channels enabled, potentially allowing impersonation or confusion in user identification.
💻 Affected Systems
- Mattermost
📦 What is this software?
Mattermost Server by Mattermost
Mattermost Server by Mattermost
Mattermost Server by Mattermost
Mattermost Server by Mattermost
⚠️ Risk & Real-World Impact
Worst Case
An attacker could impersonate legitimate users, potentially gaining unauthorized access to channels or misleading team members through spoofed identities.
Likely Case
User confusion and potential minor privilege escalation if username-based permissions are in use, though limited to shared channel contexts.
If Mitigated
Minimal impact with proper user verification processes and monitoring of shared channel activity.
🎯 Exploit Status
Requires authenticated access to a remote Mattermost server with shared channel permissions.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 9.9.1, 9.5.7, 9.7.6, 9.8.2 or later
Vendor Advisory: https://mattermost.com/security-updates
Restart Required: Yes
Instructions:
1. Backup your Mattermost installation and database. 2. Download the patched version from mattermost.com/download. 3. Stop Mattermost service. 4. Replace installation with patched version. 5. Restart Mattermost service. 6. Verify version is updated.
🔧 Temporary Workarounds
Disable Shared Channels
allTemporarily disable shared channels feature to prevent exploitation
Edit config.json: set "EnableSharedChannels" to false
Restart Mattermost service
🧯 If You Can't Patch
- Disable shared channels feature in configuration
- Implement strict monitoring of user sync events and username changes in logs
🔍 How to Verify
Check if Vulnerable:
Check Mattermost version via System Console > About, and verify if shared channels are enabled in config.jsonCheck Version:
On server: cat /opt/mattermost/version.txt or check System Console web interfaceVerify Fix Applied:
Confirm version is 9.9.1+, 9.5.7+, 9.7.6+, or 9.8.2+ via System Console > About📡 Detection & Monitoring
Log Indicators:
- Unexpected username changes in shared channel sync logs
- User sync events with unusual usernames
Network Indicators:
- Unusual shared channel synchronization patterns
SIEM Query:
source="mattermost" AND ("shared channel" OR "user sync") AND username_change