CVE-2024-39777
📋 TL;DR
This vulnerability in Mattermost allows remote attackers to forcibly share local channels without administrator consent when shared channels are enabled. Attackers can send unsolicited invites with existing local channel IDs, exposing those channels to unauthorized external access. This affects Mattermost instances with shared channels enabled across multiple version ranges.
💻 Affected Systems
- Mattermost
📦 What is this software?
Mattermost by Mattermost
Mattermost by Mattermost
Mattermost by Mattermost
Mattermost by Mattermost
⚠️ Risk & Real-World Impact
Worst Case
Sensitive internal communications and data in local channels become accessible to unauthorized external parties, potentially exposing confidential information, intellectual property, or sensitive discussions.
Likely Case
Unauthorized sharing of internal team channels with external entities, leading to data leakage and potential compliance violations.
If Mitigated
Limited exposure if shared channels feature is disabled or proper access controls are in place to restrict external connections.
🎯 Exploit Status
Exploitation requires ability to send invites to the Mattermost instance, but does not require authentication to the target channel being hijacked.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Update to Mattermost 9.9.1, 9.5.7, 9.7.6, or 9.8.2
Vendor Advisory: https://mattermost.com/security-updates
Restart Required: Yes
Instructions:
1. Backup your Mattermost instance. 2. Download the patched version from Mattermost releases. 3. Stop Mattermost service. 4. Replace with patched version. 5. Restart Mattermost service. 6. Verify version update.
🔧 Temporary Workarounds
Disable Shared Channels
allTemporarily disable the 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 immediately
- Implement network segmentation to restrict external access to Mattermost instance
🔍 How to Verify
Check if Vulnerable:
Check Mattermost version and verify if shared channels are enabled in System Console > Environment > Shared ChannelsCheck Version:
mattermost versionVerify Fix Applied:
Verify version is 9.9.1, 9.5.7, 9.7.6, or 9.8.2 or higher📡 Detection & Monitoring
Log Indicators:
- Unexpected channel sharing events
- External invites to local channels without admin approval
- Shared channel creation logs without corresponding local admin actions
Network Indicators:
- Unexpected external connections to previously internal-only channels
- Increased cross-server communication for channel sharing
SIEM Query:
source="mattermost" AND (event="channel_shared" OR event="invite_accepted") AND NOT user_role="admin"