CVE-2025-9076
📋 TL;DR
Mattermost versions 10.10.x through 10.10.1 fail to properly sanitize user data during shared channel synchronization, allowing malicious remote clusters to access sensitive user information. This affects Mattermost Server instances with shared channels enabled, potentially exposing user data across federated deployments.
💻 Affected Systems
- Mattermost Server
📦 What is this software?
Mattermost Server by Mattermost
⚠️ Risk & Real-World Impact
Worst Case
Malicious remote clusters could exfiltrate sensitive user information including email addresses, usernames, and potentially other profile data from federated Mattermost instances.
Likely Case
Compromised or malicious federated clusters could access user information they shouldn't have access to, violating data segregation between organizations.
If Mitigated
With proper network segmentation and trust controls between federated clusters, impact would be limited to authorized data sharing only.
🎯 Exploit Status
Requires control over a federated Mattermost cluster or compromise of an existing federated partner to exploit.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 10.10.2 or later
Vendor Advisory: https://mattermost.com/security-updates
Restart Required: No
Instructions:
1. Backup your Mattermost database and configuration. 2. Download Mattermost 10.10.2 or later from official channels. 3. Follow Mattermost upgrade documentation for your deployment method. 4. Verify the upgrade completed successfully.
🔧 Temporary Workarounds
Disable Shared Channels
allTemporarily disable the shared channels feature to prevent exploitation until patching can occur.
Set 'EnableSharedChannels' to 'false' in config.json
Restrict Federated Connections
allLimit shared channel connections to only trusted, verified partner organizations.
Review and restrict 'AllowedUntrustedInternalConnections' in config.json
🧯 If You Can't Patch
- Disable shared channels feature entirely in configuration
- Implement strict network segmentation between federated Mattermost instances
🔍 How to Verify
Check if Vulnerable:
Check Mattermost version via System Console > About or run 'mattermost version' command. If version is 10.10.0 or 10.10.1 with shared channels enabled, you are vulnerable.Check Version:
mattermost versionVerify Fix Applied:
Verify version is 10.10.2 or later and test shared channel functionality with a trusted partner.📡 Detection & Monitoring
Log Indicators:
- Unusual shared channel synchronization patterns
- Unexpected user data access from remote clusters
Network Indicators:
- Abnormal data transfers between federated Mattermost instances
SIEM Query:
source="mattermost" AND ("shared channel" OR "remote cluster") AND (error OR warning OR unauthorized)