CVE-2025-41436
📋 TL;DR
Mattermost versions before 11.0 fail to properly enforce the 'Allow users to view archived channels' setting, allowing regular users to access archived channel content and files via the 'Open in Channel' functionality from followed threads. This affects all Mattermost deployments with archived channels where this setting is intended to restrict access.
💻 Affected Systems
- Mattermost
📦 What is this software?
Mattermost Server by Mattermost
⚠️ Risk & Real-World Impact
Worst Case
Unauthorized users access sensitive archived channel content containing confidential information, intellectual property, or compliance data that should remain restricted.
Likely Case
Regular users inadvertently or intentionally access archived channel content they shouldn't see, potentially violating data segregation policies or compliance requirements.
If Mitigated
Minimal impact as users only access content they're already authorized to see in active channels, with archived content properly restricted.
🎯 Exploit Status
Exploitation requires authenticated user access and knowledge of the 'Open in Channel' functionality from followed threads.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 11.0 and later
Vendor Advisory: https://mattermost.com/security-updates
Restart Required: Yes
Instructions:
1. Backup Mattermost database and configuration. 2. Upgrade Mattermost to version 11.0 or later. 3. Restart Mattermost service. 4. Verify the 'Allow users to view archived channels' setting now properly restricts access.
🔧 Temporary Workarounds
Disable 'Open in Channel' functionality
allTemporarily disable the ability to open threads in channel view to prevent exploitation vector
# Modify Mattermost config.json to disable thread following features
# Consult Mattermost documentation for specific configuration changes
Restrict thread following permissions
allAdjust team/channel permissions to limit who can follow threads
# Use Mattermost System Console > Permissions to adjust thread following settings
🧯 If You Can't Patch
- Review and minimize archived channel content containing sensitive information
- Implement additional monitoring for access to archived channels and alert on suspicious patterns
🔍 How to Verify
Check if Vulnerable:
Check Mattermost version via System Console > About Mattermost. If version is below 11.0 and you have archived channels with restricted access, you are vulnerable.Check Version:
Check System Console > About Mattermost or run: mattermost version (CLI)Verify Fix Applied:
After upgrading to 11.0+, test that regular users cannot access archived channel content via 'Open in Channel' from followed threads when the setting is disabled.📡 Detection & Monitoring
Log Indicators:
- Unusual access patterns to archived channels
- Multiple failed attempts to access restricted archived content
- User activity logs showing archived channel access from thread following features
Network Indicators:
- API calls to archived channel endpoints from users without proper permissions
- Increased traffic to archived channel resources
SIEM Query:
source="mattermost" AND (event="channel_viewed" AND channel_status="archived") AND user_role="user"