CVE-2021-39392
📋 TL;DR
CVE-2021-39392 is a critical remote code execution vulnerability in MyLittleBackup management tool due to a hardcoded machineKey in web.config. This allows attackers to send serialized ASP code and execute arbitrary commands on affected systems. All users of MyLittleBackup up to version 1.7 are vulnerable.
💻 Affected Systems
- MyLittleBackup
📦 What is this software?
Mylittlebackup by Mylittletools
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise allowing attackers to execute arbitrary code, steal data, install malware, or pivot to other systems in the network.
Likely Case
Remote code execution leading to data theft, ransomware deployment, or creation of persistent backdoors on vulnerable systems.
If Mitigated
Limited impact if system is isolated, has strict network controls, and uses additional security layers, though vulnerability remains present.
🎯 Exploit Status
Proof of concept code is publicly available on GitHub gist. Exploitation is straightforward due to hardcoded cryptographic keys and no authentication requirements.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Unknown
Vendor Advisory: No official vendor advisory found
Restart Required: No
Instructions:
No official patch available. Consider migrating to alternative backup solutions or implementing workarounds.
🔧 Temporary Workarounds
Replace Hardcoded machineKey
windowsGenerate and use unique machineKey values for each installation instead of the hardcoded default
1. Open web.config
2. Locate <machineKey> section
3. Replace with unique values: <machineKey validationKey="[GENERATE_NEW]" decryptionKey="[GENERATE_NEW]" validation="SHA1" decryption="AES" />
4. Use ASP.NET machineKey generator tools to create secure keys
Restrict Network Access
allLimit access to MyLittleBackup management interface to trusted IP addresses only
1. Configure firewall rules to restrict access
2. Use IP whitelisting in IIS/application
3. Consider placing behind VPN or jump host
🧯 If You Can't Patch
- Immediately isolate the MyLittleBackup server from internet access and restrict internal network access
- Implement application-level firewall (WAF) rules to block serialized ASP payloads and monitor for exploitation attempts
🔍 How to Verify
Check if Vulnerable:
Check web.config file for hardcoded machineKey values. If validationKey and decryptionKey are the same across installations, the system is vulnerable.Check Version:
Check application version in web interface or examine installation directory for version informationVerify Fix Applied:
Verify that machineKey values are unique per installation and not using the default hardcoded values. Test that serialized payloads are rejected.📡 Detection & Monitoring
Log Indicators:
- Unusual POST requests to management endpoints
- ASP serialization payloads in request logs
- Unexpected process execution or file creation
Network Indicators:
- HTTP requests containing serialized .NET objects
- Traffic to MyLittleBackup management port from unexpected sources
SIEM Query:
source="web_logs" AND (uri="*/management/*" OR uri="*/admin/*") AND (request_body CONTAINS "Type" OR request_body CONTAINS "ObjectStateFormatter")