CVE-2021-42071
📋 TL;DR
CVE-2021-42071 is a critical remote command execution vulnerability in Visual Tools DVR VX16 software where an unauthenticated attacker can execute arbitrary commands by injecting shell metacharacters in the User-Agent HTTP header. This affects all systems running the vulnerable version of the DVR software, allowing complete system compromise. The vulnerability is particularly dangerous because it requires no authentication and has a publicly available exploit.
💻 Affected Systems
- Visual Tools DVR VX16
📦 What is this software?
Dvr Vx16 Firmware by Visual Tools
⚠️ Risk & Real-World Impact
Worst Case
Complete system takeover with root/administrator privileges, installation of persistent backdoors, data theft, ransomware deployment, and use as a pivot point for lateral movement within the network.
Likely Case
Remote code execution leading to system compromise, credential harvesting, installation of cryptocurrency miners or botnet malware, and potential data exfiltration.
If Mitigated
Limited impact with proper network segmentation, web application firewalls blocking malicious User-Agent strings, and intrusion detection systems alerting on exploitation attempts.
🎯 Exploit Status
Exploit code is publicly available on Exploit-DB (ID 50098) and requires minimal technical skill to execute. The attack can be automated and weaponized in mass scanning tools.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 4.2.29.0 or later
Vendor Advisory: https://visual-tools.com/
Restart Required: Yes
Instructions:
1. Download the latest version from Visual Tools website. 2. Backup current configuration. 3. Install the update following vendor instructions. 4. Restart the DVR service or system. 5. Verify the update was successful.
🔧 Temporary Workarounds
Network Segmentation and Access Control
linuxRestrict network access to the DVR system to only trusted IP addresses or VLANs
iptables -A INPUT -p tcp --dport 80 -s TRUSTED_IP -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
Web Application Firewall Rule
allBlock HTTP requests containing shell metacharacters in User-Agent header
ModSecurity rule: SecRule REQUEST_HEADERS:User-Agent "[;|&`$()]" "id:1001,phase:1,deny,status:403"
🧯 If You Can't Patch
- Immediately isolate the DVR system from internet access and restrict to internal network only
- Implement strict network segmentation with firewall rules blocking all unnecessary ports
🔍 How to Verify
Check if Vulnerable:
Test by sending a crafted HTTP request to the DVR's CGI endpoint with shell metacharacters in User-Agent header and observing responseCheck Version:
Check DVR web interface settings page or system information for software versionVerify Fix Applied:
Attempt the same exploit after patching - should receive error or no command execution📡 Detection & Monitoring
Log Indicators:
- HTTP requests to /cgi-bin/slogin/login.py with unusual User-Agent strings containing shell metacharacters
- System logs showing unexpected command execution or process creation
Network Indicators:
- HTTP traffic to DVR port 80/443 with malicious User-Agent patterns
- Outbound connections from DVR system to suspicious IPs post-exploit
SIEM Query:
source="dvr_logs" AND (uri="/cgi-bin/slogin/login.py" AND user_agent="*[;|&`$()]*")