CVE-2026-3787
📋 TL;DR
This vulnerability in UltraVNC 1.6.4.0 on Windows involves an uncontrolled search path weakness in cryptbase.dll that could allow local attackers to execute arbitrary code by placing malicious DLLs in specific directories. It affects Windows systems running UltraVNC 1.6.4.0 and requires local access with high complexity to exploit.
💻 Affected Systems
- UltraVNC
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Local privilege escalation leading to full system compromise, data theft, or persistent backdoor installation.
Likely Case
Limited impact due to high exploit complexity and local access requirement; potential for unauthorized code execution by malicious local users.
If Mitigated
Minimal impact with proper access controls, least privilege principles, and DLL search path hardening.
🎯 Exploit Status
Attack requires local access and high complexity; vendor unresponsive to disclosure.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Unknown - vendor did not respond
Vendor Advisory: None available
Restart Required: Yes
Instructions:
No official patch available. Consider upgrading to newer UltraVNC versions if available, or implementing workarounds.
🔧 Temporary Workarounds
Restrict DLL search path
windowsConfigure Windows to restrict DLL search paths using SafeDllSearchMode or SetDefaultDllDirectories API
reg add "HKLM\SYSTEM\CurrentControlSet\Control\Session Manager" /v SafeDllSearchMode /t REG_DWORD /d 1 /f
Remove vulnerable UltraVNC version
windowsUninstall UltraVNC 1.6.4.0 and replace with alternative remote access solutions
control panel > Programs > Uninstall UltraVNC 1.6.4.0
🧯 If You Can't Patch
- Implement strict local access controls and least privilege principles
- Monitor for suspicious DLL loading behavior and file system changes
🔍 How to Verify
Check if Vulnerable:
Check UltraVNC version: Open UltraVNC Viewer/Server > Help > About, or check installed programs list for version 1.6.4.0Check Version:
wmic product where name="UltraVNC" get versionVerify Fix Applied:
Verify UltraVNC is no longer version 1.6.4.0, or that workarounds are properly configured📡 Detection & Monitoring
Log Indicators:
- Windows Event Logs showing DLL loading from unexpected paths
- Process Monitor logs showing cryptbase.dll being loaded from non-standard locations
Network Indicators:
- Not applicable - local vulnerability
SIEM Query:
EventID=7 from Sysmon showing ImageLoaded from suspicious paths containing cryptbase.dll