CVE-2020-37134 Overview
CVE-2020-37134 is a Denial of Service vulnerability affecting UltraVNC Viewer version 1.2.4.0. The vulnerability allows attackers to crash the application by manipulating VNC Server input. Specifically, attackers can generate a malformed 256-byte payload and paste it into the VNC Server connection dialog to trigger an application crash, resulting in service disruption for users relying on the remote desktop functionality.
Critical Impact
Attackers can cause application crashes through malformed input, disrupting remote access capabilities for organizations using UltraVNC Viewer for remote administration and support purposes.
Affected Products
- UltraVNC Viewer 1.2.4.0
Discovery Timeline
- 2026-02-05 - CVE-2020-37134 published to NVD
- 2026-02-05 - Last updated in NVD database
Technical Details for CVE-2020-37134
Vulnerability Analysis
This vulnerability is classified under CWE-770 (Allocation of Resources Without Limits or Throttling). The flaw exists in how UltraVNC Viewer processes input data in the VNC Server connection dialog. The application fails to properly validate or limit the size and content of user-supplied input, allowing a specially crafted 256-byte payload to overwhelm the application's input handling mechanisms.
The local attack vector requires an attacker to have access to the target system where UltraVNC Viewer is installed. User interaction is required as the malicious payload must be pasted into the VNC Server connection dialog by a user or through social engineering techniques.
Root Cause
The root cause of this vulnerability stems from improper resource allocation handling (CWE-770). UltraVNC Viewer 1.2.4.0 does not implement adequate bounds checking or input validation when processing data entered into the VNC Server connection field. This lack of proper input sanitization allows malformed data to be processed, leading to uncontrolled resource consumption and subsequent application crash.
Attack Vector
The attack requires local access to a system running UltraVNC Viewer 1.2.4.0. An attacker must craft a specific 256-byte malicious payload and either trick a user into pasting it into the VNC Server connection dialog or have direct access to perform this action themselves. When the malformed payload is processed, the application fails to handle the unexpected input properly, resulting in a crash condition.
The vulnerability can be triggered through:
- Direct local access to paste malicious payload into the connection dialog
- Social engineering to trick users into copying and pasting malicious content
- Clipboard hijacking techniques to inject the payload when a user attempts to paste legitimate server addresses
Detection Methods for CVE-2020-37134
Indicators of Compromise
- Unexpected crashes of the vncviewer.exe process
- Windows Event Log entries indicating application failures for UltraVNC Viewer
- Clipboard activity containing unusually large or malformed connection strings (approximately 256 bytes of suspicious data)
Detection Strategies
- Monitor for repeated crashes of UltraVNC Viewer application on endpoint systems
- Implement application crash monitoring to detect unusual termination patterns
- Deploy endpoint detection rules to identify malformed clipboard content being pasted into VNC applications
Monitoring Recommendations
- Enable Windows Application Event Log monitoring for UltraVNC Viewer crash events
- Configure endpoint security solutions to alert on repeated application crashes
- Monitor user activity logs for suspicious clipboard operations targeting VNC applications
How to Mitigate CVE-2020-37134
Immediate Actions Required
- Upgrade UltraVNC Viewer to a patched version beyond 1.2.4.0 if available
- Restrict access to systems running vulnerable UltraVNC Viewer installations
- Educate users about the risks of pasting untrusted content into the VNC connection dialog
- Consider implementing application allowlisting to prevent unauthorized VNC client usage
Patch Information
Organizations should check the UltraVNC Official Site for updated versions that address this vulnerability. The VulnCheck Advisory on UltraVNC provides additional details about affected versions and remediation guidance. Technical details about the vulnerability mechanism are available via Exploit-DB #48291.
Workarounds
- Deploy alternative VNC viewer solutions that are not affected by this vulnerability
- Implement clipboard filtering or monitoring solutions to block suspicious payloads
- Restrict UltraVNC Viewer usage to controlled environments with limited external data input
- Configure Group Policy to limit clipboard functionality where VNC clients are deployed
# Configuration example - Disable clipboard in Windows Group Policy
# Navigate to: Computer Configuration > Administrative Templates > Windows Components > Remote Desktop Services > Remote Desktop Session Host > Device and Resource Redirection
# Enable: "Do not allow clipboard redirection"
# Alternative: Use registry to restrict clipboard
reg add "HKLM\SOFTWARE\Policies\Microsoft\Windows NT\Terminal Services" /v fDisableClip /t REG_DWORD /d 1 /f
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
