CVE-2020-37133 Overview
CVE-2020-37133 is a denial of service vulnerability affecting UltraVNC Launcher version 1.2.4.0. The vulnerability exists in the Repeater Host configuration field and allows attackers to crash the application by inputting an overly long string. Specifically, pasting a string of 300 characters or more into the Repeater Host property triggers an application crash due to a stack-based buffer overflow (CWE-121).
Critical Impact
Local attackers can cause application crashes and denial of service by exploiting improper input validation in the Repeater Host configuration field, potentially disrupting remote access operations that depend on UltraVNC Launcher.
Affected Products
- UltraVNC Launcher version 1.2.4.0
Discovery Timeline
- 2026-02-05 - CVE-2020-37133 published to NVD
- 2026-02-05 - Last updated in NVD database
Technical Details for CVE-2020-37133
Vulnerability Analysis
This vulnerability is classified as CWE-121 (Stack-based Buffer Overflow), a memory corruption issue that occurs when the application fails to properly validate the length of user-supplied input before copying it to a fixed-size stack buffer. When an attacker provides an excessively long string (300+ characters) in the Repeater Host configuration field, the application attempts to store this data in a buffer that cannot accommodate it, leading to memory corruption and subsequent application crash.
The local attack vector requires user interaction—the attacker must either convince a user to paste the malicious string or have local access to manipulate the application configuration. While the immediate impact is limited to denial of service through application crashes, stack-based buffer overflows can potentially be leveraged for more severe attacks depending on the application's memory layout and security mitigations in place.
Root Cause
The root cause is improper input validation in the UltraVNC Launcher application when processing the Repeater Host configuration field. The application allocates a fixed-size buffer on the stack to store the Repeater Host value but fails to verify that user input does not exceed this buffer's capacity before performing the copy operation. This classic boundary condition error allows attackers to overflow the stack buffer by providing input longer than expected.
Attack Vector
The attack vector is local, requiring either direct access to the system running UltraVNC Launcher or social engineering to convince a user to paste malicious input. The attacker crafts a string of 300 or more characters and inputs it into the Repeater Host property field within the application's configuration interface. Once this oversized string is processed, it overflows the stack buffer, corrupting adjacent memory and causing the application to crash. No authentication or special privileges are required to trigger this vulnerability, though user interaction is necessary.
The exploitation is straightforward—an attacker simply needs to paste an overly long string into the vulnerable input field. Technical details regarding the exploitation technique can be found in the Exploit-DB #48288 entry and the VulnCheck Advisory.
Detection Methods for CVE-2020-37133
Indicators of Compromise
- Unexpected crashes or termination of the UltraVNC Launcher application
- Application event logs showing abnormal termination or memory access violations
- Configuration files containing unusually long Repeater Host values (300+ characters)
Detection Strategies
- Monitor for repeated UltraVNC Launcher process crashes using endpoint detection tools
- Implement application crash monitoring to detect patterns consistent with exploitation attempts
- Review application configuration files for anomalous or excessively long field values
Monitoring Recommendations
- Enable Windows Event Log monitoring for application crashes related to UltraVNC Launcher processes
- Deploy endpoint protection solutions capable of detecting stack-based buffer overflow exploitation attempts
- Implement file integrity monitoring on UltraVNC Launcher configuration files to detect unauthorized modifications
How to Mitigate CVE-2020-37133
Immediate Actions Required
- Restrict access to systems running UltraVNC Launcher 1.2.4.0 to trusted users only
- Consider disabling or removing UltraVNC Launcher if not actively required
- Monitor the UVNC Official Site for security updates and patched versions
- Implement application allowlisting to prevent unauthorized configuration changes
Patch Information
At the time of publication, specific vendor patch information was not available in the CVE data. Users should monitor the official UVNC website and the VulnCheck Advisory for updates regarding patched versions of UltraVNC Launcher. It is recommended to upgrade to the latest available version once a fix is released.
Workarounds
- Limit local access to the UltraVNC Launcher application to prevent unauthorized configuration changes
- Implement application control policies to restrict who can modify UltraVNC Launcher settings
- Use network segmentation to isolate systems running vulnerable versions of UltraVNC Launcher
- Consider alternative VNC solutions if a patched version is not available
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
