CVE-2019-25601: UltraVNC Launcher DoS Vulnerability

CVE-2019-25601 Overview

UltraVNC Launcher version 1.2.2.4 contains a buffer overflow vulnerability in the Path vncviewer.exe property field that allows local attackers to crash the application by supplying an excessively long string. Attackers can input a 300-byte payload of repeated characters through the Properties dialog to trigger a denial of service condition.

Critical Impact

Local attackers can crash UltraVNC Launcher by exploiting a buffer overflow in the Path vncviewer.exe property field, causing denial of service to remote desktop connectivity tools.

Affected Products

• UltraVNC Launcher 1.2.2.4

Discovery Timeline

• 2026-03-22 - CVE CVE-2019-25601 published to NVD
• 2026-03-23 - Last updated in NVD database

Technical Details for CVE-2019-25601

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-Bounds Write), which occurs when the software writes data past the end or before the beginning of the intended buffer. In UltraVNC Launcher, the application fails to properly validate the length of user-supplied input in the Path vncviewer.exe property field within the Properties dialog.

The local attack vector requires the attacker to have access to the system where UltraVNC Launcher is installed. No authentication is required to exploit this vulnerability, and no user interaction is necessary beyond the attacker having local access to manipulate the application's configuration.

Root Cause

The root cause is improper input validation in UltraVNC Launcher's handling of the Path property field. When a user enters a path string in the vncviewer.exe property field through the Properties dialog, the application copies this input into a fixed-size buffer without adequate bounds checking. This allows an attacker to supply a string longer than the allocated buffer space, triggering a buffer overflow condition that corrupts adjacent memory and causes the application to crash.

Attack Vector

The attack is executed locally by an attacker with access to the UltraVNC Launcher application. The attacker navigates to the Properties dialog and enters an excessively long string (approximately 300 bytes of repeated characters) into the Path vncviewer.exe property field. When the application attempts to process this oversized input, the buffer overflow occurs, causing application instability and ultimately a denial of service condition.

The vulnerability exploitation does not require elevated privileges and can be triggered through the standard user interface. While this vulnerability results in denial of service through application crash, the out-of-bounds write condition could potentially be leveraged for more severe attacks if an attacker crafts a specific payload, though the current documented impact is limited to denial of service.

For detailed technical information about this vulnerability, refer to the Exploit-DB #46703 entry and the VulnCheck Advisory on UltraVNC.

Detection Methods for CVE-2019-25601

Indicators of Compromise

• Unexpected crashes or termination of the UltraVNC Launcher process (launcher.exe)
• Windows Event Log entries indicating application faults or access violations in UltraVNC Launcher
• Configuration files containing abnormally long path strings in the vncviewer.exe property
• Crash dump files associated with UltraVNC Launcher showing buffer overflow patterns

Detection Strategies

• Monitor for repeated UltraVNC Launcher application crashes on endpoints
• Implement application whitelisting and configuration monitoring to detect unauthorized modifications to UltraVNC settings
• Deploy endpoint detection solutions capable of identifying buffer overflow exploitation attempts
• Review Windows Application Event Logs for UltraVNC Launcher crash events

Monitoring Recommendations

• Enable crash reporting and centralized logging for UltraVNC Launcher deployments
• Implement file integrity monitoring on UltraVNC configuration files
• Set up alerts for anomalous application termination patterns
• Utilize SentinelOne's behavioral AI to detect exploitation attempts targeting desktop applications

How to Mitigate CVE-2019-25601

Immediate Actions Required

• Restrict local access to systems running vulnerable versions of UltraVNC Launcher
• Evaluate the necessity of UltraVNC Launcher in your environment and remove if not required
• Implement application control policies to prevent unauthorized users from modifying UltraVNC settings
• Monitor for updated versions of UltraVNC Launcher that address this vulnerability

Patch Information

Users should check the UVNC Official Site and the UltraVNC Download Page for updated versions that may address this buffer overflow vulnerability. Upgrading to the latest available version is recommended when patches become available.

Workarounds

• Restrict access to UltraVNC Launcher to trusted administrators only
• Implement least-privilege principles for users who require access to remote desktop tools
• Consider using alternative VNC solutions with better input validation
• Deploy application sandboxing to limit the impact of potential exploitation