CVE-2019-25600: UltraVNC Viewer DoS Vulnerability

CVE-2019-25600 Overview

CVE-2019-25600 is a denial of service vulnerability affecting UltraVNC Viewer version 1.2.2.4. The vulnerability exists due to improper input validation in the VNC Server input field, allowing attackers to crash the application by supplying an oversized string. When a malicious string containing 256 or more repeated characters is pasted into the VNC Server field and the Connect button is clicked, a buffer overflow condition occurs that terminates the viewer process.

UltraVNC is a popular open-source remote desktop software used extensively in enterprise environments for remote administration and support purposes. This vulnerability poses a risk to organizations relying on UltraVNC for remote connectivity, as attackers could disrupt remote access capabilities.

Critical Impact

Attackers can cause application crashes through buffer overflow by injecting oversized input into the VNC Server field, leading to denial of service and potential disruption of remote administration capabilities.

Affected Products

• UltraVNC Viewer 1.2.2.4

Discovery Timeline

• 2026-03-22 - CVE CVE-2019-25600 published to NVD
• 2026-03-23 - Last updated in NVD database

Technical Details for CVE-2019-25600

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-bounds Write), which occurs when the software writes data past the end of the intended buffer. In the context of UltraVNC Viewer, the application fails to properly validate the length of user-supplied input in the VNC Server connection field before processing it. When an attacker provides an input string exceeding the expected buffer size (256 characters or more), the application attempts to write beyond the allocated memory boundaries, resulting in memory corruption and subsequent application crash.

The vulnerability requires user interaction to exploit, as an attacker would need to convince a user to paste a malicious string into the VNC Server field or potentially leverage social engineering tactics. However, the attack can be executed remotely via network-based scenarios where the attacker controls the input to the VNC Server field.

Root Cause

The root cause of this vulnerability lies in insufficient input length validation within the VNC Server connection handling code. The application allocates a fixed-size buffer for storing the server address but does not enforce bounds checking when copying user input into this buffer. This allows oversized input to overflow the buffer, corrupting adjacent memory and causing the application to crash.

The lack of proper input sanitization and boundary validation represents a fundamental secure coding oversight that could have been prevented through the use of safe string handling functions that enforce length limits.

Attack Vector

The attack vector is network-based with a requirement for user interaction. An attacker can exploit this vulnerability through the following methods:

1. Direct Input Injection: An attacker with local or remote access to the UltraVNC Viewer interface pastes a crafted string of 256+ repeated characters into the VNC Server field and clicks Connect
2. Social Engineering: An attacker convinces a victim to copy-paste a malicious connection string, potentially disguised as a legitimate VNC server address
3. Clipboard Manipulation: In scenarios where clipboard contents can be controlled, an attacker could plant malicious input that gets pasted into the VNC Server field

The exploitation is straightforward and requires no special privileges or authentication. When the oversized string is processed during the connection attempt, the buffer overflow triggers an unhandled exception, causing the application to crash immediately.

For detailed technical analysis and proof-of-concept information, refer to Exploit-DB #46702.

Detection Methods for CVE-2019-25600

Indicators of Compromise

• Unexpected UltraVNC Viewer application crashes or terminations
• Windows Error Reporting events indicating UltraVNC Viewer crashes with access violation or stack buffer overflow errors
• Presence of crash dump files related to vncviewer.exe with memory corruption signatures
• Unusual clipboard activity involving long strings prior to UltraVNC crashes

Detection Strategies

• Monitor Windows Application Event Logs for repeated crash events associated with UltraVNC Viewer (vncviewer.exe)
• Deploy endpoint detection rules to identify buffer overflow patterns in UltraVNC processes
• Implement application crash monitoring to detect potential exploitation attempts through recurring crashes
• Use SentinelOne's behavioral AI to detect anomalous application terminations indicative of exploitation

Monitoring Recommendations

• Enable detailed crash reporting for UltraVNC applications to capture stack traces and memory states
• Implement centralized logging for application crash events across the enterprise
• Configure SentinelOne endpoint agents to monitor for process crashes with buffer overflow indicators
• Establish baseline crash rates to identify anomalous spikes that may indicate active exploitation attempts

How to Mitigate CVE-2019-25600

Immediate Actions Required

• Upgrade UltraVNC Viewer to the latest available version from the official UltraVNC website
• Restrict access to UltraVNC Viewer to authorized users only
• Implement application whitelisting to control which users can execute VNC client software
• Educate users about the risks of pasting untrusted content into connection dialogs
• Consider alternative VNC clients with stronger input validation until patching is complete

Patch Information

Organizations should upgrade to a patched version of UltraVNC Viewer that addresses this input validation issue. Visit the UltraVNC Download Page to obtain the latest version. Review the VulnCheck Advisory for additional guidance on remediation.

Workarounds

• Implement network segmentation to limit exposure of systems running vulnerable UltraVNC versions
• Use SentinelOne endpoint protection to monitor and alert on suspicious application behavior
• Deploy input filtering at the network perimeter to block known exploit patterns
• Restrict clipboard functionality in high-security environments where UltraVNC is used
• Consider using VPN or secure tunnels to limit network exposure of VNC connections

# Configuration example - Restrict UltraVNC Viewer access via Windows Group Policy
# Create a Software Restriction Policy to limit UltraVNC execution

# 1. Open Group Policy Editor (gpedit.msc)
# 2. Navigate to: Computer Configuration > Windows Settings > Security Settings > Software Restriction Policies
# 3. Create new policy and add path rule for vncviewer.exe
# 4. Set security level to "Disallowed" for unauthorized users

# Alternative: Use application control via SentinelOne to block or monitor vncviewer.exe execution
# SentinelOne Console: Sentinels > [Select Group] > Policy > Application Control > Add Rule