CVE-2019-25600 Overview
CVE-2019-25600 is a denial of service vulnerability in UltraVNC Viewer version 1.2.2.4. The flaw exists in the VNC Server input field, which fails to validate the length of supplied strings. An attacker who tricks a user into pasting a crafted string of 256 repeated characters into the VNC Server field and clicking Connect triggers a buffer overflow that crashes the viewer process. The issue is classified as an out-of-bounds write [CWE-787]. Successful exploitation does not result in code execution but disrupts availability of the VNC client. The vulnerability requires user interaction, which limits remote exploitability.
Critical Impact
A local user can crash the UltraVNC Viewer client by pasting an oversized 256-character string into the VNC Server input field, resulting in a denial of service condition.
Affected Products
- UltraVNC Viewer 1.2.2.4
- UltraVNC distributions bundling the affected viewer build
- Systems running the vulnerable viewer for remote desktop sessions
Discovery Timeline
- 2026-03-22 - CVE-2019-25600 published to the National Vulnerability Database (NVD)
- 2026-04-16 - Last updated in NVD database
Technical Details for CVE-2019-25600
Vulnerability Analysis
The vulnerability resides in the UltraVNC Viewer client component responsible for accepting user input in the VNC Server connection field. The application does not enforce bounds checking on the input buffer before processing the supplied string. When a user pastes a string of 256 identical characters and initiates a connection, the viewer writes past the allocated buffer boundary. This out-of-bounds write [CWE-787] corrupts adjacent memory and causes the viewer process to terminate unexpectedly. The defect impacts availability of the client only and does not compromise confidentiality or integrity, according to the CVSS metrics published with the advisory.
Root Cause
The root cause is missing input length validation in the VNC Server input field handler. The fixed-size destination buffer cannot accommodate strings of 256 characters, and the code path lacks a guard against oversized input. This pattern is consistent with classic stack or heap buffer overflows in C/C++ string handling, where functions copy attacker-controlled data without verifying destination capacity.
Attack Vector
Exploitation requires user interaction. An attacker delivers a malicious 256-character string through phishing, instant messaging, a malicious web page, or a clipboard-poisoning technique. The victim then pastes the string into the UltraVNC Viewer's VNC Server field and clicks Connect. The viewer process crashes immediately, denying the user remote desktop functionality until the application is restarted. The attack vector is described as network-reachable because the malicious payload can be delivered remotely, but exploitation depends on the victim's action within the local client.
No verified proof-of-concept code is reproduced here. Refer to the Exploit-DB #46702 entry and the VulnCheck Advisory on UltraVNC for technical reproduction details.
Detection Methods for CVE-2019-25600
Indicators of Compromise
- Repeated unexpected crashes of vncviewer.exe shortly after a user initiates a connection
- Windows Error Reporting (WER) entries referencing UltraVNC Viewer with access violation exception codes such as 0xC0000005
- Clipboard contents or chat logs containing strings of 256 repeated characters delivered to users running UltraVNC Viewer
Detection Strategies
- Inventory endpoints running UltraVNC Viewer 1.2.2.4 using software asset management or endpoint telemetry
- Monitor application crash events tied to the UltraVNC Viewer process and correlate with recent user input activity
- Flag delivery of suspiciously long single-character strings through email, chat, and web channels that target VNC operators
Monitoring Recommendations
- Enable Windows Error Reporting collection and forward crash telemetry to a centralized logging or SIEM platform
- Alert on repeated vncviewer.exe terminations on the same host within short time windows
- Track outbound connection attempts from UltraVNC Viewer to validate operational use and detect anomalous patterns
How to Mitigate CVE-2019-25600
Immediate Actions Required
- Identify all systems running UltraVNC Viewer 1.2.2.4 and prioritize them for upgrade
- Upgrade to a current UltraVNC release available from the UltraVNC download page
- Restrict UltraVNC Viewer usage to administrators with a documented need for remote desktop functionality
- Educate users to avoid pasting untrusted content into VNC Viewer input fields
Patch Information
Upgrade to a current UltraVNC release from the UVNC homepage. The vendor advisory references the 1.2.2.4 build as affected; later releases addressed input handling defects in the viewer. Confirm the installed version after upgrade by checking the viewer's About dialog.
Workarounds
- Replace UltraVNC Viewer with an alternative VNC client until upgrade is feasible
- Disable clipboard sharing or paste functionality in the viewer where supported to reduce the likelihood of malicious string injection
- Apply application allowlisting to prevent unauthorized versions of vncviewer.exe from executing on managed endpoints
# Configuration example - inventory UltraVNC Viewer versions via PowerShell
Get-CimInstance Win32_Product | Where-Object { $_.Name -like '*UltraVNC*' } | Select-Object Name, Version, InstallLocation
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
