CVE-2026-50538 — LibVNCClient is a library for easy implementation of a VNC client.¶

Key facts

CVE-2026-50538 is a High (8.8) vulnerability: LibVNCClient is a library for easy implementation of a VNC client.. This entry ships a detection-only PoC and a Docker test lab.


CVE	CVE-2026-50538
Severity	High (8.8)
Detection / prevention	—
PoC	poc.py
Source	2026/CVE-2026-50538/

Details¶

English | 한국어

Description¶

LibVNCClient is a library for easy implementation of a VNC client. In versions 0.9.12 through 0.9.15, a malicious (or man-in-the-middle) VNC server can force a connecting libvncclient to write attacker-controlled data past the end of its framebuffer. This is an out-of-bounds heap write with attacker-controlled length, contents, and offset. It needs no authentication (the attacker is the server), works in a default build with default settings, and fires from a single FramebufferUpdate the moment the victim connects. It crashes any client unconditionally (denial of service); we also demonstrated it overwriting an application callback pointer and redirecting execution to attacker-chosen code (code execution) under the default configuration. Commit 540332be3e0acc566fa64da6f1b4680c72c724dd patches the issue.

Vulnerability Details¶

Property	Value
CVE ID	CVE-2026-50538
Type	RCE
CVSS Score	8.8

Proof of Concept¶

Download¶

# Download PoC script
curl -O https://raw.githubusercontent.com/fankh/vulnerability-poc/main/2026/CVE-2026-50538/poc.py

Usage¶

# Run PoC against target
python3 poc.py --target <TARGET_URL>

# With authentication
python3 poc.py --target <TARGET_URL> --username <USER> --password <PASS>

⚠️ Warning: This PoC is for authorized security testing only. It uses detection-only techniques and does not exploit or damage target systems.

Test Lab¶

This repository includes a Docker-based test environment with both vulnerable and patched applications.

Quick Start¶

# Clone and navigate to CVE directory
cd CVE-2026-50538

# Start both applications
docker-compose up -d

# Run tests
./run-tests.sh    # Linux/Mac
./run-tests.ps1   # Windows PowerShell

Application Endpoints¶

Application	URL	Status
Vulnerable App	http://localhost:8080	Shows vulnerability
Patched App	http://localhost:8081	Demonstrates fix

Directory Structure¶

├── README.md           # English documentation
├── README_KO.md        # Korean documentation
├── poc.py              # PoC detection script
├── docker-compose.yml  # Container orchestration
├── run-tests.sh        # Linux/Mac test runner
├── run-tests.ps1       # Windows test runner
├── vulnerable-app/     # Vulnerable version
│   ├── app.py
│   ├── Dockerfile
│   └── requirements.txt
└── patched-app/        # Fixed version
    ├── app.py
    ├── Dockerfile
    └── requirements.txt

References¶

https://github.com/LibVNC/libvncserver/commit/540332be3e0acc566fa64da6f1b4680c72c724dd
https://github.com/LibVNC/libvncserver/security/advisories/GHSA-v9pm-47h4-jcq8

Generated by PoC Generator on 2026-08-22T10:11:15.475681226

Frequently asked questions¶

What is CVE-2026-50538?¶

CVE-2026-50538 is a High (8.8) vulnerability: LibVNCClient is a library for easy implementation of a VNC client.. This entry ships a detection-only PoC and a Docker test lab.

How do I detect CVE-2026-50538?¶

Run the detection-only PoC included with this entry against your Docker test lab to confirm whether a target is affected; pair it with your own IDS logging.

How do I block or mitigate CVE-2026-50538?¶

Upgrade to a fixed version per the vendor advisory; the NVD record lists the affected versions. Use the included PoC to verify your remediation.

For authorized security testing and research only. See the full source and NVD record.