VNC Pentesting

A skill for enumerating, connecting to, and exploiting VNC (Virtual Network Computing) services during penetration testing.

When to Use This Skill

Use this skill when:

• You discover VNC services on ports 5800, 5801, 5900, or 5901
• You need to enumerate VNC configurations and vulnerabilities
• You want to connect to a VNC server for remote desktop access
• You have a VNC password file that needs decryption
• You're performing brute force attacks against VNC authentication
• You need to search for exposed VNC services using Shodan

Basic Information

Virtual Network Computing (VNC) is a graphical desktop-sharing system using the Remote Frame Buffer (RFB) protocol. It enables remote control by transmitting keyboard and mouse events bidirectionally.

Common VNC Ports:

• 5800/tcp

  - VNC with HTTP tunneling

• 5801/tcp

  - VNC with HTTP tunneling (display :1)

• 5900/tcp

  - Standard VNC (display :0)

• 5901/tcp

  - Standard VNC (display :1)

Enumeration

Nmap Scanning

Run comprehensive VNC enumeration with Nmap:

# Basic service detection
nmap -sV -p 5900 <TARGET_IP>

# Advanced VNC enumeration with scripts
nmap -sV --script vnc-info,realvnc-auth-bypass,vnc-title -p <PORT> <IP>

# Full enumeration with version detection
nmap -sV -sC --script vnc-* -p 5800,5801,5900,5901 <TARGET_IP>

Metasploit Scanner

Use Metasploit's VNC scanner to find instances without authentication:

msfconsole
use auxiliary/scanner/vnc/vnc_none_auth
set RHOSTS <TARGET_IP>
set RPORT 5900
run

Shodan Search

Find exposed VNC services on the internet:

# Search for VNC on port 5900
shodan search "port:5900 RFB"

# Search for VNC with no authentication
shodan search "port:5900 RFB" "no authentication"

Connection

Using vncviewer (Kali Linux)

Connect to a VNC server:

# Basic connection
vncviewer <IP>::5901

# Connection with password file
vncviewer -passwd passwd.txt <IP>::5901

# Connection with display number
vncviewer <IP>:1

Note: Use

::

for port specification (e.g.,

192.168.1.100::5901

) and

:

for display number (e.g.,

192.168.1.100:1

).

Password Decryption

VNC passwords are encrypted using a weak 3DES cipher. If you obtain a VNC password file, you can decrypt it.

Linux/macOS

The default VNC password file location is

~/.vnc/passwd

.

Use the

vncpwd

tool to decrypt:

# Clone and build the tool
git clone https://github.com/jeroennijhof/vncpwd.git
cd vncpwd
make

# Decrypt the password
./vncpwd <path-to-password-file>

Windows

Use the VNC Password Decryptor tool available at:

• https://www.raymond.cc/blog/download/did/232/

Brute Force Attacks

If authentication is required, attempt brute force attacks:

# Using nmap
nmap -sV --script vnc-brute -p 5900 <TARGET_IP>

# Using metasploit
msfconsole
use auxiliary/attack/vnc/vnc_brute
set RHOSTS <TARGET_IP>
set RPORT 5900
set USER_FILE /path/to/usernames.txt
set PASS_FILE /path/to/passwords.txt
run

Common VNC Passwords to Try

Default or weak passwords often include:

• password

  ,

  admin

  ,

  root

  ,

  vnc

  ,

  123456

• guest

  ,

  test

  ,

  demo

  ,

  vncviewer

• Empty password (no authentication)

Security Considerations

• VNC with no authentication is a critical vulnerability
• VNC passwords are weakly encrypted and easily cracked
• Exposed VNC services on the internet are common attack vectors
• Always check for authentication bypass vulnerabilities
• Consider using SSH tunneling for secure VNC access

Workflow Summary

1. Discover - Identify VNC services via port scanning (5800, 5801, 5900, 5901)
2. Enumerate - Use Nmap scripts and Metasploit to gather information
3. Test Authentication - Check for no-auth or weak authentication
4. Connect - Use vncviewer to establish remote desktop session
5. Decrypt - If you have password files, decrypt them using vncpwd
6. Brute Force - If needed, attempt credential attacks

Tips

• Always try connecting without a password first (many VNC servers have no auth)
• The

  ::

  syntax in vncviewer specifies the port,

  :

  specifies the display
• VNC password files are typically 16 bytes when encrypted
• Use

  nmap --script vnc-info

  to get version and configuration details
• Check for RealVNC authentication bypass vulnerabilities