How to remediate – VNC Server Security Type Detection

1. Introduction

A VNC server is running on the remote host. This means a Virtual Network Computing service is accessible, potentially allowing remote graphical access to systems. It matters because an unsecure VNC server can allow unauthorised access to sensitive data and system control. Systems commonly affected include servers, workstations, and embedded devices with a GUI. A successful attack could compromise confidentiality, integrity, and availability.

2. Technical Explanation

This vulnerability indicates the presence of a VNC server without specific security measures being confirmed. Attackers can exploit weak or missing authentication to gain control of affected systems. The preconditions needed for exploitation include network connectivity to the VNC port (typically 5900 and above) and potentially known or guessable passwords. There is no CVE associated with simply running a VNC server, but vulnerabilities exist in specific implementations.

• Root cause: Lack of enforced authentication or use of weak default credentials on the VNC server.
• Exploit mechanism: An attacker connects to the VNC server using a client and attempts to authenticate with common usernames and passwords, or exploits known vulnerabilities in the protocol version being used. For example, an attacker could attempt brute-force attacks against the VNC password.
• Scope: Any system running a VNC server including Windows, Linux, macOS, and other operating systems supporting VNC implementations such as TightVNC, RealVNC, UltraVNC.

3. Detection and Assessment

Confirming a vulnerable system involves checking for the presence of a listening VNC service and identifying its security settings. A quick check can determine if the service is running at all. Thorough assessment requires examining the server configuration.

• Quick checks: Use netstat -tulnp | grep 5900 on Linux or netstat -ano | findstr "LISTENING" | findstr 5900 on Windows to check for a process listening on port 5900.
• Scanning: Nessus plugin ID 83461 (VNC Authentication Check) can identify VNC servers and attempt basic authentication tests. This is an example only.
• Logs and evidence: Examine system logs for VNC server startup events or connection attempts. Log file locations vary by operating system and VNC implementation.

netstat -tulnp | grep 5900

4. Solution / Remediation Steps

Fixing this issue involves securing the VNC server, disabling it if not needed, or restricting network access. These steps aim to prevent unauthorised remote control of systems.

4.1 Preparation

• Ensure you have alternative methods for accessing the system in case of connection issues. Roll back by restoring the snapshot or restarting the VNC service with its original configuration.
• Changes should be made during scheduled maintenance windows, and approved by the IT security team.

4.2 Implementation

1. Step 1: If the VNC server is not required, disable it completely using the operating system’s service management tools (systemctl stop vncserver on Linux or Services app on Windows).
2. Step 2: If the VNC server must remain active, configure strong authentication. This includes setting a complex password and enabling encryption if available.
3. Step 3: Restrict network access to the VNC port (5900+) using firewalls or network ACLs to only trusted IP addresses.

4.3 Config or Code Example

Before

# /etc/vnc/config.d/default (example - no password)
# No password set, allowing anyone with network access to connect

After

# /etc/vnc/config.d/default (example - strong password)
Password=your_strong_password
SecurityTypes=High

4.4 Security Practices Relevant to This Vulnerability

• Practice 1: Least privilege – limit access to the VNC server only to authorized users and systems, reducing impact if compromised.
• Practice 2: Strong authentication – enforce complex passwords and multi-factor authentication where possible to prevent brute-force attacks.

4.5 Automation (Optional)

# Example Ansible playbook snippet to set VNC password
- name: Set VNC Password
  copy:
    dest: /etc/vnc/config.d/default
    content: |
      Password=your_strong_password
      SecurityTypes=High
  notify: Restart VNC Server
- name: Restart VNC Server
  service:
    name: vncserver
    state: restarted

5. Verification / Validation

Confirm the fix by verifying that strong authentication is enabled and network access is restricted. A negative test should confirm unauthorized connections are blocked.

• Post-fix check: Run netstat -tulnp | grep 5900 to ensure the VNC server is still running, then attempt a connection from an untrusted host – it should be refused.
• Re-test: Re-run the Nessus scan (ID 83461) and confirm that authentication tests now fail without valid credentials.
• Smoke test: Verify authorized users can still connect to the VNC server using their new credentials.
• Monitoring: Monitor system logs for failed VNC connection attempts from untrusted IP addresses.

netstat -tulnp | grep 5900

6. Preventive Measures and Monitoring

Suggest only measures that are relevant to the vulnerability type. Use “for example” to keep advice conditional, not prescriptive.

• Baselines: Update security baselines or policies to require strong authentication for all remote access services (for example, CIS control 5).
• Pipelines: Add checks in CI/CD pipelines to ensure default passwords are not used and that network configurations restrict access.
• Asset and patch process: Implement a regular review cycle for remote access service configurations to identify and address potential vulnerabilities.

7. Risks, Side Effects, and Roll Back

• Risk or side effect 2: Restricting network access too aggressively may disrupt valid remote administration tasks. Mitigation: Carefully plan and document allowed IP addresses.

8. References and Resources

• Vendor advisory or bulletin: RealVNC Security Information – https://www.realvnc.com/en/security/
• NVD or CVE entry: No specific CVE for running VNC, but search for vulnerabilities in your specific implementation at https://nvd.nist.gov/
• Product or platform documentation relevant to the fix: RealVNC Configuration Guide – https://help.realvnc.com/hc/en-us