How to remediate – VNCviewer in Listen Mode Detection

1. Introduction

VNCviewer in Listen Mode Detection indicates a remote control service is accepting incoming connections on your network. This allows a VNC server to connect back to a client, potentially giving an attacker access to the system. Systems running VNC viewer software are usually affected. A successful exploit could compromise confidentiality, integrity and availability of the host.

2. Technical Explanation

• Root cause: The service is configured in listen mode without appropriate network restrictions.
• Exploit mechanism: An attacker connects from a VNC server to the listening VNC viewer instance on the target host, initiating a remote control session.
• Scope: Systems running any version of VNCviewer with listen mode enabled are affected.

3. Detection and Assessment

You can confirm if a system is vulnerable by checking for the listening service and its associated port. A quick check involves using command line tools to identify open ports, while thorough assessment requires network scanning.

• Quick checks: Use netstat -tulnp or ss -tulnp to list listening ports and processes. Look for VNCviewer listening on port 5900 or other non-standard ports.
• Scanning: Nessus plugin ID 16843 can detect VNC services in listen mode, but results should be verified manually.
• Logs and evidence: Check system logs (e.g., syslog) for VNCviewer startup events with listen mode enabled.

netstat -tulnp | grep vnc

4. Solution / Remediation Steps

To fix this issue, limit incoming traffic to the port used by VNC viewer or disable listen mode if it is not required. These steps are small and can be rolled back easily.

4.1 Preparation

• The roll back plan involves restoring the system snapshot or restarting the stopped services.
• A change window may be needed depending on service criticality and impact of downtime. Approval from the IT manager is recommended.

4.2 Implementation

1. Step 1: Configure your firewall to block incoming traffic to the VNC viewer port (typically 5900).
2. Step 2: If listen mode is not required, disable it in the VNCviewer configuration settings.
3. Step 3: Restart the VNCviewer service for changes to take effect.

4.3 Config or Code Example

Before

# /etc/systemd/system/vncserver@:service
ExecStart=/usr/bin/vncserver :1 -localhost no

After

# /etc/systemd/system/vncserver@:service
ExecStart=/usr/bin/vncserver :1 -localhost yes

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue. Least privilege reduces the impact if exploited, and network segmentation limits exposure.

• Practice 1: Implement least privilege by running VNCviewer with a non-administrative user account.
• Practice 2: Use network segmentation to isolate systems running VNC viewer from sensitive networks.

4.5 Automation (Optional)

# Example Bash script for blocking port 5900 using UFW
sudo ufw deny 5900/tcp comment 'Block incoming VNC connections'
sudo ufw enable

5. Verification / Validation

Confirm the fix by checking that incoming traffic is blocked to the VNC viewer port and re-running the detection methods. A service smoke test should verify basic functionality remains intact.

• Post-fix check: Run netstat -tulnp again; VNCviewer should no longer be listening on the external interface.
• Re-test: Re-run the earlier detection methods (e.g., Nessus scan) to confirm the vulnerability is no longer reported.
• Smoke test: Attempt a basic VNC connection from an internal host to verify connectivity if required.
• Monitoring: Monitor firewall logs for blocked connections to port 5900 as an example of regression detection.

netstat -tulnp | grep vnc

6. Preventive Measures and Monitoring

Update security baselines to include restrictions on VNC viewer ports, and add checks in deployment pipelines to prevent listen mode being enabled. A regular patch cycle is also sensible.

• Baselines: Update your security baseline or policy (e.g., CIS control 5) to require blocking incoming traffic to port 5900 by default.
• Pipelines: Add checks in CI/CD pipelines to scan for VNCviewer configuration files with listen mode enabled.
• Asset and patch process: Implement a regular review cycle (e.g., monthly) of system configurations to identify and remediate any instances of VNC viewer in listen mode.

7. Risks, Side Effects, and Roll Back

Blocking incoming traffic may prevent legitimate VNC connections. The roll back steps involve restoring the firewall rule or re-enabling listen mode.

• Risk or side effect 1: Blocking port 5900 could disrupt existing remote access workflows.
• Risk or side effect 2: Disabling listen mode may require reconfiguration of VNC servers.
• Roll back: Step 1: Restore the original firewall rule using sudo ufw allow 5900/tcp. Step 2: Re-enable listen mode in the VNCviewer configuration settings and restart the service.

8. References and Resources

• Vendor advisory or bulletin: http://www.nessus.org/u?ec9f0272
• NVD or CVE entry: Not applicable at this time.
• Product or platform documentation relevant to the fix: http://gentoo-wiki.com/VNC#Reverse_VNC