How to remediate – XSTUNT Server Detection

1. Introduction

XSTUNT Server Detection indicates a service capable of TCP NAT traversal is listening on your network. XSTUNT extends STUN to work over TCP, potentially allowing applications behind firewalls to establish connections. This could indicate an unintended open port or the presence of software designed for peer-to-peer communication without explicit approval. A successful exploit could allow remote systems to initiate connections to internal hosts, impacting confidentiality, integrity and availability depending on the application using the service.

2. Technical Explanation

XSTUNT servers assist clients behind Network Address Translation (NAT) in discovering their public IP address and port for incoming connections. The server listens for connection attempts and responds with information to facilitate communication. Exploitation typically involves an attacker using a client application to connect to the XSTUNT server, then leveraging this information to establish unwanted connections to internal systems. There is no specific CVE associated with simply *running* an XSTUNT server; risk depends on the application it supports.

• Root cause: The presence of an XSTUNT service listening for incoming TCP connections.
• Exploit mechanism: An attacker connects to the XSTUNT server, discovers internal IP and port information, then attempts to establish a connection with a vulnerable application using that information.
• Scope: Any system running an XSTUNT server implementation. This is most commonly found in peer-to-peer applications or services requiring NAT traversal.

3. Detection and Assessment

Confirming the presence of an XSTUNT service can be done through network scanning and process inspection.

• Quick checks: Use netstat -tulnp to list listening TCP ports and associated processes. Look for any unexpected services listening on standard or non-standard ports.
• Scanning: Nessus plugin ID 16825 can detect XSTUNT servers, but results should be verified manually.
• Logs and evidence: Examine firewall logs for connections to port 443 (or other unusual ports) from external sources attempting to establish TCP connections with internal hosts.

netstat -tulnp | grep 

4. Solution / Remediation Steps

Remediating this issue involves assessing the necessity of the XSTUNT service and, if not required, disabling or removing it.

4.1 Preparation

• Ensure you have access to reinstall the service if needed. A roll back plan involves restoring from the snapshot or re-installing the application.
• Changes should be reviewed by the relevant application owner.

4.2 Implementation

1. Step 1: Identify the process associated with the XSTUNT service using netstat -tulnp.
2. Step 2: Stop the identified service using systemctl stop or equivalent command for your operating system.
3. Step 3: Disable the service from starting automatically on boot using systemctl disable or equivalent.

4.3 Config or Code Example

Before

systemctl status   # Shows service is active (running)

After

systemctl status  # Shows service is inactive (disabled)

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue.

• Practice 1: Least privilege – only allow necessary services to run, reducing the attack surface.
• Practice 2: Network segmentation – isolate sensitive systems to limit the impact of potential compromises.

4.5 Automation (Optional)

# Example Bash script to stop and disable a service
#!/bin/bash
service_name="xstunt-server"
if systemctl is-active --quiet $service_name; then
  systemctl stop $service_name
fi
systemctl disable $service_name
echo "Service $service_name stopped and disabled."

5. Verification / Validation

Confirm the fix by verifying that the XSTUNT service is no longer listening on the network.

• Post-fix check: Run netstat -tulnp | grep . The output should not show the XSTUNT service listening on the previously identified port.
• Re-test: Re-run the initial detection method (netstat -tulnp) to confirm the service is no longer present.
• Smoke test: Verify that any applications dependent on this service are still functioning as expected, or have been decommissioned if appropriate.
• Monitoring: Monitor firewall logs for unexpected connections to the previously used port.

netstat -tulnp | grep  # Should return no results

6. Preventive Measures and Monitoring

Implement security baselines and monitoring to prevent recurrence.

• Baselines: Update your system hardening baseline to explicitly disallow unnecessary services like XSTUNT servers.
• Pipelines: Integrate vulnerability scanning into your CI/CD pipeline to identify potentially exposed ports during deployment.
• Asset and patch process: Regularly review installed software and network configurations for unexpected or unauthorized services.

7. Risks, Side Effects, and Roll Back

Stopping the XSTUNT service may impact applications that rely on it.

• Roll back: Step 1: Re-enable the service using systemctl enable . Step 2: Start the service using systemctl start .

8. References and Resources

• Vendor advisory or bulletin: http://www.cis.nctu.edu.tw/~gis87577/xDreaming/XSTUNT/index.html