How to remediate – NAT-PMP Detection (remote network)

1. Introduction

NAT-PMP Detection concerns the Network Address Translation Port Mapping Protocol. This protocol allows applications on a local network to request port mappings, potentially exposing services externally. Affected systems are typically routers and firewalls running NAT. A successful exploit could allow an attacker to gain information about your internal network or create dynamic port mappings for malicious purposes, impacting confidentiality, integrity, and availability.

2. Technical Explanation

The vulnerability occurs when the NAT-PMP protocol remains enabled on a device. This allows any application on the internal subnet to request external access through port mappings. An attacker can exploit this by sending requests to create port mappings, potentially gaining access to services behind the firewall or gathering network information. There is no specific CVE associated with simply having the service enabled but it’s often referenced in broader security assessments.

• Root cause: The NAT-PMP protocol is active and responding to external requests.
• Exploit mechanism: An attacker sends a NAT-PMP request to map an external port to an internal host, potentially bypassing firewall rules. For example, they could attempt to map port 8080 on the public IP address to an internal server at 192.168.1.100 port 80.
• Scope: Routers and firewalls with NAT-PMP enabled are affected. Specific models vary; check vendor documentation for details.

3. Detection and Assessment

Confirming the vulnerability involves checking if the NAT-PMP service is listening on UDP port 5351. A thorough method would involve a network scan.

• Quick checks: Use `netstat` to check for listening services.
• Scanning: Nessus ID 184540 can detect this vulnerability. Other scanners may have similar signatures.
• Logs and evidence: Check firewall logs for UDP traffic on port 5351 originating from external sources.

netstat -an | grep ".5351"

4. Solution / Remediation Steps

The solution is to filter incoming traffic to UDP port 5351, effectively disabling the NAT-PMP service from external access.

4.1 Preparation

• Dependencies: Ensure you understand the impact of blocking port 5351 on any legitimate applications using NAT-PMP. A roll back plan involves restoring the original firewall configuration.
• Change window: Coordinate with network owners for approval, especially during peak hours.

4.2 Implementation

1. Step 1: Create a firewall rule to block incoming UDP traffic on port 5351 from all external sources.
2. Step 2: Apply the new firewall rule and verify it’s active.

4.3 Config or Code Example

Before

# No specific rule blocking UDP port 5351

After

# Block incoming UDP traffic on port 5351
rule block_natpmp {
  direction: input
  protocol: udp
  port: 5351
  source: any
  action: drop
}

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue.

• Practice 1: Least privilege – only enable necessary services and protocols on firewalls.
• Practice 2: Safe defaults – configure devices with the most restrictive settings by default, disabling unnecessary features like NAT-PMP.

4.5 Automation (Optional)

# Example Ansible playbook snippet
- name: Block NAT-PMP on firewall
  firewalld:
    port: 5351/udp
    permanent: true
    state: disabled
    immediate: yes

5. Verification / Validation

Confirm the fix by checking that incoming traffic to UDP port 5351 is blocked.

• Post-fix check: Run `netstat -an | grep “.5351″` again; it should show no listening services on that port.
• Re-test: Re-run the Nessus scan (ID 184540); it should no longer report the vulnerability.
• Smoke test: Ensure other firewall rules are still functioning as expected, such as allowing outbound traffic and established connections.
• Monitoring: Monitor firewall logs for any blocked UDP traffic on port 5351 to confirm the rule is active.

netstat -an | grep ".5351" # Expected output should be empty

6. Preventive Measures and Monitoring

Update security baselines and consider adding checks in CI/CD pipelines.

• Baselines: Update your firewall baseline to include a rule blocking incoming UDP traffic on port 5351.
• Pipelines: Add a check during deployment to ensure NAT-PMP is disabled or blocked by default.
• Asset and patch process: Review firewall configurations regularly as part of your asset management process.

7. Risks, Side Effects, and Roll Back

Blocking port 5351 may disrupt applications relying on NAT-PMP.

• Roll back: Restore the original firewall configuration from your backup.

8. References and Resources

• Vendor advisory or bulletin: Check your router/firewall vendor’s website for specific guidance.
• NVD or CVE entry: No specific CVE, but see https://nvd.nist.gov/ for related vulnerabilities.
• Product or platform documentation relevant to the fix: Consult your router/firewall’s documentation on firewall rule configuration.