How to remediate – PCI DSS Compliance : Scan Interference

1. Introduction

PCI DSS Compliance : Scan Interference means that a network security device is blocking attempts to validate PCI DSS compliance using scanning tools like Nessus. This impacts an organisation’s ability to demonstrate adherence to the Payment Card Industry Data Security Standard, potentially leading to fines and loss of merchant status. Systems usually affected are those in scope for PCI DSS, including web servers, database servers, and network infrastructure components. Impact on confidentiality is possible if scans cannot verify data protection measures; integrity may be compromised if configuration changes go undetected; availability could be indirectly impacted by compliance failures.

2. Technical Explanation

The interference prevents a complete scan of the system, making certification impossible. This usually happens because a firewall or intrusion detection/prevention system (IDS/IPS) is incorrectly identifying the Nessus scanner as malicious traffic. Preconditions include a running Nessus scan and active network security devices configured to inspect inbound traffic. An attacker could exploit this by deliberately misconfiguring network devices to block legitimate scans, masking vulnerabilities on the protected systems. Affected platforms are those running firewalls or IDS/IPS solutions that intercept network traffic.

• Root cause: Incorrectly configured firewall rules or IDS signatures blocking Nessus scan traffic.
• Exploit mechanism: An attacker could alter firewall rules to drop packets from the scanning IP address, preventing validation of PCI DSS requirements.
• Scope: Systems running firewalls (e.g., iptables, pfSense, Cisco ASA) and intrusion detection/prevention systems (e.g., Snort, Suricata).

3. Detection and Assessment

Confirming vulnerability involves checking firewall logs for blocked Nessus traffic and verifying the scanner’s ability to reach the target system. A quick check is to see if Nessus reports incomplete scan results.

• Quick checks: Check Nessus scan status – look for errors indicating connection issues or dropped packets.
• Scanning: Nessus itself will report scan interference, but this isn’t definitive proof of the cause.
• Logs and evidence: Examine firewall logs for blocked connections from the Nessus scanner’s IP address to the target system on ports used by PCI DSS scans (e.g., 22, 80, 443). Event IDs will vary depending on the firewall vendor.

# Example command placeholder: iptables -L INPUT | grep nessus_scanner_ip

4. Solution / Remediation Steps

Fixing this requires adjusting firewall or IDS/IPS rules to allow Nessus scan traffic, while maintaining overall network security. The steps below aim for minimal disruption.

4.1 Preparation

• Dependencies: Access to the firewall or IDS/IPS management interface. Roll back by restoring the previous firewall configuration.
• Change window: Schedule during off-peak hours, with approval from the security team.

4.2 Implementation

1. Step 1: Identify the Nessus scanner’s IP address.
2. Step 2: Add a rule to the firewall allowing inbound traffic from the Nessus scanner’s IP address on ports used for PCI DSS scans (e.g., 22, 80, 443).
3. Step 3: If using an IDS/IPS, update signatures to exclude the Nessus scanner’s IP address or scan patterns.
4. Step 4: Restart the firewall service if required by your system.

4.3 Config or Code Example

Before

# iptables example - blocking all traffic on port 22
iptables -A INPUT -p tcp --dport 22 -j DROP

After

# iptables example - allowing Nessus scanner IP on port 22
iptables -I INPUT -s nessus_scanner_ip -p tcp --dport 22 -j ACCEPT
iptables -A INPUT -p tcp --dport 22 -j DROP

4.4 Security Practices Relevant to This Vulnerability

Practices that directly address this vulnerability type include least privilege and regular rule reviews.

• Practice 1: Least privilege – only allow necessary traffic, reducing the attack surface.
• Practice 2: Regular firewall rule reviews – identify and remove unnecessary or overly permissive rules.

4.5 Automation (Optional)

If using infrastructure-as-code tools like Ansible, you can automate firewall rule updates.

# Example Ansible playbook snippet - use with caution!
- name: Allow Nessus scanner IP on port 22
  iptables:
    chain: INPUT
    protocol: tcp
    destination_port: 22
    jump: ACCEPT
    source: nessus_scanner_ip

5. Verification / Validation

Confirm the fix by re-running the PCI DSS scan and verifying that it completes successfully. A smoke test involves checking basic system functionality.

• Post-fix check: Check Nessus scan status – confirm a successful completion with no errors related to blocked connections.
• Re-test: Re-run the original PCI DSS scan to verify full validation.
• Smoke test: Verify that web servers respond to HTTP/HTTPS requests and database servers accept connections.
• Monitoring: Monitor firewall logs for any continued blocking of Nessus scanner traffic (example query: search for ‘nessus_scanner_ip’ and ‘DROP’).

# Post-fix command and expected output: Nessus scan completes successfully with all ports scanned.

6. Preventive Measures and Monitoring

Preventive measures include updating security baselines and incorporating checks into CI/CD pipelines.

• Baselines: Update firewall rules to align with a security baseline (for example, CIS benchmarks).
• Pipelines: Add static analysis of firewall configurations in CI/CD pipelines.
• Asset and patch process: Review firewall rule changes as part of the regular asset management process.

7. Risks, Side Effects, and Roll Back

Risks include inadvertently opening up unintended access to systems. Roll back by restoring the previous firewall configuration.

• Risk or side effect 1: Opening ports too widely could increase attack surface – restrict rules to only necessary traffic.
• Roll back: Restore the firewall configuration from backup.

8. References and Resources

Link only to sources that match this exact vulnerability.

• Vendor advisory or bulletin: Check your firewall vendor’s documentation for guidance on allowing scan traffic.
• NVD or CVE entry: Not applicable – this is a configuration issue, not a specific software flaw.
• Product or platform documentation relevant to the fix: Refer to your firewall/IDS/IPS product documentation for rule syntax and best practices.