How to remediate – Open Port Re-check

1. Introduction

The Open Port Re-check vulnerability indicates that ports previously identified as open are now closed or unresponsive. This suggests a change in network availability, service status, or security controls. It’s usually found on servers and network devices. A successful scan interruption could lead to incomplete audit results, potentially masking active vulnerabilities. The likely impact is reduced availability of services and incomplete information about the system’s security posture.

2. Technical Explanation

• Root cause: The root cause is interrupted communication between the scanner and the target host during the scan process.
• Exploit mechanism: An attacker would repeatedly scan the system, attempting to identify a brief window where a service is accessible for exploitation.
• Scope: Any network device or server running services subject to port scanning is potentially affected.

3. Detection and Assessment

Confirming whether a system was temporarily unavailable requires re-running the scan. A quick check involves pinging the target host to verify basic connectivity. For thorough assessment, use a network scanner to perform a full port scan.

• Quick checks: Use `ping ` to confirm basic reachability.
• Scanning: Nessus or Nmap can be used for port scanning. Signature ID examples may vary depending on the scanner configuration.
• Logs and evidence: Check firewall logs for blocked scan traffic, intrusion detection system (IDS) alerts related to the scanner’s IP address, or service restart events around the time of the scan.

ping 

4. Solution / Remediation Steps

4.1 Preparation

• Backups are not typically required for this issue, but ensure recent system snapshots exist as a precaution. No services need to be stopped.
• Change window needs may apply if firewall rules are modified; approval from network administrators might be required.

4.2 Implementation

1. Step 1: Increase the `checks_read_timeout` setting in your scanning tool configuration. This allows more time for responses.
2. Step 2: Reduce the `max_checks` value to decrease scan intensity and reduce potential impact on the target host.
3. Step 3: Temporarily disable any Intrusion Prevention Systems (IPS) during the Nessus scan, if possible and safe to do so.

4.3 Config or Code Example

Before

checks_read_timeout = 5

After

checks_read_timeout = 10

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help mitigate this issue and improve overall network visibility. Least privilege helps limit the impact of potential exploitation attempts. Network segmentation reduces the attack surface by isolating critical systems.

• Practice 1: Implement least privilege access controls to reduce the potential damage from compromised services.
• Practice 2: Regularly review firewall rules and network configurations to ensure they are appropriate and effective.

4.5 Automation (Optional)

Automation is not typically suitable for directly resolving this issue, but scan scheduling can be automated to run during periods of low network activity.

# Example cron job schedule
0 2 * * * /path/to/scanner --schedule

5. Verification / Validation

Confirm the fix by re-running the scan and verifying that all expected ports are reachable. A negative test involves confirming that no new vulnerabilities are identified during the re-scan. Perform a basic service smoke test to ensure critical functionality remains operational.

• Post-fix check: Re-run the port scan; confirm all previously open ports now show as open and responsive.
• Re-test: Repeat the original scan configuration to verify that no ports are reported as closed or unresponsive.
• Smoke test: Verify key services like web access, email delivery, or database connectivity still function correctly.
• Monitoring: Monitor firewall logs for blocked traffic from the scanner’s IP address; investigate any unexpected blocks.

nmap -p  

6. Preventive Measures and Monitoring

Preventive measures include maintaining a stable network environment and regularly reviewing security baselines. For example, update your firewall rules to allow scan traffic from trusted sources. Implement regular patch management for all network devices.

• Baselines: Update security baselines or policies to reflect acceptable port configurations and scan traffic allowances.
• Pipelines: Integrate vulnerability scanning into CI/CD pipelines to identify potential issues early in the development lifecycle.
• Asset and patch process: Implement a regular patch management cycle for all network devices and servers.

7. Risks, Side Effects, and Roll Back

• Risk or side effect 1: Increasing timeout values can extend scan times.
• Risk or side effect 2: Disabling IPS temporarily reduces intrusion detection capabilities.
• Roll back: 1. Restore original firewall rules. 2. Revert `checks_read_timeout` to the default value. 3. Re-enable any disabled security controls.

8. References and Resources

• Vendor advisory or bulletin: Not applicable for this general issue.
• NVD or CVE entry: Not applicable for this general issue.
• Product or platform documentation relevant to the fix: Nessus documentation on scan configuration options.