Performing Vulnerability Scanning With Nessus

# Performing Vulnerability Scanning with Nessus

## When to Use

- Conducting initial vulnerability assessment during the reconnaissance phase of a penetration test
- Performing periodic vulnerability scans to maintain compliance with PCI-DSS (requirement 11.2), HIPAA, or SOC 2 standards
- Validating that remediation efforts have successfully addressed previously identified vulnerabilities
- Establishing a baseline of known vulnerabilities before targeted manual exploitation
- Auditing patch compliance and configuration drift across server and workstation fleets

**Do not use** as a substitute for manual penetration testing, against systems without written authorization, or against fragile systems (medical devices, legacy SCADA) where scanning may cause service disruption.

## Prerequisites

- Tenable Nessus Professional or Nessus Expert with current plugin updates (plugins should be less than 24 hours old)
- Network connectivity to all target hosts on all ports (no firewall restrictions between scanner and targets)
- Administrative credentials for authenticated scanning (domain admin or local admin for Windows, root/sudo for Linux, SNMP community strings for network devices)
- Target IP ranges and hostnames documented in the scope agreement
- Change management approval for scanning during authorized windows

## Workflow

### Step 1: Scan Configuration

Configure the Nessus scan policy based on engagement requirements:

- **Scan type selection**: Choose "Advanced Scan" for full control over plugin families, or "Credentialed Patch Audit" for patch compliance. Avoid "Basic Network Scan" for penetration tests as it uses a limited plugin set.
- **Discovery settings**: Configure port scanning to scan all 65,535 TCP ports and top 1,000 UDP ports. Set host discovery to use ARP (local), TCP SYN, and ICMP for maximum coverage.
- **Authentication**: Add Windows credentials (domain account with local admin), SSH credentials (key-based preferred over password), SNMP credentials (v3 with authPriv preferred), and database credentials for database-specific checks.
- **Plugin configuration**: Enable all plugin families relevant to the target environment. For penetration testing, ensure "Denial of Service" plugins are disabled unless explicitly authorized. Enable CGI scanning for web servers.
- **Performance settings**: Set maximum concurrent hosts per scanner (default 30, reduce for sensitive networks), maximum concurrent checks per host (4-5 for production, higher for test environments), and network timeout values appropriate for the target network.

### Step 2: Scan Execution and Monitoring

Launch the scan and monitor for issues:

- Start the scan during the authorized testing window
- Monitor scan progress through the Nessus web interface, checking for hosts timing out, authentication failures, or plugins causing errors
- Watch for credential failures indicated by "Authentication Failure" results; these mean the authenticated scan fell back to unauthenticated mode, producing incomplete results
- If specific hosts are crashing or becoming unresponsive, pause the scan, exclude those hosts, and report the issue to the client
- For large networks (1,000+ hosts), consider splitting scans into smaller subnets to manage load and allow restartability

### Step 3: Results Analysis and Validation

Analyze scan results to separate true positives from false positives:

- **Sort by severity**: Start with Critical and High findings; these represent the most exploitable and impactful vulnerabilities
- **Validate authentication**: Verify that plugin 19506 (Nessus Scan Information) shows "Credentialed checks: yes" for each host. Unauthenticated results miss local vulnerabilities.
- **Eliminate informational noise**: Filter out informational findings unless they reveal useful information for manual testing (service banners, SSL certificate details, open ports)
- **Cross-reference CVEs**: For each Critical/High finding, verify the CVE in the National Vulnerability Database. Check if the vulnerability has a public exploit (Exploit-DB, Metasploit module).