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).
• False positive identification: Common false positives include version-based detection where backported patches make the software appear vulnerable (common in RHEL/CentOS). Check rpm -q --changelog <package> on the target to verify.
• Group by remediation: Organize findings by the action needed to fix them (e.g., "Apply Windows KB5034441" affects 47 hosts) rather than listing each instance individually

Step 4: Vulnerability Prioritization

Rank validated vulnerabilities for remediation using risk-based prioritization:

• CVSS score: Use the CVSS v3.1 base score as the starting point. Scores 9.0-10.0 are Critical, 7.0-8.9 High, 4.0-6.9 Medium, 0.1-3.9 Low.
• Exploit availability: Increase priority for vulnerabilities with publicly available exploit code, especially Metasploit modules or weaponized PoCs
• Network exposure: A critical vulnerability on an internet-facing system is higher priority than the same vulnerability on an isolated internal server
• Asset criticality: Consider the business value of the affected system. Domain controllers, databases with PII, and payment processing systems warrant higher priority.
• Compensating controls: Reduce priority if the vulnerability is mitigated by network segmentation, WAF rules, or EDR protections (document the compensating control)

Step 5: Report Generation

Generate a comprehensive vulnerability scan report:

• Export the Nessus report in both executive (PDF) and detailed (CSV/HTML) formats
• Create a custom report that includes only validated findings with false positives removed
• Include a remediation priority matrix mapping each vulnerability to its recommended fix, affected hosts, and timeline
• Add context from manual validation (e.g., "This finding was confirmed exploitable during the penetration test")
• Include scan metadata: date/time, scanner version, plugin set date, scan policy used, authentication success rate

Key Concepts

Tools & Systems

• Nessus Professional: Commercial vulnerability scanner by Tenable with over 200,000 plugins covering CVEs, misconfigurations, and compliance checks
• Nessus Expert: Extended version including external attack surface scanning, IaC scanning, and cloud infrastructure assessment
• Tenable.io: Cloud-hosted vulnerability management platform for enterprise deployments with asset tracking, trend analysis, and prioritization
• OpenVAS (Greenbone): Open-source alternative vulnerability scanner with community-maintained vulnerability tests for comparison scanning

Common Scenarios

Scenario: Quarterly PCI-DSS Vulnerability Scan for a Retail Company

Context: A retailer processes credit card payments and must comply with PCI-DSS requirement 11.2, which mandates quarterly internal and external vulnerability scans. The cardholder data environment (CDE) consists of 200 servers across 3 VLANs. All hosts run either Windows Server 2019/2022 or RHEL 8/9.

Approach:

1. Configure authenticated scan with domain service account for Windows and SSH key for Linux hosts
2. Use the PCI-DSS scan policy template with all relevant plugin families enabled
3. Scan all 200 CDE hosts during the Saturday maintenance window (02:00-06:00)
4. Identify 847 findings: 12 Critical, 34 High, 189 Medium, 612 Low/Informational
5. Validate Critical findings: 3 are false positives (backported patches on RHEL), 9 are confirmed vulnerabilities
6. Group remaining findings by remediation action: 6 require Windows patches, 2 require Apache upgrades, 1 requires TLS configuration hardening
7. Generate PCI-compliant report showing no Critical or High vulnerabilities remain unaddressed (after remediation and rescan)

Pitfalls:

• Running unauthenticated scans and missing the majority of local vulnerabilities, producing an incomplete compliance report
• Not updating Nessus plugins before scanning, missing recently published CVEs
• Scanning fragile legacy systems without reducing scan intensity, causing crashes or service disruption
• Accepting Nessus results at face value without manually validating critical findings for false positives

Output Format

## Vulnerability Scan Summary - CDE Environment

**Scan Date**: 2025-11-15 02:00-05:47 UTC
**Scanner**: Nessus Professional 10.8.3 (Plugins: 2025-11-14)
**Hosts Scanned**: 200 (198 authenticated, 2 authentication failed)
**Scan Policy**: PCI-DSS Internal Scan

### Findings Summary
| Severity | Count | Validated |
|----------|-------|-----------|
| Critical | 12    | 9 (3 FP)  |
| High     | 34    | 31 (3 FP) |
| Medium   | 189   | 178       |
| Low/Info | 612   | N/A       |

### Top Critical Findings

**1. CVE-2024-21762 - Fortinet FortiOS Out-of-Bounds Write (CVSS 9.8)**
- Affected Hosts: fw-cde-01.corp.example.com (10.50.1.1)
- Exploit Available: Yes (Metasploit module)
- Remediation: Upgrade FortiOS to 7.4.3 or later
- Priority: Immediate - internet-facing device protecting CDE

**2. CVE-2024-6387 - OpenSSH regreSSHion (CVSS 8.1)**
- Affected Hosts: 14 Linux servers (see Appendix A)
- Exploit Available: Yes (public PoC)
- Remediation: Upgrade OpenSSH to 9.8p1 or later
- Priority: Within 7 days - authenticated remote code execution