Performs authenticated and unauthenticated vulnerability scanning using Tenable Nessus to identify known vulnerabilities, misconfigurations, default credentials, and missing patches across network infrastructure, servers, and applications. The scanner correlates findings with CVE databases and CVSS scores to produce prioritized remediation guidance. Activates for requests involving vulnerability scanning, Nessus assessment, patch compliance checking, or automated vulnerability detection.
Performs authenticated and unauthenticated vulnerability scanning using Tenable Nessus to identify known vulnerabilities, misconfigurations, default credentials, and missing patches across network infrastructure, servers, and applications. The scanner correlates findings with CVE databases and CVSS scores to produce prioritized remediation guidance. Activates for requests involving vulnerability scanning, Nessus assessment, patch compliance checking, or automated vulnerability detection.
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.
Most Often Missed & How to Confirm
Authenticated scanning, not just unauthenticated — the single biggest gap. Unauthenticated scans miss the majority of local vulnerabilities (missing patches, local config). Always supply Windows local-admin/SSH/SNMPv3 creds and verify they actually worked.
Confirming credentials landed — check plugin 19506 (Nessus Scan Information) shows Credentialed checks: yes per host; an "Authentication Failure" silently downgrades to a near-useless surface scan. This is routinely overlooked.
Fresh plugins and full port coverage — plugins older than 24h miss new CVEs; scanning only the top ports misses services. Configure all 65535 TCP + top UDP.
Manual false-positive validation — version/banner detection flags backported-patch distros (RHEL/CentOS) as vulnerable. Verify with rpm -q --changelog, vendor advisories, or a manual PoC before reporting.
Risk-based prioritization over raw CVSS — fold in exploit availability (Metasploit/PoC), internet exposure, and asset criticality; a CVSS 7 on an internet-facing DC can outrank a CVSS 9 on an isolated box.
How to confirm: every reported finding should cite the plugin ID + CVE, the authenticated-scan confirmation (19506), and for Critical/High a manual validation note (PoC result, version check, or "confirmed exploitable during testing"). Don't conclude a host is clean until you've confirmed the scan was credentialed; don't pass a Nessus result straight into the report until you've manually ruled out the common backported-patch and version-detection false positives.
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
Term
Definition
Authenticated Scan
A vulnerability scan that uses valid credentials to log into target hosts and perform local checks, detecting significantly more vulnerabilities than unauthenticated scanning
Plugin
A Nessus script that checks for a specific vulnerability, misconfiguration, or compliance item; Nessus maintains over 200,000 plugins updated daily
CVSS
Common Vulnerability Scoring System; a standardized framework for rating the severity of vulnerabilities from 0.0 to 10.0 based on exploitability and impact metrics
False Positive
A vulnerability reported by the scanner that does not actually exist on the target, often caused by version-based detection without exploit verification
Credentialed Patch Audit
A scan type focused specifically on identifying missing operating system and application patches by comparing installed versions against known vulnerability databases
Plugin Family
A logical grouping of Nessus plugins by category (e.g., Windows, Ubuntu Local Security Checks, Web Servers, Databases)
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:
Configure authenticated scan with domain service account for Windows and SSH key for Linux hosts
Use the PCI-DSS scan policy template with all relevant plugin families enabled
Scan all 200 CDE hosts during the Saturday maintenance window (02:00-06:00)
