Nessus for Beginners: Vulnerability Scanning Done Properly

Nessus, from Tenable, is one of the most widely used vulnerability scanners. It checks systems for known vulnerabilities, missing patches and weak configurations. Running a scan takes minutes; turning the results into something useful is the skill. Scan only systems you own or are authorized to assess.

The Editions

Tenable offers a free edition for learning that scans a small number of IP addresses, and paid editions for professional use. The free edition's limits have changed over the years, so check Tenable's site for the current terms. For a home lab, the free edition is enough.

Scan Types

• Host discovery: find which machines are alive
• Basic network scan: a broad vulnerability scan of hosts and services
• Web application tests: basic checks of web servers; dedicated web testing still needs tools like Burp Suite
• Compliance and configuration audits: compare settings against benchmarks, usually in paid editions

Credentialed vs Uncredentialed Scans

An uncredentialed scan sees a system as an outsider would: open ports and the versions services reveal. A credentialed scan logs in, with an account you are given, and checks installed software and patches directly. Credentialed scans find far more and produce fewer false positives, so professional VAPT engagements use them wherever the client agrees.

Reading the Results

Findings are grouped by severity (critical, high, medium, low and informational), usually based on CVSS scores; see CVSS explained. Each finding shows the affected host and port, a description, the evidence Nessus saw, references and a suggested fix. Sort by severity, but do not stop there: a medium finding on an internet-facing server may matter more than a high one on an isolated test machine.

False Positives and Verification

Scanners guess from version numbers and responses, and they are sometimes wrong, for example when a vendor has back-ported a fix without changing the version. Verify important findings manually before reporting them. Explaining which results were false positives, and why, is part of professional work.

From Scan to Report

Do not paste the scanner's export as your report. Group related findings, confirm the important ones, rank them by real risk and give clear fixes. See what is VAPT for how scanning fits into an assessment, and how to write a penetration testing report.

Learning Vulnerability Assessment

Nessus is taught in our network and Active Directory course, together with Nmap for discovery and Metasploit for validating findings in the lab.