In computer protection, (Nessus – A Vulnerability
Scanning Tool) Nessus is a exclusive complete weeknesses checking program. It is cost-free for personal use in a non-enterprise atmosphere. Its objective is to identify prospective weaknesses on the examined techniques.
According to internet surveys done by sectools.org, Nessus is the most popular weeknesses reader, taking first place in the 2000, 2003, and 2006 protection resources study. Tenable reports that it is used by over 75,000 companies globally.

Nessus allows scans for the following types of vulnerabilities:
Vulnerabilities that allow a remote cracker to control or access sensitive data on a system.
Misconfiguration (e.g. open mail relay, missing patches, etc.).
Default passwords, a few common passwords, and blank/absent passwords on some system accounts. Nessus can also call Hydra (an external tool) to launch a dictionary attack.
Denials of service against the TCP/IP stack by using mangled packets
Preparation for PCI DSS audits