What is a Nessus Scan?
Nessus plug-ins are very much like virus signatures in a common virus scanner application. Each plug-in is written to test for a specific vulnerability. These can be written to actually exploit the vulnerability or just test for known vulnerable software versions. Plug-ins can be written in most any language but usually are written in the Nessus Attack Scripting Language (NASL). NASL is Nessus’ own language, specifically designed for vulnerability test writing. Each plug-in is written to test for a specific known vulnerability and/or industry best practices. NASL plug-ins typically test by sending very specific code to the target and comparing the results against stored vulnerable values. Nessus can scan some of the well known worm backdoors. For example, W32.Dabber propagates by exploiting a vulnerability in the FTP server component of W32.Sasser.Worm and its variants. It installs a backdoor on infected hosts and tries to listen on port 9898. If the attempt fails, W32Dabber.A tries to
