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Nessus : A security vulnerability scanning tool


What is Nessus?

Nessus is a remote security scanning tool, which scans a computer and raises an alert if it discovers any vulnerabilities that malicious hackers could use to gain access to any computer you have connected to a network.  It does this by running over 1200 checks on a given computer, testing to see if any of these attacks could be used to break into the computer or otherwise harm it.

Who would use a tool like this?

If you are an administrator in charge of any computer (or group of computers) connected to the internet, Nessus is a great tool help keep their domains free of the easy vulnerabilities that hackers and viruses commonly look to exploit. 



What Nessus is NOT

Nessus is not a complete security solution, rather it is one small part of a good security strategy.  Nessus does not actively prevent attacks, it is only a tool that checks your computers to find vulnerabilities that hackers COULD exploit.  IT IS UP TO THE SYSTEM ADMINISTRATOR TO PATCH THESE VULNERABILITIES IN ORDER TO CREATE A SECURITY SOLUTION.

Nessus is an open-source network vulnerability scanner that uses the Common Vulnerabilities and Exposures architecture for easy cross-linking between compliant security tools. Nessus employs the Nessus Attack Scripting Language (NASL), a simple language that describes individual threats and potential attacks




Nessus has a modular architecture consisting of centralized servers that conduct scanning, and remote clients that allow for administrator interaction. Administrators can include NASL descriptions of all suspected vulnerabilities to develop customized scans. Significant capabilities of Nessus include:

Compatibility with computers and servers of all sizes.

Detection of security holes in local or remote hosts.

Detection of missing security updates and patches.

Simulated attacks to pinpoint vulnerabilities.

Execution of security tests in a contained environment.

Scheduled security audits



Why Nessus?

If you are familiar with other network vulnerability scanners, you might be wondering what advantages Nessus has over them.  Key points include:

    - Unlike other scanners, Nessus does not make assumptions about your server configuration (such as assuming that port 80 must be the only web server) that can cause other scanners to miss real vulnerabilities.

    - Nessus is very extensible, providing a scripting language for you to write tests specific to your system once you become more familiar with the tool. Its also provides a plug-in interface, and many free plug-ins are available from the Nessus plug-in site.   These plugs are often specific to detecting a common virus or vulnerability. 

    - Up to date information about new vulnerabilities and attacks.  The Nessus team updates the list of what vulnerabilities to check for on a daily basis in order to minimize the window between an exploit appearing in the wild, and you being able to detect it with Nessus.

    - Open-source.  Nessus is open source, meaning it costs nothing, and you are free to see and modify the source as you wish.

    - Patching Assistance:  When Nessus detects a vulnerability, it is also most often able to suggest the best way you can mitigate the vulnerability.

    For other advantages/features of Nessus, see: http://www.nessus.org/features.html

How Nessus Works

To learn how Nessus and other port-scanning security tools work, it is necessary to understand different services (such as a web server, SMTP server, FTP server, etc) are accessed on a remote server.  Most high-level network traffic, such as email, web pages, etc reach a server via a high-level protocol that is transmitted reliably by a TCP stream.  To keep different streams from interfering with each other, a computer divides its physical connection to the network into thousands of logical paths, called ports.  So if you want to talk to a web server on a given machine, you would connect to port #80 (the standard HTTP port), but if you wanted to connect to an SMTP server on that same machine you would instead connect to port #25.   

Each computer has thousands of ports, all of which may or may not have services (ie: a server for a specific high-level protocol) listening on them.  Nessus works by testing each port on a computer, determining what service it is running, and then testing this service to make sure there are no vulnerabilities in it that could be used by a hacker to carry out a malicious attack.  Nessus is called a "remote scanner" because it does not need to be installed on a computer for it to test that computer.  Instead, you can install it on only one computer and test as many computers as you would like.

Installation:

Nessus comes in two parts, a server called nessusd and a client, which can by any of several options.  The server is the part of Nessus that actually runs the tests, and the client is used to tell the server what tests to run on what computers.

The server exists only for Unix/Linux platforms, but there are clients available for Unix/Linux, Windows and Mac.  Therefore, once the server is set up and running, an administrator can run regularly scheduled Nessus tests using a client written for almost any platform.

Go to www.nessus.org to download the most release release of Nessus.  As of this date, the current release can be found at http://www.nessus.org/nessus_2_0.html along with extremely simple installation instructions.  This will install the Nessus server app and a client on the unix based machine (note: this includes Mac OS X and above with developer tools installed).  After installing the server you will have to do a couple quick configuration options, such as adding a user, described here: http://www.nessus.org/demo/first.html.

To download and install a Windows client, look at: http://nessuswx.nessus.org/


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An Ethical hacker should know the penalties of unauthorized hacking into a system. Read more at: Legality and Ethics


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