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Writing Nessus Plugins

by Nitesh Dhanjani

In a previous article, I showed you how to install and use the Nessus scanner. If the bundled security checks aren't enough for you, you can write your own Nessus plugins in NASL (Nessus Attack Scripting Language). This article demonstrates how to write your own custom vulnerability checks.

Why Write Your Own Vulnerability Checks?

Before you can patch systems affected by specific vulnerabilities, you must first scan for them. There are commercial vulnerability scanners from vendors who promise to update you with new plugins that scan for the latest vulnerabilities, but in reality, many commercial scanner vendors do not release these new checks until it's too late. In addition, your organization may know of a vulnerability that exists in a homegrown application. In those cases, you'll need to develop custom, in-house vulnerability checks.

Quite often, organizations that need to scan their networks for the latest vulnerabilities end up writing their own scanners from scratch, reinventing the wheel with every new vulnerability found. Nessus provides an excellent framework in which to write custom vulnerability checks, so there is no need to write scanners from scratch. Besides, Nessus is free and open source, which makes the task even easier.

The NASL Interpreter

With Nessus installed, take a look at the NASL scripts in the /usr/local/lib/nessus/plugins/ directory. The Nessus server executes these scripts when performing scans. To run a specific script on the command line, use the NASL interpreter, nasl, with the -t switch to signify the host to scan. For example, the finger.nasl connects to a remote host on port 79 to check if the fingerd daemon is running. To run this script against your own host, run:

[notroot]$ nasl -t finger.nasl

Writing a Plugin

Before we begin to write a Nessus plugin to scan for a vulnerability, we need to define the vulnerability. We'll use the following definition. A homegrown web application serves the file /src/ when a client requests the URL http://host/src/ This file contains usernames and hashes of passwords, so the server should not serve it. The following NASL script scans for this specific vulnerability:

if (description)
   script_version ("$Revision: 1.00 $");
   script_name(english:"Checks for /src/");
   desc["english"]="/src/ is usually installed by XYZ web 
application and contains username and password information in clear text. 

Solution: Configure your web browser to not serve .inc files.

Risk factor: High";

   script_summary(english:"Checks for the existence of /src/");

   script_copyright(english:"This script is Copyright (c)2004 Nitesh 
   script_family(english:"CGI abuses");


include ("");



When the Nessus server queries the plugin for summary information, the description variable will hold a non-zero value. Therefore, we define information about the plugin with description in this case. The script_id function sets a unique ID for the plugin. Every plugin's value must be unique. In this case, we use a high number, 99999, to ensure a distinct value. The script_version function sets the version number of the plugin. It is a good idea to update this number to reflect the latest version of the plugin. The script_description function sets the description of the plugin. The Nessus client will show this description when the user queries a plugin. Similarly, the script_summary function produces a summary description of the plugin.

The script_category function sets the plugin's category as required by Nessus. In our case, we have set it to ACT_GATHER_INFO because the plugin gathers information about a remote service by checking for the existence of the /src/ file on a remote web server. The script_copyright function sets author copyright information.

Nessus categorizes plugins into different families to help sort the vulnerability check plugins. We have set it to CGI abuses to indicate an abuse of a CGI-based web application. The Nessus web site has a list of publicly available plugins, categorized by family.

Nessus can optimize scans, if you select the appropriate checkbox in the Scan Option tab of the GUI client. When this option is enabled, Nessus scans for vulnerabilities related to the applications running on the open ports of the target host. We use the script_require_ports function to set the port related to the vulnerability, which in our case is set www, for HTTP traffic.

The functions described so far set various description values for the plugin. Click on the appropriate plugin name from the list of plugins displayed in the Plugins tab of the Nessus client to view them, as Figure 1 shows.

plugin details
Figure 1. Plugin details.

The file contains a list of useful HTTP functions for our convenience, so we include it using the include call. Next, we define port to hold a value that is relevant to the web server port number — 80. We use the is_cgi_installed function to verify the existence of the file /src/ If this file exists, is_cgi_installed returns true, and we call the security_hole function to tell Nessus that we've found a security hole.

Installing and Running the Plugin

Place the preceding script in the /usr/local/lib/nessus/plugins directory. The script must end with the extension .nasl. Run the Nessus GUI client and select the Plugins tab. Make sure that CGI Abuses is selected. Highlight CGI Abuses and make sure the plugin with the name "Checks for /src/" is enabled. If you have trouble finding the plugin, click the Filter button and search for a filter with the ID of 99999.

To get a positive result from the plugin, you need to have a local web server configured to serve the /src/ file. If you have an Apache server running on a host, create a file called src/passwd in the web root of the web server. Now, enter the IP address or hostname of the target web server in the Target Selection tab and click the Start the Scan button. If everything goes well, Nessus will detect and report our custom vulnerability, as shown in Figure 2.

Nessus report with output from our plugin
Figure 2. Nessus report with output from our plugin.


That is all there is to writing a Nessus plugin using NASL! As you can see, NASL is a language specifically tailored to make it easy to write vulnerability checks. For more details on NASL, read the NASL reference manual (PDF).

Nitesh Dhanjani is a well known security researcher, author, and speaker. Dhanjani has been invited to talk at various information security events such as the Black Hat Briefings, RSA, Hack in the Box, Microsoft Blue Hat, and OSCON.

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