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Introduction

Nessus is a high-quality vulnerability scanner. One of its main advantages is its extensive and continually evolving plugin database of vulnerability checks. Additionally, one can configure Nessus to constantly scan a network with a continuously updated collection of plugins with minimal user interaction -- an extremely appealing notion!

One drawback, though, is that plugins not explicitly listed in the client configuration file -- eg, new plugins -- are enabled by default (unless you've enabled safe_checks, in which case dangerous plugins are disabled). Further, the only way to update the configuration file out-of-the-box is via the GUI manually.

To get around this shortcoming, I've written update-nessusrc, which queries a Nessus server for its list of available plugins and updates a Nessus client configuration file named on the commandline. Specifically, it completely updates the sections SCANNER_SET and PLUGIN_SET whenever it is run. When used periodically along with nessus-update-plugins, it ensures your client configuration files are as current as possible.

update-nessusrc is written in Perl and calls the Nessus client to obtain a list of current plugins (using the option -qp). It should work on any unix-like system with Perl 5.003 or better and Nessus 1.1.13 or better. It also requires the following Perl modules:

  • Carp
  • Getopt::Long
  • IPC::Open2
  • LWP::Debug
  • LWP::UserAgent
  • Safe

If your system does not have these modules installed already, visit CPAN. Note that Safe must be at least version 2.0, which does not work with versions of Perl older than 5.003. Note also that LWP::Debug and LWP::UserAgent are not included with the default Perl distribution so you may need to install them yourself; they're included as part of the LWP library.

Note: Jay Jacobson of Edgeos wrote a Python script, also named update-nessusrc, that offers functionality similar to this script. I'm not sure it's still available, though.

Installation

  • Retrieve the script and save it locally.
  • Verify ownership and permissions on the script - it can (and probably should) be invoked as an ordinary user rather than root and will hold a userid and password used to connect with a Nessus server.
  • Edit the script and set $nessusd_host, $nessusd_port, $nessusd_user, $nessusd_user_pass, and $proxy according to your environment. Also, you may wish to adjust the location of the perl interpreter in the first line, $ENV{PATH}, @plugin_cats, @plugin_fams, @plugin_excludes, @plugin_includes, @plugin_risks, and/or $script_config to suit your tastes.
  • Have each user create a script configuration file with personalized settings, if desired.

Use

update-nessusrc offers considerable control in the selection of plugins - you can include entire categories and families of plugins, include specific plugins, exclude specific plugins, or even include based on the risk factor of the vulnerabilities scanned for by plugins.

Much of the script's behaviour is controlled by variables set either in the script itself or in a separate script configuration file, specified by $script_config. If it exists, the script configuration file is treated as Perl code and evaluated in a sandbox, which supports only variable definitions. Use of a separate script configuration file makes it possible for several people to share update-nessusrc on the same system and promises to make upgrading the script much easier.

There are several commandline arguments you can use to override variables defined in the script or the script configuration file:

Option Meaning
-c, --categories Enable plugins in the specified categories, overriding @plugin_cats. _all_ can be used to represent all plugin categories and the prefix ! to skip specific ones.
-d, --debug Display debugging messages while running. Creates a scratch configuration file but doesn't actually replace the original one.
-f, --families Enable plugins in the specified families, overriding @plugin_fams. _all_ can be used to represent all plugin families and ! to skip specific ones.
-i, --includes Include the specified plugin ids, overriding @plugin_includes. _all_ can be used to represent all plugin ids, ! to skip specific ones, and x-y to cover the range of plugins between ids x and y inclusive.
-r, --risks Enable plugins that scan for vulnerabilities with the specified risk factors, overriding @plugin_risks. Note: unlike other options, risk factors specified are regarded as regular expressions and matched case-insensitively against the risk factors appearing in plugin descriptions.
-s, --summary Display a summary of the changes, detailing plugins added, removed, enabled, or disabled.
-x, --excludes Exclude the specified plugin ids, overriding @plugin_excludes. _all_ can be used to represent all plugin ids, ! to skip specific ones, and x-y to cover the range of plugins between ids x and y inclusive.

Notes:

  • For a list of plugin families, see http://www.tenable.com/plugins/index.php?view=all; and for plugin categories, see the file doc/WARNING.En in the nessus-core source for Nessus 2.x. Unfortunately, risk factors are not standardized; while Low, Medium, and High are common ones, they are by far from the only ones used. Further, some risk factors depend on the outcome of the plugin. Possible category and family names will both change over time, especially the latter: category names may vary with the version of Nessus you use while family names are specified by plugin writers as an arbitrary string using the script_family function. Thus, you may wish to periodically review the specific categories and families you use with this script.
  • Commandline arguments take precedence over variables defined in a script configuration file, which in turn take precedence over variables defined in the script itself. For example, you can disable all plugin categories by using the commandline argument -c "".
  • Plugin categories, families, and risk factors are considered together when deciding whether to include plugins. For example, choosing -c denial -f "SMTP problems" -r "High" will run only denial of service attacks for high-risk vulnerabilities specifically associated with SMTP servers.
  • To negate a range of plugin ids, prefix the first id only with '!'; eg, !10000-100010.
  • Nessus will run plugins in the category settings as needed regardless of what's in the configuration file. These plugins are required to initialize knowledge bases; they do not generally send any packets.
  • The '--top20' option on one hand and the '--categories', '--families', and '--risks'` options on the other are mutually exclusive.
  • Plugins explicitly excluded will never be used regardless of the other variables or commandline options.
  • Multiple categories / families / risks / plugin ids can be specified either by a comma-delimited string or by multiple argument pairs. For example, -c "settings,infos" is equivalent to -c settings -c infos.

Examples:

Invocation Meaning
update-nessusrc ~/.nessusrc updates nessusrc using default settings (eg, non-dangerous plugins and ping / tcp_connect scanners in the script as distributed).
update-nessusrc -s ~/.nessusrc same as above but also print a summary of the changes.
update-nessusrc -d ~/.nessusrc produces an alternate nessusrc without replacing original and also prints lots of debugging info.
update-nessusrc -c "" -f "" -r "" -i "10335,11835" .nessusrc-ms03-039 updates a special nessusrc to use tcp_connect scanner (plugin #10335) and test just for MS RPC interface buffer overruns (plugin #11835).
update-nessusrc -c "denial,destructive_attack,flood,kill_host" -i 10335 ~/.nessusrc-destructive updates a special nessusrc w/ destructive plugins and tcp connect scanner.
update-nessusrc -c "_all_" -f "SMTP problems" ~/.nessusrc-smtp updates a special nessusrc w/ plugins associated with SMTP servers in all categories.
update-nessusrc -c "_all_" -c !destructive_attack -f "SMTP problems" ~/.nessusrc-smtp updates a special nessusrc w/ plugins associated with SMTP servers in all categories except destructive_attack.
`update-nessusrc -r "(Critical High)" ~/.nessusrc-risky`

Known Bugs and Caveats

This script may hang indefinitely if paranoia_level is not set in your config file (for example, if you've exported a policy from NessusClient 3.2+) or it is set but the SSL certificate presented by the server has changed for some reason. If this happens, either update the script so it calls the client with -x, which disables certificate verification, or run nessus manually and resolve SSL_paranoia / certificate issue outside of this script, at least until I can figure out a satisfactory way to handle such cases.

This script is not a substitute for the Nessus client in terms of managing a configuration file. On one hand, it requires that a configuration file already exists. On the other, several plugins require additional configuration - simply adding them to the list of plugins used may not be optimal.

There is a limit to the size of the arguments passed to script_cve_id(), which sets the CVE IDs of the flaws tested by each plugin. Additional CVE IDs, which by convention are listed in comments, are not handled by this script since they can not be reliably identified. Thus, you would do well to review the report of Top 20 Vulnerabilities for which no plugins were found and update the configuration file manually after examining plugins available on your server. Otherwise, you risk generating a configuration file that's not as comprehensive as it could be.

Finally, realize that this script along with its script configuration files may hold userids and passwords used to connect to a Nessus server; protect them accordingly!

Copyright and License

Copyright (c) 2003-2016, George A. Theall. All rights reserved.

This script is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

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This script queries a Nessus server for its list of available plugins and updates a Nessus client configuration file

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