Networking/Computing Tips/Tricks

In this article we continue our nmap series (just click on the nmap tag at the bottom of the article to see the other articles), this time we discuss an advanced nmap feature called the NSE: nmap Scripting Engine.

There are series of nmap scripting categories that are installed as a baseline set when you enabled nmap on your machine.  Each script category is made up of multiple scripts.  You can see what each category contains using the script help command discussed below.

Currently defined categories are auth, broadcast, brute, default. discovery, dos, exploit, external, fuzzer, intrusive, malware, safe, version, and vuln. Category names are not case sensitive. The following list describes each category.

auth

These scripts deal with authentication credentials (or bypassing them) on the target system. Examples include x11-access, ftp-anon, and oracle-enum-users. Scripts which use brute force attacks to determine credentials are placed in the brute category instead.

broadcast

Scripts in this category typically do discovery of hosts not listed on the command line by broadcasting on the local network. Use the newtargets script argument to allow these scripts to automatically add the hosts they discover to the Nmap scanning queue.

brute

These scripts use brute force attacks to guess authentication credentials of a remote server. Nmap contains scripts for brute forcing dozens of protocols, including http-brute, oracle-brute, snmp-brute, etc.

default

These scripts are the default set and are run when using the -sC or -A options rather than listing scripts with --script. This category can also be specified explicitly like any other using --script=default. Many factors are considered in deciding whether a script should be run by default:

  • Speed
    • A default scan must finish quickly, which excludes brute force authentication crackers, web spiders, and any other scripts which can take minutes or hours to scan a single service.
  • Usefulness
    • Default scans need to produce valuable and actionable information. If even the script author has trouble explaining why an average networking or security professional would find the output valuable, the script should not run by default.
  • Verbosity
    • Nmap output is used for a wide variety of purposes and needs to be readable and concise. A script which frequently produces pages full of output should not be added to the default category. When there is no important information to report, NSE scripts (particularly default ones) should return nothing. Checking for an obscure vulnerability may be OK by default as long as it only produces output when that vulnerability is discovered.
  • Reliability
    • Many scripts use heuristics and fuzzy signature matching to reach conclusions about the target host or service. Examples include sniffer-detect and sql-injection. If the script is often wrong, it doesn't belong in the default category where it may confuse or mislead casual users. Users who specify a script or category directly are generally more advanced and likely know how the script works or at least where to find its documentation.
  • Intrusiveness
    • Some scripts are very intrusive because they use significant resources on the remote system, are likely to crash the system or service, or are likely to be perceived as an attack by the remote administrators. The more intrusive a script is, the less suitable it is for the default category. Default scripts are almost always in the safe category too, though we occasionally allow intrusive scripts by default when they are only mildly intrusive and score well in the other factors.
  • Privacy
    • Some scripts, particularly those in the external category described later, divulge information to third parties by their very nature. For example, the whois script must divulge the target IP address to regional whois registries. We have also considered (and decided against) adding scripts which check target SSH and SSL key fingerprints against Internet weak key databases. The more privacy-invasive a script is, the less suitable it is for default category inclusion.
    • We don't have exact thresholds for each of these criteria, and many of them are subjective. All of these factors are considered together when making a decision whether to promote a script into the default category. A few default scripts are identd-owners (determines the username running remote services using identd), http-auth (obtains authentication scheme and realm of web sites requiring authentication), and ftp-anon (tests whether an FTP server allows anonymous access).

discovery

These scripts try to actively discover more about the network by querying public registries, SNMP-enabled devices, directory services, and the like. Examples include html-title (obtains the title of the root path of web sites), smb-enum-shares (enumerates Windows shares), and snmp-sysdescr (extracts system details via SNMP).

dos

Scripts in this category may cause a denial of service. Sometimes this is done to test vulnerability to a denial of service method, but more commonly it is an undesired by necessary side effect of testing for a traditional vulnerability. These tests sometimes crash vulnerable services.

exploit

These scripts aim to actively exploit some vulnerability. Examples include jdwp-exec and http-shellshock.

external

Scripts in this category may send data to a third-party database or other network resource. An example of this is whois-ip, which makes a connection to whois servers to learn about the address of the target. There is always the possibility that operators of the third-party database will record anything you send to them, which in many cases will include your IP address and the address of the target. Most scripts involve traffic strictly between the scanning computer and the client; any that do not are placed in this category.

fuzzer

This category contains scripts which are designed to send server software unexpected or randomized fields in each packet. While this technique can useful for finding undiscovered bugs and vulnerabilities in software, it is both a slow process and bandwidth intensive. An example of a script in this category is dns-fuzz, which bombards a DNS server with slightly flawed domain requests until either the server crashes or a user specified time limit elapses.

intrusive

These are scripts that cannot be classified in the safe category because the risks are too high that they will crash the target system, use up significant resources on the target host (such as bandwidth or CPU time), or otherwise be perceived as malicious by the target's system administrators. Examples are http-open-proxy (which attempts to use the target server as an HTTP proxy) and snmp-brute (which tries to guess a device's SNMP community string by sending common values such as public, private, and cisco). Unless a script is in the special version category, it should be categorized as either safe or intrusive.

malware

These scripts test whether the target platform is infected by malware or backdoors. Examples include smtp-strangeport, which watches for SMTP servers running on unusual port numbers, and auth-spoof, which detects identd spoofing daemons which provide a fake answer before even receiving a query. Both of these behaviors are commonly associated with malware infections.

safe

Scripts which weren't designed to crash services, use large amounts of network bandwidth or other resources, or exploit security holes are categorized as safe. These are less likely to offend remote administrators, though (as with all other Nmap features) we cannot guarantee that they won't ever cause adverse reactions. Most of these perform general network discovery. Examples are ssh-hostkey (retrieves an SSH host key) and html-title (grabs the title from a web page). Scripts in the version category are not categorized by safety, but any other scripts which aren't in safe should be placed in intrusive.

version

The scripts in this special category are an extension to the version detection feature and cannot be selected explicitly. They are selected to run only if version detection (-sV) was requested. Their output cannot be distinguished from version detection output and they do not produce service or host script results. Examples are skypev2-version, pptp-version, and iax2-version.

vuln

These scripts check for specific known vulnerabilities and generally only report results if they are found. Examples include realvnc-auth-bypass and afp-path-vuln.

How To Run Scripts in nmap

Be very careful with scripts - they tend to be quite aggressive.  As we have repeated many times in this nmap series: be sure you have permission to execute this type of network scanning.  Even the "safe" scripts above are not 'safe'.

Let's start with the general nmap help and man page:

nmap -h

or

man nmap

To get specific help on a script use the following command replacing the script group name of interest:

nmap --script-help default

To actually run the script - let's start with default - enter the following command example (you can replace the address or address range or subnet as you wish):

nmap --script=default 192.168.1.1

Expect scripts to take a while to execute.  Here is my output for running the default script against my gateway:

nse1

 You can combine script groups with 'and' and 'or'.  The script groups need to be in double quotes.  For example:

nmap --script="default and discovery" 192.168.1.0/24
nmap --script="default or discovery" 192.168.1.0/24

NSE allows users to write (and share) simple scripts (using the Lua programming language ) to automate a wide variety of networking tasks. Those scripts are executed in parallel with the speed and efficiency you expect from Nmap. Users can rely on the growing and diverse set of scripts distributed with Nmap, or write their own to meet custom needs.  Want to find more than what comes with nmap?  Just do a Google search for "nmap script repository" as a starting point.

Pretty powerfull stuff.  We encourage you to play with these, and ofcourse use Wireshark to uncover what they send and what the targets respond with.

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