A small and quick introduction to ARP poisoning

This article won’t be about something new nor something extraordinary for any experienced computer security or even the average hacker, but since I’ve been ask this question quite often by some of my friends, I decided to explain how to sniff passwords from a network.  Moreover, I’m well aware I haven’t been writing anything for a while, and I want to get back to it once all my personal matters are resolved. I’ll concentrate on WEP wireless networks since they are almost certain to be cracked easily. Although those a deprecated, there are still used in many household as the out-of-the-box default configuration, so it’s still pertinent in my opinion. Then I will explain the ARP (Address Resolution Protocol) poisoning attack, which will be used to intercept packets between the target and the Internet.

Attacking the WEP wireless network

Packets in a WEP network are encryted, so in order to sniff packets off from it, you’ll first need to acquire the WEP key. This can be done easily with a wireless network adapter that supports monitor mode and the aircrack suite. For the adapter, I’m using the Linksys  Compact Wireless-G USB adapter, model no WUSB54GC. Plug your adapter into a USB connector and boot up your machine. Once you have booted up, make sure Backtrack or any other distribution has detected your adapter:

ifconfig rausb0 up

and then put the adapter in “Monitor Mode”

iwconfig rausb0 mode monitor

The goal of a WEP attack is to capture as many initialization vectors (IVs) as possible. IVs are random numbers used with a either 64, 128 and 256-bit key to encrypt a stream cipher. Those are used so that two exact same plain text do not produce the same ciphertext. The problem with WEP is that IVs are very short, and on a busy network, the same vectors get reused quickly. The IV is 24 bit long, therefore there are 16 777 216 possibilities1. Moreover, changing the IV for each packet is optional. The keys are also quite short, therefore opening the possibility of finding the key with some brute force calculation. No matter what is they key length, you will just need more packets.

The WEP protocol then use the randomly generated IV, the WEP key and pass it throught the RC4 cipher to produce a keystream. The keystream is then XORed with the plain text stream to produce the cipher text, as shown in the picture below:

So basically, if you get many packets with the same Ivs, different ciphertext, you can now try to brute-force the WEP key. And to get those packets, you need traffic on the network. Now if there are already some people connected and surfing the web, you can easily capture packets and replay them to get more IVs, otherwise, you need to generate the traffic yourself.

Once you’ve tell airodump to capture IVs, we will use aireplay to generate more traffic, and therefore capture more IVs quickly. If you look at the airodump screen, you’ll see it capturing packets.

Once you have the key, you can finally start the poisoning process. As you have seen, I have not detailed how to crack a WEP network as it is widely described all over the net. You can find find good video tutorials from InfinityExists here and here. The last 2600 issue also had a good article about it.

The ARP poisoning attack

The concept behind this is simple. ARP is the protocol that maintains network devices tables up-to-date by associating an IP address with a MAC address. The problem with ARP is that it doesn’t really care about who answered, it will gladly update the tables from whoever says so. Most of the time, it won’t even ask. So the idea behind the attack, is to send the client an ARP answer saying “hey, I’m the gateway, send stuff to me” and a second ARP answer to the real gateway saying “hey there, I’m this guy, send me his stuff”. Then you just have to relay the packets between the victim and the gateway.Those schemas are more simply to understand:

In Linux, the rerouting can be done using the following iptables commands:

iptables -t nat -A PREROUTING -i <interface> -p tcp –dport <port> -j REDIRECT –to-port <redirection port>

iptables -t nat -D PREROUTING -i <interface> -p tcp –dport <port> -j REDIRECT –to-port <redirection port>

I’m showing those commands because you can do a lot with those. Many web applications such as some Flash applications use RTMP (Real-time messaging protocol) to control web applications, which run locally.  Flash server send commands to the application using message. Using those commands, you can filter the packets send or receive from the Flash server. Simply use a sniffer first, then locate which packets you wish to drop, alter or whatever.

For example, some sites gives you samples of live music or videos for 30 seconds, then nag you to pay. Using a sniffer, analyze the traffic and find that RTMP Invoke packet that closes the connection with the server. Code a quick proxy that will let all packets go to the flash application except for the connection closing RTMP packet. Then use the commands above to redirect traffic to your proxy.

Of course you could just use Ettercap, which does exactly what have been mentioned above. Start Ettercap with the following:

sudo ettercap -G -W 128:p:25AAAAC18DEADDADA433332B65

This will open the graphical interface (-G), that is if you have installed the GTK interface to Ettercap. -W specify to listen for wireless networks and to use a 128-bit key with key found earlier. I don’t know what the p is really for. You can also use the text mode.

Then select Sniffing > Unified Sniffing > select on which interface you want to sniff. Then start the sniffing: File > Start Sniffing. Now let’s specify which targets you wanna sniff. Go to Hosts > Scan for hosts. That will locate the hosts on the current network. Then popup the hosts list, Hosts > Show Hosts List.

On the list, add the router to target 2 and the hosts you wanna sniff to target 1. Only one step left: MITM > ARP poisoning.  Select Sniff Remote Connections > OK.

Then you wait for users to connect to pages like MySpace or Hotmail etc…and Ettercap will find out the sensitive information for you.

See also:

Wireless Networking, Praphul Chandra, Alan Bensky, Ron Olexa, Daniel Mark Dobkin, David A. Lide, Farid Dowla

RFC 826 – Ethernet Address Resolution Protocol, David C. Plummer, November 1982, http://www.faqs.org/rfcs/rfc826.html

Wired Equivalent Protocol, Wikipedia, http://en.wikipedia.org/wiki/Wired_Equivalent_Privacy

Ettercap, http://ettercap.sourceforge.net/