How To Crack WEP - Part 2: Performing the Crack

Starting from scratch

In real-life, someone trying to break into a wireless network usually would have to obtain the information needed (MAC address of the AP and Target PC and wireless channel). Professionals who do penetration testing of networks describe this attack as a "Zero Knowledge" attack, for obvious reasons. If the attacker already has all the information needed, that’s called a "Full Knowledge" attack, which is nowhere near as challenging! We’ll assume that we know nothing and describe how to get the information we need.

Finding the MAC Address of the AP with Kismet

Figure 1: Navigating Kismet
(click image to enlarge)

Finding the MAC Address of the AP is extremely easy with either Kismet or Netstumbler. Start Backtrack with its Wi-Fi card and CD inserted. Once Backtrack is up, start Kismet, just like you did in Part 1, and you will see a list of APs. Type s and then c to sort the APs by channel and using the arrow keys, move the highlight bar to your target AP’s SSID. Then hit the Enter key. This will bring up a detailed screen (Figure 2) that will show the selected AP’s SSID, MAC address and channel. Voila! "Zero knowledge" has been transformed into almost all the information needed to run a WEP crack.

Figure 2: Kismet easily finds the SSID, Channel and MAC address
(click image to enlarge)

Tip: Some "security professionals" suggest cloaking your SSID / disabling SSID broadcasts. While this will defeat a Netstumbler scan, Kismet will easily detect "cloaked" SSIDs. Kismet captures more network information than Netstumbler and can find AP SSID’s by following conversations between associated clients and the AP.

Finding the MAC Address of the Client

We need one last piece of information to begin our cracking - the MAC address of a wireless client associated to the AP of our Target WLAN. Go back to Kismet and type q to quit out of the details menu. The highlight bar should still be on your AP, if it isn’t, then use the arrow keys again. Typing shift-C will bring up a list of clients. The MAC addresses are listed on the left side (Figure 3).

Figure 3: Client MAC address found by Kismet

(click image to enlarge)

If you don’t see the MAC address of the TARGET computer, check to make sure it’s on and associated with the Target AP (boot the TARGET into Windows, have it connect to the AP and start browsing the web). In about 10-30 seconds, you should see the MAC address of the TARGET computer pop up in Kismet. A prudent cracker would probably record all the client MAC addresses found so as not to be thwarted if a client isn’t present when the time comes to start the cracking process.

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