How to Crack a Wi-Fi Network’s WEP Password with BackTrack
You already know that if you want to lock down your Wi-Fi network, you should
opt for WPA encryption because WEP is easy to crack. But did you know how easy? Take a look.
Note: This post demonstrates how to crack WEP passwords, an older
and less often used network security protocol. If the network you want
to crack is using the more popular WPA encryption, see our guide to cracking a Wi-Fi network's WPA password with Reaver instead.
Today we're going to run down, step-by-step, how to crack a Wi-Fi
network with WEP security turned on. But first, a word: Knowledge is
power, but power doesn't mean you should be a jerk, or do anything
illegal. Knowing
how to pick a lock doesn't make you a thief. Consider this post educational, or a proof-of-concept intellectual exercise.
Dozens of tutorials on how to crack WEP are already all over the
internet using this method. Seriously—Google it. This ain't what you'd
call "news." But what is surprising is that someone like me, with
minimal networking experience, can get this done with free software and a
cheap Wi-Fi adapter. Here's how it goes.
What You'll Need
Unless
you're a computer security and networking ninja, chances are you don't
have all the tools on hand to get this job done. Here's what you'll
need:
- A compatible wireless adapter—This is the biggest
requirement. You'll need a wireless adapter that's capable of packet
injection, and chances are the one in your computer is not. After
consulting with my friendly neighborhood security expert, I purchased an
Alfa AWUS050NH USB adapter, pictured here, and it set me back about $50
on Amazon. Update: Don't do what I did. Get the Alfa AWUS036H, not the US050NH, instead. The guy in this video below is using a $12 model he bought on Ebay (and is even selling his router of choice). There are plenty of resources on getting aircrack-compatible adapters out there.
- A BackTrack Live CD. We already took you on a full screenshot tour of how to install and use BackTrack 3,
the Linux Live CD that lets you do all sorts of security testing and
tasks. Download yourself a copy of the CD and burn it, or load it up in
VMware to get started.
- A nearby WEP-enabled Wi-Fi network. The signal should be
strong and ideally people are using it, connecting and disconnecting
their devices from it. The more use it gets while you collect the data
you need to run your crack, the better your chances of success.
- Patience with the command line. This is an ten-step process
that requires typing in long, arcane commands and waiting around for
your Wi-Fi card to collect data in order to crack the password. Like the
doctor said to the short person, be a little patient.
Crack That WEP
To crack WEP, you'll need to launch Konsole, BackTrack's built-in
command line. It's right there on the taskbar in the lower left corner,
second button to the right. Now, the commands.
First run the following to get a list of your network interfaces:
airmon-ng
The only one I've got there is labeled
ra0. Yours may be
different; take note of the label and write it down. From here on in,
substitute it in everywhere a command includes (interface).
Now, run the following four commands. See the output that I got for them in the screenshot below.
airmon-ng stop (interface)
ifconfig (interface) down
macchanger --mac 00:11:22:33:44:55 (interface)
airmon-ng start (interface)
If you don't get the same results from these commands as pictured here,
most likely your network adapter won't work with this particular crack.
If you do, you've successfully "faked" a new MAC address on your
network interface, 00:11:22:33:44:55.
Now it's time to pick your network. Run:
airodump-ng (interface)
To see a list of wireless networks around you. When you see the one
you want, hit Ctrl+C to stop the list. Highlight the row pertaining to
the network of interest, and take note of two things: its BSSID and its
channel (in the column labeled CH), as pictured below. Obviously the
network you want to crack should have WEP encryption (in the ENC)
column, not WPA or anything else.
Like I said, hit Ctrl+C to stop this listing. (I had to do this once or
twice to find the network I was looking for.) Once you've got it,
highlight the BSSID and copy it to your clipboard for reuse in the
upcoming commands.
Now we're going to watch what's going on with that network you chose and capture that information to a file. Run:
airodump-ng -c (channel) -w (file name) --bssid (bssid) (interface)
Where (channel) is your network's channel, and (bssid) is the BSSID
you just copied to clipboard. You can use the Shift+Insert key
combination to paste it into the command. Enter anything descriptive for
(file name). I chose "yoyo," which is the network's name I'm cracking.
You'll get output like what's in the window in the background
pictured below. Leave that one be. Open a new Konsole window in the
foreground, and enter this command:
aireplay-ng -1 0 -a (bssid) -h 00:11:22:33:44:55 -e (essid) (interface)
Here the ESSID is the access point's SSID name, which in my case is
yoyo. What you want to get after this command is the reassuring "Association successful" message with that smiley face.
You're almost there. Now it's time for:
aireplay-ng -3 -b (bssid) -h 00:11:22:33:44:55 (interface)
Here we're creating router traffic to capture more throughput faster
to speed up our crack. After a few minutes, that front window will start
going crazy with read/write packets. (Also, I was unable to surf the
web with the
yoyo network on a separate computer while this
was going on.) Here's the part where you might have to grab yourself a
cup of coffee or take a walk. Basically you want to wait until enough
data has been collected to run your crack. Watch the number in the
"#Data" column—you want it to go above 10,000. (Pictured below it's only
at 854.)
Depending on the power of your network (mine is inexplicably low at -32 in that screenshot, even though the
yoyo
AP was in the same room as my adapter), this process could take some
time. Wait until that #Data goes over 10k, though—because the crack
won't work if it doesn't. In fact, you may need more than 10k, though
that seems to be a working threshold for many.
Once you've collected enough data, it's the moment of truth. Launch a
third Konsole window and run the following to crack that data you've
collected:
aircrack-ng -b (bssid) (file name-01.cap)
Here the filename should be whatever you entered above for (file
name). You can browse to your Home directory to see it; it's the one
with .cap as the extension.
If you didn't get enough data, aircrack will fail and tell you to try again with more. If it succeeds, it will look like this:
The WEP key appears next to "KEY FOUND." Drop the colons and enter it to log onto the network.
Problems Along the Way
With this article I set out to prove that cracking WEP is a
relatively "easy" process for someone determined and willing to get the
hardware and software going. I still think that's true, but unlike the
guy in the video below, I had several difficulties along the way. In
fact, you'll notice that the last screenshot up there doesn't look like
the others—it's because it's not mine. Even though the AP which I was
cracking was my own and in the same room as my Alfa, the power reading
on the signal was always around -30, and so the data collection was very
slow, and BackTrack would consistently crash before it was complete.
After about half a dozen attempts (and trying BackTrack on both my Mac
and PC, as a live CD and a virtual machine), I still haven't captured
enough data for aircrack to decrypt the key.
So while this process is easy in theory, your mileage may vary
depending on your hardware, proximity to the AP point, and the way the
planets are aligned. Oh yeah, and if you're on deadline—Murphy's Law
almost guarantees it won't work if you're on deadline.
To see the video version of these exact instructions, check out this dude's YouTube video.
Got any experience with the WEP cracking courtesy of BackTrack? What do you have to say about it? Give it up in the comments.
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