Penetration Testing

Aamir Lakhani

February 2015

In this article by Aamir Lakhani and Joseph Muniz, authors of the book Penetration Testing with Raspberry Pi, we will see the various LAN- and wireless-based attack scenarios, using tools found in Kali Linux that are optimized for a Raspberry Pi. These scenarios include scanning, analyzing and capturing network traffic.

(For more resources related to this topic, see here.)

The Raspberry Pi has limited performance capabilities due to its size and processing power. It is highly recommended that you test the following techniques in a lab prior to using a Raspberry Pi for a live penetration test.

Network scanning

Network reconnaissance is typically time-consuming, yet it is the most important step when performing a penetration test. The more you know about your target, the more likely it is that you will find the fastest and easiest path to success. The best practice is starting with reconnaissance methods that do not require you to interact with your target; however, you will need to make contact eventually. Upon making contact, you will need to identify any open ports on a target system as well as map out the environment to which it's connected. Once you breach a system, typically there are other networks that you can scan to gain deeper access to your target's network.

One huge advantage of the Raspberry Pi is its size and mobility. Typically, Kali Linux is used from an attack system outside a target's network; however, tools such as PWNIE Express and small systems that run Kali Linux, such as a Raspberry Pi, can be placed inside a network and be remotely accessed. This gives an attacker a system inside the network, bypassing typical perimeter defenses while performing internal reconnaissance. This approach brings the obvious risks of having to physically place the system on the network as well as create a method to communicate with it remotely without being detected; however, if successful, this can be very effective.

Let's look at a few popular methods to scan a target network. We'll continue forward assuming that you have established a foothold on a network and now want to understand the current environment that you have connected to.

Nmap

The most popular open source tool used to scan hosts and services on a network is Nmap (short for Network Mapper). Nmap's advanced features can detect different applications running on systems as well as offer services such as the OS fingerprinting features. Nmap can be very effective; however, it can also be easily detected unless used properly. We recommend using Nmap in very specific situations to avoid triggering a target's defense systems.

For more information on how to use Nmap, visit http://nmap.org/.

To use Nmap to scan a local network, open a terminal window and type nmap (target), for example, nmap www.somewebsite.com or nmap 192.168.1.2. There are many other commands that can be used to tune your scan. For example, you can tune how stealthy you want to be or specify to store the results in a particular location. The following screenshot shows the results after running Nmap against www.thesecurityblogger.com. Note that this is an example and is considered a noisy scan. If you simply type in either of the preceding two commands, it is most likely that your target will easily recognize that you are performing an Nmap scan.

Penetration Testing with Raspberry Pi

There are plenty of online resources available to learn how to master the various features for Nmap. Here is a reference list of popular nmap commands:

  • nmap 192.168.1.0/24: This scans the entire class C range
  • nmap -p <port ranges>: This scans specific ports
  • nmap -sP 192.168.1.0/24: This scans the network/find servers and devices that are running
  • nmap –iflist: This shows host interfaces and routes
  • nmap –sV 192.168.1.1: This detects remote services' version numbers
  • nmap –sS 192.168.1.1: This performs a stealthy TCP SYN scan
  • nmap –sO 192.168.1.1: This scans for the IP protocol
  • nmap -192.168.1.1 > output.txt: This saves the output from the scan to the text file
  • nmap –sA 192.168.1.254: This checks whether the host is protected by a firewall
  • nmap –PN 192.168.1.1: This scans the host when it is protected by a firewall
  • nmap --reason 192.168.1.1: This displays the reason a port is in a particular state
  • nmap --open 192.168.1.1: This only shows open or possibly open ports

The Nmap GUI software Zenmap is not included in the Kali Linux ARM image. It is also not recommended over using the command line when running Kali Linux on a Raspberry Pi.

Wireless security

Another attack vector that can be leveraged on a Raspberry Pi with a Wi-Fi adapter is targeting wireless devices such as mobile tablets and laptops. Scanning wireless networks, once they are connected, is similar to how scanning is done on a LAN; however, typically a layer of password decryption is required before you can connect to a wireless network. Also, wireless network identifier known as Service Set Identifier (SSID) might not be broadcasted but will still be visible when you use the right tools. This section will cover how to bypass wireless onboarding defenses so that you can access a target's Wi-Fi network and perform the penetration testing steps.

Looking at a Raspberry Pi with Kali Linux, one of the use cases is hiding the system inside or near a target's network and launching wireless attacks remotely. The goal will be to enable the Raspberry Pi to access the network wirelessly and provide a remote connection back to the attacker. The attacker can be nearby using wireless to control the Raspberry Pi until it gains wireless access. Once on the network, a backdoor can be established so that the attacker can communicate with the Raspberry Pi from anywhere in the world and launch attacks.

Cracking WPA/WPA2

A commonly found security protocol for protecting wireless networks is Wi-Fi Protected Access (WPA). WPA was later replaced by WPA2 and it will be probably what you will be up against when you perform a wireless penetration test.

WPA and WPA2 can be cracked with Aircrack. Kali Linux includes the Aircrack suite, which is one of the most popular applications to break wireless security. Aircrack works by gathering packets seen on a wireless connection to either mathematically analyze the data to crack weaker protocols such as Wired Equivalent Privacy (WEP), or use brute force on the captured data with a wordlist.

Cracking WPA/WPA2 can be done due to a weakness in the four-way handshake between the client and the access point. In summary, a client will authenticate to an access point and go through a four-step process. This is the time when the attacker is able to grab the password and use a brute force approach to identify it. The time-consuming part in this is based on how unique the network password is, how extensive your wordlist that will be used to brute force against the password is, and the processing power of the system. Unfortunately, the Raspberry Pi lacks the processing power and the hard drive space to accommodate large wordlist files. So, you might have to crack the password off-box with a tool such as John the Ripper. We recommend this route for most WPA2 hacking attempts.

Here is the process to crack a WPA running on a Linksys WRVS4400N wireless router using a Raspberry Pi on-box options. We are using a WPA example so that the time-consuming part can be accomplished quickly with a Raspberry Pi. Most WPA2 cracking examples would take a very long time to run from a Raspberry Pi; however, the steps to be followed are the same to run on a faster off-box system.

The steps are as follows:

  1. Start Aircrack by opening a terminal and typing airmon-ng;

    Penetration Testing with Raspberry Pi

  2. In Aircrack, we need to select the desired interface to use for the attack. In the previous screenshot, wlan0 is my Wi-Fi adapter. This is a USB wireless adapter that has been plugged into my Raspberry Pi.
  3. It is recommended that you hide your Mac address while cracking a foreign wireless network. Kali Linux ARM does not come with the program macchanger. So, you should download it by using the sudo apt-get install macchanger command in a terminal window. There are other ways to change your Mac address, but macchanger can provide a spoofed Mac so that your device looks like a common network device such as a printer. This can be an effective way to avoid detection.
  4. Next, we need to stop the interface used for the attack so that we can change our Mac address. So, for this example, we will be stopping wlan0 using the following commands:
    airmon-ng stop wlan0
    ifconfig wlan0 down
  5. Now, let's change the Mac address of this interface to hide our true identity. Use macchanger to change your Mac to a random value and specify your interface. There are options to switch to another type of device; however, for this example, we will just leave it as a random Mac address using the following command:
    macchanger -r wlan0

    Our random value is b0:43:3a:1f:3a:05 in the following screenshot. Macchanger shows our new Mac as unknown.

    Penetration Testing with Raspberry Pi

  6. Now that our Mac is spoofed, let's restart airmon-ng with the following command:
    airmon-ng start wlan0
  7. We need to locate available wireless networks so that we can pick our target to attack. Use the following command to do this:
    airodump-ng wlan0
  8. You should now see networks within range of your Raspberry Pi that can be targeted for this attack. To stop the search once you identify a target, press Ctrl + C. You should write down the Mac address, also known as BSSID, and the channel, also known as CH, used by your target network. The following screenshot shows that our target with ESSID HackMePlease is running WPA on CH 6:

    Penetration Testing with Raspberry Pi

  9. The next step is running airodump against the Mac address that you just copied. You will need the following things to make this work:
    • The channel being used by the target
    • The Mac address (BSSID) that you copied
    • A name for the file to save your data

    Let's run the airodump command in the following manner:

    airodump-ng –c [channel number] –w [name of file] –-bssid [target ssid] wlan0

    This will open a new terminal window after you execute it. Keep that window open.

    Open another terminal window that will be used to connect to the target's wireless network. We will run aireplay using the following command:

    aireplay-ng-deauth 1 –a [target's BSSID] –c [our BSSID] [interface]

    For our example, the command will look like the following:

    aireplay-ng -–deauth 1 –a 00:1C:10:F6:04:C3 –c 00:0f:56:bc:2c:d1 wlan0

    The following screenshot shows the launch of the preceding command:

    Penetration Testing with Raspberry Pi

    You may not get the full handshake when you run this command. If that happens, you will have to wait for a live user to authenticate you to the access point prior to launching the attack. The output on using Aircrack may show you something like Opening [file].cap a few times followed by No valid WPA handshakes found, if you didn't create a full handshake and somebody hasn't authenticated you by that time. Do not proceed to the next step until you capture a full handshake.

  10. The last step is to run Aircrack against the captured data to crack the WPA key. Use the –w option to specify the location of a wordlist that will be used to scan against the captured data. You will use the .cap file that was created earlier during step 9, so we will use the name capturefile.cap in our example. We'll do this using the following command:
    Aircrack-ng –w ./wordlist.lst wirelessattack.cap

    The Kali Linux ARM image does not include a wordlist.lst file for cracking passwords. Usually, default wordlists are not good anyway. So, it is recommended that you use Google to find an extensive wordlist (see the next section on wordlists for more information). Make sure to be mindful of the hard drive space that you have on the Raspberry Pi, as many wordlists might be too large to be used directly from the Raspberry Pi. The best practice for running process-intensive steps such as brute forcing passwords is to do them off-box on a more powerful system.

    You will see Aircrack start and begin trying each password in the wordlist file against the captured data. This process could take a while depending on the password you are trying to break, the number of words in your list, and the processing speed of the Raspberry Pi. We found that it ranges from a few hours to days, as it's a very tedious process and is possibly better-suited for an external system with more horsepower than a Raspberry Pi. You may also find that your wordlist doesn't work after waiting a few days to sort through the entire wordlist file.

    If Aircrack doesn't open and start trying keys against the password, you either didn't specify the location of the .cap file or the location of the wordlist.lst file, or you don't have the captured handshake data. By default, the previous steps store files in the root directory. You can move your wordlist file in the root directory to mimic how we ran the commands in the previous steps since all our files are located in the root directory folder. You can verify this by typing ls to list the current directory files. Make sure that you list the correct directories of each file that are called by each command.

    If your attack is successful, you should see something like the following screenshot that shows the identified password as sunshine:

    Penetration Testing with Raspberry Pi

    It is a good idea to perform this last step on a remote machine. You can set up a FTP server and push your .cap files to that FTP server.

You can learn more about setting up an FTP server at http://www.raspberrypi.org/forums/viewtopic.php?f=36&t=35661.

Creating wordlists

There are many sources and tools that can be used to develop a wordlist for your attack. One popular tool called Custom Wordlist Generator (CeWL), allows you to create your own custom dictionary file. This can be extremely useful if you are targeting individuals and want to scrape their blogs, LinkedIn, or other websites for commonly used words. CeWL doesn't come preinstalled on the Kali Linux ARM image, so you will have to download it using apt-get install cewl.

To use CeWL, open a terminal window and put in your target website. CeWL will examine the URL and create a wordlist based on all the unique words it finds. In the following example, we are creating a wordlist of commonly used words found on the security blog www.drchaos.com using the following command:

cewl www.drchaos.com -w drchaospasswords.txt

The following screenshot shows the launch of the preceding command:

Penetration Testing with Raspberry Pi

You can also find many examples of popular wordlists used as dictionary files on the Internet. Here are a few wordlist examples sources that you can use; however, be sure to research Google for other options as well:

Here is a dictionary that one of the coauthors put together:

http://www.drchaos.com/public_files/chaos-dictionary.lst.txt

Capturing traffic on the network

It is great to get access to a target network. However, typically the next step, once a foothold is established, is to start looking at the data. To do this, you will need a method to capture and view network packets. This means turning your Raspberry Pi into a remotely accessible network tap.

Many of these tools could overload and crash your Raspberry Pi. Look out for our recommendations regarding when to use a tuning method to avoid this from happening.

Tcpdump

Tcpdump is a command line based packet analyzer. You can use tcpdump to intercept and display TCP/IP and other packets that are transmitted and seen attached by the system This means the Raspberry Pi must have access to the network traffic that you intend to view or using tcpdump won't provide you with any useful data. Tcpdump is not installed with the default Kali Linux ARM image, so you will have to install it using the sudo apt-get install tcpdump command.

Once installed, you can run tcpdump by simply opening a terminal window and typing sudo tcpdump. The following screenshot shows the traffic flow visible to us after the launch of the preceding command:

Penetration Testing with Raspberry Pi

As the previous screenshot shows, there really isn't much to see if you don't have the proper traffic flowing through the Raspberry Pi. Basically, we're seeing our own traffic while being plugged into an 802.1X-enabled switch, which isn't interesting. Let's look at how to get other system's data through your Raspberry Pi.

Running tcpdump consumes a lot of the Raspberry Pi's processing power. We found that this could crash the Raspberry Pi by itself or while using it with other applications. We recommend that you tune your data capture to avoid this from happening.

Man-in-the-middle attacks

One common method to capture sensitive information is by performing a man-in-the-middle attack. By definition, a man-in-the-middle attack is when an attacker makes independent connections with victims while actively eavesdropping on the communication. This is typically done between a host and the systems. For example, a popular method to capture passwords is to act as a middleman between login credentials passed by a user to a web server.

Summary

This article introduced us to the various attack scenarios of penetration testing used with the tools available in Kali Linux, over a Raspberry Pi. This article also gave us detailed description of tools like Nmap, CeWL, and tcpdump, which are used for network scanning, creating wordlists, and analyzing network traffic respectively.

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