2600 Cover

Foreword

       The following is "Mobile Hacking with Android", as it appeared on page 15 of 2600 issue 28:2. This article starts off by taking a look at the origins of Android, and a basic rundown on the internal structure of the OS. It then moves on to some choice software selections, and finally, details a real-world man in the middle attack using a rooted Android smartphone.

This is the first article in which I actually detail a functional attack; in this case I thought it was important to demonstrate the capabilities of Android in an offensive capacity. It is also the first article featuring QR codes to appear in 2600. I was happy the QRs made it into the final print, though not without issue. Between the time I wrote the article and the time it was published, Wireless Tether was removed from the Android Market, making it's QR a dead link. I made the concious decision to not use a URL shortener when generating the QR codes, as I believe the 2600 readers would be rightly suspicious of a QR pointing them to a shortener service. A shortener would have allowed me to fix the link after publication, but I am still not sure it is worth the risk to the reader. Chalk this one up to a learning experience I guess.

This article has been made available as an eBook on the Amazon Kindle and Barnes & Noble Nook. Purchases of the eBook version of this article are considered a donation to support my research and projects; and are greatly appreciated.

Amazon Kindle Version
Barnes & Noble Nook Version

Introduction

If you have been following the mobile industry for the last year or so, you have already heard about Android. Google's mobile Linux operating system has taken the industry by storm, and analysts predict that by the end of 2011 it will have overtaken Apple's iOS as the number two mobile operating system in the world. Some even say that by 2015, it should overtake Nokia's Symbian OS as the number one mobile OS.

The continued success of Android is of particular importance to hackers, as it is more proof that a large scale open source project can not only compete with proprietary software, but excel beyond it if properly supported. Perhaps more importantly, the open nature of Android allows it's more technically inclined users to peer into the workings of their mobile devices and modify them however they wish. Finally, the dream of an open mobile device that started with the OpenMoko FreeRunner is very close to realization for the mass market.

Of course, we know that every story has two sides. With increased hardware performance, storage capacity, and software capability, mobile devices have become increasingly tantalizing targets for attackers and criminals over the last few years. But with flexible operating systems like Android under the hood, mobile devices are now becoming practical attack platforms; allowing an attacker to scan for and engage targets from the palm of his hand.

This article will take a look at a few Android applications of interest to both the hacker and the criminal alike, and detail a proof of concept attack using nothing more than a rooted Android mobile phone and publicly available software. The information herein is provided entirely for educational purposes so that the reader may have a better understanding of the capabilities and realistic applications of this technology. It in no way condones or suggests attempting to use these techniques in a malicious manner.

What is Android?

To get a better understanding of what Android is capable of, we should first get a good handle on what it actually is.

In 2005, Google acquired a little known company called "Android, Inc", which had been developing software for mobile phones. Soon after, Google began filing various patents with a focus on mobile phone technology. This prompted the media to begin speculating that Google was planning on releasing a "G-Phone" to go head-to-head with Apple's immensely popular (and largely unchallenged) iPhone.

But in 2007, rather than announcing a single phone they intended to bring to market, Google brought together a group of some of the most important companies in the mobile industry and created the Open Handset Alliance (OHA), a consortium designed to develop open standards for mobile devices. The OHA revealed that their first product would be an open source mobile OS, called Android, designed to run on the full gambit of mobile devices (phones, tablets, netbooks, etc), rather than an OS tied to a specific piece of hardware (like Apple's iOS). In October of 2008, the HTC Dream (more commonly referred to as the G1) was released and became the first official Android device.

Android is made up of several software layers which are intended to make the OS more modular and easier to develop for. Android is based on the 2.6.x Linux kernel which handles hardware interaction, GNU userspace utilities for low-level system management, and various open source libraries such as OpenGL, SQLite, and FreeType.

While this technically makes Android a GNU/Linux OS, Android applications (or "apps" as they are usually referred to as) are not native Linux binaries. Rather, Google has developed a Java virtual machine called Dalvik and a large framework of libraries which developers can use without ever touching the underlaying Linux system. This means that developing for Android requires no previous knowledge of Linux programming, and allows the developer to work within a well documented and defined environment, regardless of what device their code will eventually run on.

The idea that a developer should be able to write one application and be able to deploy it on essentially every piece of hardware Android itself supports is a core element of the OHA. In theory, this should be a boon for developers, but in practice it introduces a number of problems, one of which being that Android applications are never truly optimized for a specific device, and are always limited by the capabilities of the Dalvik VM. Updates to Dalvik and the introduction of the Native Development Kit (NDK), which allows developers to bundle in native C code with their Java applications, are beginning to alleviate the issue, but hardware intensive applications like 3D games are still noticeably absent from Android's software library.

While not a viable option for large-scale Android development, it is also possible to write (or adapt) Linux C code for use with Android. In theory, this means you could take existing Linux tools and applications and cross-compile them for the ARM architecture most Android devices are running on. In practice however, there are a number of limitations imposed by the abridged nature of Android's Linux implementation that make things more difficult. Most notably, Android doesn't include libc, but rather uses it's own library known as Bionic. All native Linux code must be compiled against Bionic, but as Bionic is not 100% compatible with libc, there is no guarantee code will work as expected (or at all). In addition, Android doesn't use an X server, so graphical Linux applications are out of the question.

As with all Unix-like operating systems, Android has a very strict set of permissions; which in this case extend from the core Linux components all the way up to the Dalvik VM. Since anyone can write an Android application and publish it in the Android Marketplace, it is extremely important for the system to monitor and limit the capabilities of everything the user installs. Every application must list it's capabilities in regards to the Dalvik VM for the user upon installation, and Linux's standard per-user filesystem permissions prevent even rogue applications from accessing the underlaying OS and doing system-wide damage.

While that is fine for the average user, those of us who want more control over our systems can start to feel a little caged in. Just like in a full Linux OS, if you want to get complete access to the system you need to elevate your user level to root. Gaining root privileges is not technically supported by Android, and doing so usually requires making use of some exploit or glitch in that particular device's build of Android. Accordingly, the process of "rooting" an Android device differs greatly depending on the hardware and what version of Android it's running, which makes it considerably out of the scope of this particular article. I can say that, as far as I am aware, all Android devices currently on the market can be rooted, with varying degrees of difficulty or Linux knowledge required. A simple Google search of your device name along with the term "rooting" should get you started.

Android Software

Even though Android has been on the market since 2008, it wasn't until relatively recently that it started to take off. Android's surge in popularity (at least in the US) is considered to be due in large part to Verizon Wireless and their DROID lineup of phones, specifically the Motorola Droid which more or less became the de facto Android 2.0 handset. With an increased userbase comes more developers, and as such, Android software has started to mature and offer legitimate tools and applications rather than the drivel that populated the Android Marketplace for the first couple years of the OS's life.

At the same time, Google's release of the NDK and the fact that Google doesn't prevent or discourage rooting Android has lead to some very powerful and useful software that anyone can install and run without fear of persecution from Google or their device's manufacturer.

In the following sections I will briefly go over some applications of particular interest to the hacker. All of these applications are available on any Android device that has access to the Android Marketplace, though some do require your device to be rooted as mentioned in the previous section.

WiFi Analyzer

       QR Code: Show
       Android Market: View

WiFi Analyzer is one of the most popular applications in the Android Marketplace, which is really a testament to how wildly useful this tool is for both the average user and the more technically inclined. In the most basic of terms, WiFi Analyzer is a tool to scan the area for WiFi networks and determine which channel is the least populated so you can adjust your own hardware to a less congested part of the spectrum.

But as the application has evolved, it has picked up a number of other helpful features. For every detected network it offers multiple detailed graphs of signal strength (strength over time, comparison to other networks in the area, etc), MAC address, and encryption used. There is even a function where you can lock on to a specific network and view the signal strength as an analog gauge, complete with an audio tone which increases in frequency as the signal gets stronger; an absolutely invaluable tool for locating WiFi devices in the field.

That said, it is important to realize that WiFi Analyzer is NOT a full fledged WiFi scanner or "wardriving" tool. As of this writing, there is no method to export the list of detected networks to file, and some functions (like the signal strength versus time graph) won't even retain their data when switching to one of the application's other modes.

ConnectBot

       QR Code: Show
       Android Market: View

ConnectBot is an exceptionally well done SSH/Telnet client, which also acts as a terminal emulator for the local Linux sub-system. While there are better terminal emulators (though not for free), there is no question that ConnectBot is the absolute best SSH client available for Android.

In addition, ConnectBot also allows you to setup SSH port forwarding from your device to a remote server, otherwise known as SSH tunneling; a topic that has been covered numerous times on these pages. Tunneling is an exceptionally useful technique for circumventing firewalls or protecting your data on public networks, both of which are very useful on mobile devices. The SSH forwarding in ConnectBot is not quite as polished as I would like, such as needing to keep an interactive shell open when using the tunnel instead of doing it in the background, but it works well enough.

Network Discovery

       QR Code: Show
       Android Market: View

Network Discovery is a handy tool for finding and enumerating devices on public WiFi networks. Network Discovery uses a simple ping scan to find hosts on the network, and then allows the user to select one of the found hosts to target for a TCP connect() scan.

The execution is pretty basic, but Network Discovery does have a few nice touches such as a NIC vendor database which shows who manufactured the network interface of the discovered devices, service detection (by reading the service banner), and the ability to directly connect to services which Android supports or has known applications for (WWW, FTP, SSH, etc). Future plans include database storage for scan results, OS fingerprinting, NAT traversal, and root-enabled functions like SYN scans.

Wireless Tether

       QR Code: Show

Wireless Tether is a mainstay of rooted Android phones, as it allows any Android phone to share it's cellular Internet connection out over either Ad-Hoc WiFi or Bluetooth PAN. It does this in such a way that prevents your carrier from differentiating between the traffic generated from the Android device itself and any devices connected to it, the upside being that you are able to share the cellular Internet access you already pay (dearly) for without having to sign up for the nonsensical "tethering" charges which many carriers have begun implementing. This is an excellent tool for setting up temporary Internet access for small groups of people, such as at hackerspaces or 2600 meetings.

Shark for Root

       QR Code: Show
       Android Market: View

Shark for Root is a port and front-end for the venerable tcpdump. I suspect the use and function of tcpdump is well known enough that I don't need to go into explicit detail, but to put it briefly, it allows the user to examine and log all of the TCP/IP packets going into and out of the Linux kernel. As the name implies, Shark only works properly if it is run as the root user, which allows it complete access over the kernel's networking subsystems.

Shark isn't much to look at, and in fact has a few rather annoying bugs in the user interface, but the UI itself is the last thing you are going to be worried about. Installing Shark is the easiest way to get a working tcpudmp binary installed on an Android device (though some custom ROMs do include it out of the box), so it's an absolute must have if you want to do any kind of mobile network analysis.

Remote Exploit Applications

This is more of an "Honorable Mention" category; there are currently a handful of applications in the Android Market which are designed to use documented remote exploits against various operating systems and server applications. For example, there are a few applications designed to use the recent Windows Vista and Windows 7 remote SMB exploit. These applications can be used to trigger a BSOD on any unpatched Windows system on the same WiFi network as the Android device.

While this type of software is still fairly rare on Android, it is going to become more common as developers get better acquainted with the intricacies of developing software for Android. This area of development certainly warrants a close watch from the community, both offensively and defensively.

Mobile MITM Attack

So we have covered a few very useful tools you can download on your Android device, but you still might be wondering how these seemingly innocuous applications could possibly be used maliciously. A powerful mobile device running Android could be used by an attacker in thousands of different ways, but for this example we will be focusing on a specific case that involves a few of the applications we just discussed; using a rooted Android phone as part of a man-in-the-middle attack.

The idea is really rather simple, we will be setting up Wireless Tether to make our phone appear to be a public WiFi AP (access point) to our victims, and then once they connect to our phone (and through it, the Internet) we can capture their traffic for later analysis and data retrieval.

The first step is to scout out a good location. Tools like WiFi Analyzer are helpful here as they can be used to find important information about the existing WiFi coverage in the area. Ideally, the best place to attempt an attack like this would be locations with a high density of users, and a relatively low number of existing WiFi APs. Once an attacker finds a location where there are many potential targets, he can use WiFi Analyzer to determine the signal strength of surrounding APs and how many of them are currently running open networks. If there are many strong and free APs in the area, the attack will be less likely to work, but if the only APs with strong signal are encrypted, users will be much more likely to connect to the attacker's open AP with excellent signal strength. Therefore, areas such as coffee shops and hotels would be particularly good candidates for this sort of attack as the users in the building will quickly jump at the chance for free WiFi when presented with the paid access model the business is likely running.

Once a location has been selected, Wireless Tether needs to be configured to appear as an innocent public AP. The SSID can be changed from within Wireless Tether by pressing the Menu key, selecting "Setup", then scrolling down to "Change SSID". Wireless Tether doesn't allow spaces in SSID names, and also has an unusually short character limit, but in practice you can still get the point across. Changing the SSID to something like "free_wifi" should get the desired results, but for added effect it could be more contextually relevant to the target location, such as "hilton_wifi". With the SSID set as something sufficiently approachable, start Wireless Tether by pressing the large icon in the center of the screen (Wireless Tether must be running to complete the next steps).

With Wireless Tether up and running, the next step would be to get Shark setup and begin logging packets. Normally, tcpdump will listen on the default interface, which in the case of a phone would be the 3G radio. Capturing packets from 3G is just going to get us a big log file of gibberish, so Shark needs to be setup so that it runs tcpdump against the phone's WiFi interface where the victims will be connecting.

To do that, you need to figure out what the WiFi interface is actually called. Just like on desktop Linux, some WiFi drivers rename the interface instead of leaving it as the standard wlan0, so you need to do a little digging to figure out what your particular phone is running. The easiest way to do this is by using a terminal emulator (such as ConnectBot) and running the command "netcfg", which will list the device's networking interfaces like so:

# netcfg
lo       UP    127.0.0.1       255.0.0.0       0x00000049
dummy0   DOWN  0.0.0.0         0.0.0.0         0x00000082
usb0     DOWN  0.0.0.0         0.0.0.0         0x00001002
ppp0     UP    75.206.123.22   255.255.255.255 0x000010d1
tiwlan0  UP    192.168.2.254   255.255.255.0   0x00001043
Here we can see the two important interfaces, ppp0 which is the 3G Internet connection, and the tiwlan0 interface which is running Wireless Tether. You can tell them apart easily as one is running a public IP (ppp0), and the other is using a private IP (tiwlan0). The WiFi interface in this case is called tiwlan because the phone in question is using a Texas Instruments chipset. Different devices will be running different hardware, so don't be surprised if you see something completely different.

With the WiFi interface name in hand, you can start up Shark and add in the proper tcpdump parameters. To specify a different interface from the default, you use the "-i" option, so add "-i tiwlan0" to the parameters already listed in Shark (substituting your particular WiFi device name, if necessary). Then press "Start", and make sure it begins logging packets. You should see a line at the bottom that says "Got xx", where "xx" is the numbers of packets currently captured.

Shark Capture

Now all that is left to do is wait. With patience and a little luck, a client device should connect to the phone and attempt to get online. Wireless Tether can be setup with various notifications when new devices connect, including a vibrate option that would let the attacker know a client has connected without making a sound or even having to glance at the phone. Once a client device connects, they will be routed to the Internet just as they expected. Their experience will be identical to that of a regular public WiFi connection, and they would have no reason to suspect anything is wrong.

Obviously, there are some constraints due to the device's relatively limited processing power and bandwidth, but as long as you keep your expectations reasonable (such as using Wireless Tether's access control options to limit yourself to 2 or 3 simultaneous connections) the illusion will hold together well enough for the victims.

Once you feel you have captured enough data, simply stop Shark and shutdown Wireless Tether. You can then open up the PCAP file that was created under /sdcard in Shark Reader (a basic PCAP analyzer created as a companion for Shark), or better yet, pull the file off of the phone's SD card and open it up in Wireshark. Assuming everything went according to plan, you should be looking at a complete log of everything your victim(s) did while connected to your "free" WiFi AP.

A Step Further: Data Siphon

Being able to log all of the plain text traffic to and from the victim's computer is certainly bad, but there are limitations to the Android platform that keep the attack from moving much beyond that. As flexible and widely supported as it is, Android still only has a fraction of the tools available for x86 operating systems. Even if more advanced tools were available for Android, the processing and storage limitations of mobile devices would make it difficult to do much in the way of real-time data manipulation while still delivering the content fast enough to keep the victim from suspecting anything.

But what if, rather than attempting to manipulate the data stream on the mobile device itself, he simply redirected all of the traffic from his rogue AP to another network of which he had full control over; a network which housed machines with the software and processing capability to manipulate the victim's data in real-time? As it turns out, it only takes a few more steps to adapt the man-in-the-middle setup from the previous example into a "siphon" which can redirect all of the traffic on the rouge AP to any network the attacker wishes.

The first step is to bridge the connection between the Android device and the destination network by way of a VPN. Android comes with support for various types of VPNs out of the box, but there are some long-standing bugs in it's implementation that make it all but useless in many software configurations. Luckily, the community rose to the challenge and ported over OpenVPN, which offers incredible amounts of customization and capability. Some custom Android ROMs include OpenVPN, but if yours doesn't you can download it from the Marketplace by way of the "OpenVPN Installer" application by Friedrich Schaeuffelhut. The same developer also put out an application called "OpenVPN Settings" which aims to make configuring and managing OpenVPN connections as easy as the built-in VPN functions, which you may also want to grab.

The actual configuration of an OpenVPN server is outside the scope of this document, but the general idea is that you setup an Internet-facing server in bridge mode; this will let you connect your VPN client (the Android device) to the server from a remote location and give it an IP that is within the subnet of the destination network. I personally used a Linksys WRT56GL running DD-WRT as my OpenVPN server, but any other implementation will work just as well.

With OpenVPN correctly configured on both sides, and Wireless Tether running, the output of "netcfg" should now look something like this:

# netcfg
lo       UP    127.0.0.1       255.0.0.0       0x00000049
dummy0   DOWN  0.0.0.0         0.0.0.0         0x00000082
usb0     DOWN  0.0.0.0         0.0.0.0         0x00001002
tap0     UP    192.168.1.50    255.255.255.0   0x00001043
ppp0     UP    75.206.123.22   255.255.255.255 0x000010d1
tiwlan0  UP    192.168.2.254   255.255.255.0   0x0000104
Notice the addition of the "tap0" interface, with an IP address in the middle of the WRT54G's 192.168.1.x network. The Android device is now connected to three separate networks simultaneously: the primary 3G Internet connection on ppp0, the rouge AP running on tiwlan0, and now the VPN on tap0.

The goal now is to get traffic from our rouge AP on tiwlan0 to go through the VPN, rather than straight through 3G to the Internet. If we run a traceroute from the Android device now, we will see this is currently not the case:

# traceroute 75.206.123.22            
traceroute to 75.206.123.22 (75.206.123.22), 30 hops max, 38 byte packets
 1  66.174.112.129 (66.174.112.129)  143.097 ms  75.959 ms  70.312 ms
 2  66.174.112.127 (66.174.112.127)  62.164 ms  85.510 ms  69.916 ms
 ...
So what we need to do now is setup a new default route that will take all traffic out through the 192.168.1.x network's primary router (in this case, 192.168.1.1) To do this you will use the "route" command:
# route add default gw 192.168.1.1 dev tap0
Note that, unlike the desktop Linux equivalent, the Android "route" command requires you give it an interface name.

Re-running the traceroute command from before, we can see that the path packets are taking through the phone has changed:

# traceroute 75.206.123.22            
traceroute to 75.206.123.22 (75.206.123.22), 30 hops max, 38 byte packets
 1  192.168.1.50 (192.168.1.50)  300.831 ms 365.326 ms  265.656 ms
 2  66.174.112.127 (66.174.112.127)  257.843 ms  257.507 ms  265.930 ms
 ...
The first hop is now the tap0 interface, so we can see that data is traveling through the 192.168.1.x network to get to the Internet, rather than directly out 3G. The keen eye will also note the increased travel time, as data now has to run through the VPN before it gets out to the Internet. Though it is worth noting that the travel times shown here are rather high because my phone had poor signal when I ran this particular test. In ideal conditions, performance over the VPN is not much different than 3G alone.

With the victim's data now traveling through the attacker's personal network, there is no limit to what he can do. A server on the network could provide the victims spoofed DNS entries and forged login pages, or sslstrip could be used to hijack HTTPS connections and get their plain-text content. A combination of these techniques could be used to present the victim with a convincing looking "Critical Update" page that instructs the user to "Download and install the following important system update..." before allowing them to continue on to the Internet at large.

Conclusion

For those of us interested in technical exploration, Android offers nearly unlimited possibilities. Not only can an Android device be used to explore and examine the world around us, we are even given the freedom to explore and modify Android itself by virtue of its open nature. While the installation and use of security related tools on a mobile device is certainly nothing new, older devices primarily used close source proprietary operating systems the user had no control over. Even in the few previous mobile devices that actually shipped with an open source OS, you were still limited by the relative rarity of supported devices and the small userbase. The fact that you can walk into the store of essentially every cellular carrier in the US and purchase a handset that runs an open source OS with development tools baked right in is completely without precedent.

Of course, the same opportunity is available for criminals, and if Android continues it's meteoric rise in popularity as analysts predict, it won't be long until they start getting on the Android bandwagon too. Whether it is to develop malicious applications or remote exploits (at the time of this writing, proof of concepts exist in both cases), criminals will attempt to exploit Android's open nature for their own gains.

For hackers, Android represents not only an excellent platform for personal use and an ideal worthy of our support, but also a future battleground. As smartphones approach the ubiquity that was once reserved for wristwatches, mobile security research and development will be key in protecting user's data and privacy. The hacker ethics of exploration, experimentation, and dissemination of knowledge can aid in Android's evolution just as they once helped shape the telephone itself.