2600 Cover

Foreword

       The following is Bluetooth Hacking Primer, an introduction to Bluetooth hacking published on page 54 of 2600 issue 27:1.

This document is not a guide about actually attacking Bluetooth devices, but rather walks the reader through Bluetooth's basic principles of operation, and touches on the hardware and software setup required to start doing security related research. Some simple attacks are mentioned at the end of the document to illustrate the realistic threat of Bluetooth devices, but it is not intended as a full documentation of those weaknesses.

The original ASCII text file for this document can be found in the "Text" section of the Downloads page.

Introduction

Originally conceived in 1994 by Ericsson, Bluetooth was set to revolutionize the computing and consumer electronics world. It promised to rid us of wires and provide a method by which all of our devices could communicate seamlessly. Unfortunately, early versions of the protocol were so beleaguered by problems that consumers were all too happy to keep their spider web of cables. Besides, most technology of the mid-ninety's was not exactly designed with mobility in mind in the first place.

But today, Bluetooth has come back in a big way. Mobile technology has dominated this decade, and the need for a standardized method of low-power communication has never been greater. At the same time, newer versions of the Bluetooth protocol have all but eliminated the poor range, transfer rate, and interoperability issues that plagued earlier implementations. Bluetooth has now become so popular in the mass market that it has even attained a sort of brand association, to the point that most people simply refer to all wireless headsets as "a Bluetooth."

However, with the resurgence of Bluetooth has come a dangerous, if predictable, complacency. Millions of people are now using the technology without any clear understanding of how it works and what it is capable of.

This article is not written as a hyper-technical look at the Bluetooth protocol, nor does it detail any one particular attack against Bluetooth devices. Instead, it is intended to give the reader some information on how Bluetooth works, what you can do with it, and the risks associated. Hopefully this article will give you enough information to start exploring Bluetooth and allow you to form your own opinions on the technology.

Low-Level Communication

Bluetooth operates in the ISM band between 2.4 and 2.4835 GHz, which is divided into 79 channels that are each 1 MHz wide. Connected Bluetooth devices hop channels at up to 1600 times per second in a pattern derived by the master device's clock. By rapidly changing channels like this, Bluetooth devices are able to avoid interference with other devices in the 2.4 GHz band, such as WiFi networks and cordless telephones, and remain segmented from other Bluetooth networks in the area.

When one Bluetooth device wants to connect to another, it must go through a few steps to learn about and authenticate with the remote device. The eventual master device first scans the band to find other devices which are in so-called "discoverable" mode, and then preforms an inquiry on each one. This gives the device a list of hardware addresses (which are in the familiar MAC-48 format), human-friendly device names (which the owner of the device assigns, or more often than not, leaves as the default), device class IDs (to determine what the device actually is), and clock offsets (used in calculating channel hopping operations). This provides the master device with enough information to begin establishing an actual connection with one or more of the devices it found. The master sends out what is known as a frequency-hop synchronization (FHS) packet, which the slaves use to get locked on to the correct channels and start the authentication process.

While the Bluetooth protocol is a master/slave arrangement, there is a provision which allows for multiple devices to be connected together in what is known as a piconet. In a piconet, up to 8 Bluetooth devices can communicate simultaneously by timing their transmissions to fall on even or odd channel hops. The device currently marked as master can communicate with any of the slaves in the piconet, as well as add or remove devices from the network. It is also possible to connect multiple piconets together by having certain devices act as a master in one piconet and a slave in the other, which is referred to as a scatternet.

Even though only 8 devices can be active in the piconet, there can be up to 255 slaves waiting for their turn to be activated. In addition to the standard "Active" mode, a slave in a piconet can be in 3 modes: "Sniff", "Hold", and "Park". Each of these modes involves progressively less data transmission and therefore lower power consumption (important on portable devices). Devices in these inactive modes still remain synchronized with the piconet master, but do not actively participate unless they are brought back to "Active" status.

High-Level Protocols

There are a few core protocols that all Bluetooth services make use of in some way or another. The most fundamental of these is the Logical Link Control and Adaptation Protocol (L2CAP), which could be thought of as the Bluetooth equivalent of TCP. L2CAP handles the creation, sequencing, and reassembling of packets, QoS, and the channel identifiers (CIDS). CIDS are like TCP ports, they are the endpoints between two devices through which processes can communicate. Like TCP, L2CAP also features a number of signalling commands which are used to control communication over the CIDS.

The next protocol is known as Radio Frequency Communication (RFCOMM). At it's core, RFCOMM is designed as a replacement for RS-232 connections; anything that uses serial communications can be adapted to RFCOMM very easily. RFCOMM provides up to 60 emulated serial ports per device, which are usually referred to as RFCOMM channels. Bluetooth services bind to an open RFCOMM channel, and remote devices address that particular service with a combination of MAC and channel number.

The last major protocol you should be aware of is the Service Discovery Protocol (SDP). SDP is the method by which two Bluetooth devices can determine which services the other is running and how they would connect to them. Each SDP entry contains the name of the service, which protocols it relies on, and which RFCOMM channel it is bound to. With this information, the device can inform the user about the remote device's capability, and internally store the channel and protocol information for later use.

On top of all of these protocols are the highest-level functions, which are provided by what are known as profiles or services. These applications are what the end user is actually interacting with when they send a picture to a phone or connect a headset. There are many Bluetooth services available, certainly more than I would want to list here, but the main ones would be things like Dial-up Networking (DUN), File Transfer Profile (FTP), Headset Profile (HSP), Object Push Profile (OPP), etc.

Hardware Options

Bluetooth hardware is rated in three Classes, which determine the output power (and therefore the approximate range) of the device:

       Class 1       100 mW (20 dBm)       ~100 meters
       Class 2       2.5 mW (4 dBm)          ~10 meters
       Class 3       1 mW (0 dBm)              ~1 meter

If you don't mind spending a little money, try to get a Class 1 adapter that has an external antenna, such as the Linksys USBBT100. Adapters with external antennas are obviously going to have a better range out of the box, but are also easier to modify for use with a larger antenna. One of the nice things about working with Bluetooth hardware is that, since it uses the 2.4 GHz band, you can use WiFi antennas by simply hacking in the appropriate connector.

On the other side of the spectrum, you can get a low-end adapter for as little as $3 shipped from a number of overseas retailers. While the price is certainly right, you need to be careful when buying these cheap adapters for use in research. Manufacturers will often mislabel these devices as Class 1, when they are actually Class 2 or even sometimes Class 3. It is also common for the very cheap adapters to have duplicate MAC addresses; rather than writing a new MAC address to each device's firmware as it rolls off the line, it is cheaper for the manufacturer to leave them all with the default.

Don't be fooled by very cheap adapters with external antennas either. I have purchased many of these devices online, and every one of them had either a fake antenna (nothing more than a plastic stick), or just a bare wire poorly soldered to the existing internal antenna of a generic adapter.

The last thing you want to be aware of when buying Bluetooth hardware is the chipset it is using. While all of them are fairly good, the best supported and documented is the Cambridge Silicon Radio (CSR) chipset. There are a number of tools written specifically for this chipset, and with firmware modifications it is possible to get enhanced scanning and sniffing capabilities. While any adapter will let you scan and enumerate; if you want to get into more advanced techniques like sniffing the pairing process and cracking PINs, a CSR-based device is a must.

BlueZ Basics

It probably won't come as much of a surprise to hear that the Linux Bluetooth stack, BlueZ, is one of the most advanced and capable Bluetooth implementations available on any operating system. Unfortunately, not all parts of it are well documented, and it is currently in a state of transition between the widely supported 3.x branch and the next generation 4.x branch. As of this writing, very little software supports the BlueZ 4.x branch; BlueZ 3.x is still the standard and is what all of the software and guides are written for. This document will be no different, so the following information and recommended software is not guaranteed to work under the newer BlueZ 4.x releases.

The easiest way to get started with BlueZ is to run BackTrack 3 (BackTrack 4 has switched to BlueZ 4.x, and dropped a lot of Bluetooth tools in the process), which includes a wealth of Bluetooth software and the proper libraries to make it all work. Even if you already have a Linux system up and running, it may be easier for you to run BackTrack as it will already have all of the tools and support software ready to go, which may or may not be true for your distribution's package repository.

The capabilities provided by BlueZ could take up a few articles by itself, so I'm not going to detail every possible configuration and function of the whole library, but let's take a brief look at the most important commands and how they work.

The first tool, hciconfig, is the Bluetooth equivalent to ifconfig. With this tool you can bring Bluetooth devices up and down, set their operating modes, and various other low-level functions. The most useful functions of hciconfig in the context of Bluetooth hacking are probably the ability to change the device's name and class. For example, you could make your adapter appear to be a Bluetooth headset to the casual observer:

bash# hciconfig hci0 name "Motorola H700" class 0x200404
The second tool we will cover is hcitool. You will be using hcitool quite a bit when working with Bluetooth, as this command is what you use to scan for, inquire, and ultimately pair with other devices. hcitool also shows any current connections to and from a specific Bluetooth interface, as well as details like signal quality and power levels. A scan for other Bluetooth devices looks like this:
bash# hcitool scan
Scanning ...
	00:21:FB:5F:B3:21	LG VX9600
	00:1B:AF:DB:CB:72	Nokia 6555b
	00:15:A8:2D:4C:A2	Motorola Phone
	00:1F:E3:77:E3:1F	Dare
Here you can see that my Bluetooth adapter is currently connected to a remote device (in this case, my mouse):
bash# hcitool con
Connections:
	> ACL B0:73:08:09:10:57 handle 42 state 1 lm MASTER
Once connected to a device, hcitool can perform a number of other neat tricks, such as displaying the received signal strength indication (RSSI) for a given MAC; which can be used as a crude form of proximity detection. Here you can see how the RSSI differs between my mouse sitting right next to the keyboard and my phone charging across the room:
bash# hcitool rssi B0:73:08:09:10:57
RSSI return value: 0
bash# hcitool rssi 00:1F:E3:77:E3:1F
RSSI return value: -3
Unfortunately, due to the different output ratings of various devices you can't directly equate RSSI to a set distance. With targets of unknown transmission power, the best you can do is determine if your distance from the target is increasing or decreasing.

Another exceptionally useful tool is sdptool. This tool allows you to not only query the SDP records of remote devices, but also add, delete, and edit the SDP records being advertised for your adapter. Getting the SDP records for a target device looks like this (truncated greatly for space):

bash# sdptool browse 00:1F:E3:77:E3:1F
Browsing 00:1F:E3:77:E3:1F ...
Service RecHandle: 0x10000
Service Class ID List:
  "PnP Information" (0x1200)

Service Name: Object Push
Service RecHandle: 0x10001
Service Class ID List:
  "OBEX Object Push" (0x1105)
Protocol Descriptor List:
  "L2CAP" (0x0100)
  "RFCOMM" (0x0003)
    Channel: 1
  "OBEX" (0x0008)
Profile Descriptor List:
  "OBEX Object Push" (0x1105)
    Version: 0x0100
Here you can see the wealth of information returned by an SDP query. We not only see the name and class of each service being offered, but also the protocols and services they rely on, the channels they use, and which version of the service is being run.

Not only is sdptool invaluable for enumerating possible targets, it can also be used to advertise bogus services to remote devices. To go back to the hciconfig example, after changing your adapter's name and device class to that of a Bluetooth headset, you could then use sdptool to advertise the headset and handsfree profiles; in practice making your machine almost completely indistinguishable from a standard headset.

Finally, a few words about rfcomm, which is (rather obviously) the tool used to setup and maintain RFCOMM links under BlueZ. This tool is used when you want to create a direct link to an RFCOMM channel on the remote device. You might use this to try and pass different commands to a Bluetooth service to see how it reacts, or you might need to legitimately connect to a device over the Serial Port Protocol (SPP). For example, binding a Bluetooth GPS to /dev/rfcomm0 over SPP would look something like:

bash# rfcomm bind rfcomm0 00:0B:0D:6F:88:3E 
bash# cat /dev/rfcomm0
$GPGGA,190505.558,0000.0000,N,00000.0000,E,0,00,,0.0,M,0.0,M,,0000*43

Recommended Software

The following are a few tools that anyone interested in Bluetooth hacking should take a look at. This list is by no means exhaustive, but should give you some ideas as to what is possible. To make things a little easier, I made sure that all of these tools can be found on the aforementioned BackTrack 3 Linux live CD.

carwhisperer

If you are looking for a quick way to scare your friends, this would be it. carwhisperer is an absolutely brilliant piece of software that exploits a design flaw in many Bluetooth headsets: essentially, if the phone the headset is paired with is not in range or otherwise unavailable, the headset goes back into discoverable mode. carwhisperer scans for any headsets that are in discoverable mode, connects to them by using the included list of common headset PINs, and then makes the headset believe the "phone" has received a call. The end result? Your victim is now unwittingly wearing a bug strapped to the side of his head.

Considering the number of people who walk around with a Bluetooth headset in their ear all day, this is a staggering security issue. Coupled with a high-gain directional antenna, an attacker could use this software to listen in on a meeting taking place in the office across the street; or just record all the audio from all the headsets picked up in a coffee shop or other public place to be analyzed for personal information at their leisure. If you show this to your friends and they are not at least partially concerned, get new friends.

Bluetooth Stack Smasher (BSS)

BSS is a tool to send malformed L2CAP packets to a given MAC, which can do anything from completely crashing the target to simply impairing it's ability to communicate. In my research, I found that BSS could remotely reboot a number of older phones within 5 seconds of launching a random attack (BSS cycles through it's list of fuzzed packets, which causes the most possible confusion in the least amount of time) on them, and most headsets I tested it against would either disconnect from the host phone or simply restart themselves.

btscanner

As the name suggests, this is a tool to continuously scan for nearby devices and extract as much information as possible from them. Technically, btscanner doesn't do anything you couldn't already do with hcitool (in fact, it's heavily based on hcitool), but the simple fact that it compresses the output from multiple commands into a clean Kismet-inspired ncurses UI is enough to win over most users.

BT Audit

This suite of tools contains rfcomm_scan and psm_scan, which are port scanners for RFCOMM and L2CAP, respectively. These scanners allow you to see which ports are open on the target device, which can help in finding services that are not advertised via SDP records.

rfcomm_shell

This is a simple tool that lets you bind an interactive shell to an RFCOMM channel on the remote device. This can be used to pass arbitrary data to a listening service, which could be used for things like passing AT commands to a phone or causing a buffer overflow.

Real World Implications

Phone Screen As an experiment, next time you are out in a public place like the mall or a restaurant, pull out your phone and have it search for nearby devices. You will almost certainly pull up a few devices that have been left in discoverable mode, most of them still running the default device name. From there you could try to find a device-specific exploit, but more likely you could just use the ignorance of the user to gain access.

Imagine if you changed the device name of your Bluetooth adapter to "Facebook friend, enter 1234 to", and then attempted to pair with the target phone. Most phones will prompt the user about new Bluetooth connections with a line like:

       "Connection from DEVICE_NAME. Allow?"

Which, when combined with your new device name, would look something like:

       "Connection from Facebook friend, enter 1234 to. Allow?"

Admittedly, this isn't exactly the King's English; but in the modern "click first and ask questions later" world of shakily financed Nigerian princes, poor grammar alone is unlikely to set off any mental alarms in the average person's head. Given Facebook's exploding application library and the questionable mental capacity of many social networking denizens, a message like this could fool a decent amount of the targeted users. This particular attack can be even more effective if used contextually. For example, imagine if you were at a concert and advertised yourself as having free ringtones for the band currently on stage.

Another possible threat that doesn't get nearly the attention it deserves is tracking and identification. There is a huge fear of RFID being used to track a person's location without their knowledge or consent, to the point that people are now buying shielded wallets to prevent an attacker from sniffing any RFID chips that may be present in their ID cards. I have always found it rather ironic that a good deal of these people are likely carrying an active transmitter (which just happens to contain a wealth of personal information) in the pocket opposite their shielded wallet. In fact, there is a budding industry (especially overseas) for Bluetooth proximity marketing, which is a technology that sends unsolicited advertisements to any Bluetooth device that comes into radio range. The technical differences between pushing out a ringtone you didn't ask for and logging your device's unique MAC along with the current time and geographical location of the transmitter is very slight, and indeed could both be happening at the exact same time.

Conclusion

With so many Bluetooth devices in consumer's hands, and the increasing use of mobile devices for personal and financial data management, the incentive is certainly there for attackers to look into new ways to exploit the Bluetooth protocol. It is also worth mentioning that devices running the new Bluetooth 3.0 protocol are slated for production soon, and as we all know, first run devices using new technologies are very likely to include a poor implementation at no extra charge; especially considering that the new specifications involve routing data over WiFi for increased range and speed.

Vendor implementations of the current protocol are improving, but are still not perfect. While many new devices default to non-discoverable mode, a lot still offer the option to leave the device permanently discoverable instead of using a time-limited discoverable mode. This means that if a user wants to put his device into discoverable mode to legitimately connect with his friend's device, he will remain discoverable if he forgets to turn it back off (or just doesn't know any better). Newer smartphones like the Blackberry allow the user to specify which Bluetooth services they wish to advertise, but this excellent feature doesn't seem to be making it's way into many other devices.

On the other hand, if used properly, Bluetooth is an incredibly useful technology for hackers and consumers alike. For example, I have scripts on my machine that backup system configuration and personal documents to my phone every night. Another script downloads the latest "Off the Hook" MP3 and pushes it into my phone's media player application. I've been tinkering with a setup that sends my wife's phone an SMS alert if her laptop detects that the phone isn't within a certain proximity of her desk past a set time of day so she remembers to put it on charge.

The possibilities for a low power, low cost, and widely available wireless communication technology are nearly endless with a little imagination and a bit of hacking. All you have to do is get out of the pervasive mindset that Bluetooth is solely capable of connecting a wireless headset to a mobile phone, and hopefully reading this has gotten a few people a bit closer to that realization.

Thanks:
Special thanks to all those who have donated their old Bluetooth-capable phones and other hardware to me for research.