Laundry Laundering: A Security Analysis of Laundromat Washing Machines at the University of Michigan

Transcription

1 Laundry Laundering: A Security Analysis of Laundromat Washing Machines at the University of Michigan Eric Wustrow, Jahan Khanna University of Michigan {ewust,jahan}@umich.edu Abstract Coin-operated laundromats are commonplace across college campuses and cities alike, providing students or the community with accessible laundry at any time. In recent years, washing and drying machines have been fitted to allow for more modern forms of payments, including credit cards, prepaid charge cards, and in some cases, student ID cards linked to university accounts. In this paper, we analyze the security of one such system currently in use at the University of Michigan, and present several attacks to which it is vulnerable. Using the attacks described in this paper, we show a sufficiently knowledgeable attacker is able to obtain free laundry in an untraceable way, using a simple device whose parts are both widely and inexpensively available. Furthermore, we demonstrate that this device could be designed to covertly fit inside an attacker's pocket, with the attacker needing only 15 seconds with the machine to carry out the attack. Finally, we present ways to defend against our attacks, and further secure the system against such simple yet effective attacks. 1 Introduction Historically, Laundry Machines have not been the typical ground for computer-based attacks. The entire system has typically been enclosed inside a one-piece box that is relatively inaccessible to attackers, and provided that false coins can be effectively detected and weeded out, there was not much else for an attacker to do. However, as new devices are integrated into laundry centers for the sake of customer convenience, the attack surface increases. Specifically, we choose to analyze the security of the nearest Laundromat to our department building, one of the Northwood University of Michigan graduate student housing Laundromats. puts. The machine can respond to either a coin being placed into the coin receptacle or a signal from an external credit card reader. In our case, the University has taken advantage of the external connector, and placed a centralized card reader at the back of each Laundromat facility (see Fig 2). This allows students to swipe their student ID card and have the fare deducted from their student account, rather than requiring everyone to pay in quarters. The card reader is physically connected to each washer/dryer via a port in the back. This connection is where we choose to concentrate our attack efforts. 2.1 Card Reader The rest of this paper will be as follows: Section 2 describes an overview of the laundromat we analyzed; Section 3 describes our reverse engineering of the protocol used; Section 4 describes our implementation of an attack using the reversed protocol; Section 5 discusses some possible defenses. Finally, Section 6 provides a conclusion, and recommendations for the University of Michigan. 2 Laundromat Overview The Laundromat we analyzed consists of 15 Washers and 10 Dryers, all commercial machines manufactured by Maytag (Fig 1). From the substantial information provided by a quick search of Maytag's website [1], we can see that all the parts inside the machine are fairly primitive, and the machine only has two monetary in- At the rear of the laundromat facility, two Diebold card readers are mounted on a wooden box. Students use these card readers to swipe their Student Identification cards (known at Michigan as MCards ). The student then selects a washer or dryer number on the keypad on the card reader. The card reader then checks to see if the machine is in use, and if not, deducts the credit due from the student's corresponding university account. The card reader then sends a series of pulses over the connecting cable to the appropriate machine. At each of these half-second separated pulses, the laundry machine's credit-needed amount decrements by $0.25. Once the credit-needed display on the laundry machine reaches $0.00, the machine allows the student to select cycle options and start their load of laundry.

2 Figure 1: Maytag washing machine. Visible on the right is coin-slot mechanism that accepts quarters. Figure 2: Diebold card reader. Visible on the right side of the device is the card swipe slot, and in the center the keypad. 2.2 Connecting Cable Each laundry machine is connected to the card reader individually through a bundle of 4 wires. The connection is made to the back of the laundry machine with a plastic screw-on connector, shown in Fig 5. On the connector face, is a part number, which we were able to use to locate and buy an exact part match, for $2.91 from an online part distributer. The four pins on the connector are labeled with numbers 1-4, on both the cable side (male pins) and laundry machine side (female sockets). The two connectors have corresponding notches cut, such that it is only possible to connect them when the corresponding pin numbers line up, preventing mis-aligned connectors from connecting and potentially damaging the equipment. Using a voltmeter, we were able to determine the relative voltages between the pins, when disconnected from the laundry machine. Since pin 1 had the lowest potential relative to the other pins, we made the assumption that this pin corresponded to ground (0V). We observed the voltages of pins 2-4 relative to this ground to be 10.5V, 12V, and 5V correspondingly. We also observed that when the cable is unplugged from the machine, the card reader would not allow a swiped card to credit money to the machine, as it reported the machine in use. 3 Reverse Engineering the Protocol Using only the knowledge of the idle voltages from above, we were able to construct a simple circuit on a breadboard that allowed us to watch these signals using a logic analyzer. We observed the wires (excluding ground) while the card reader was sending pulses to the machine. We discovered that the 5V line (pin 4) was Figure 3: Overall view of logic analyzer result. Visible are the four pairs of pulses for each quarter. Here the controller is attempting to credit $1.00 to the machine.

3 Figure 4: Detailed view of a single pulse pair. weakly pulled low by the washing machine when it was available. By pulling this line low ourselves via a 1K ohm resistor, we were able to convince the card reader that our machine was available, allowing us to observe the signals. After capturing this data, we found that the 12V line (pin 3) had not changed at all during the transaction, leading us to believe that this is a power line. We also discovered that the laundry machine would not accept credit over the cable if this line was disconnected. Using the logic analyzer, we found that the only line that changed during the transaction was the 10.5V line (pin 2). This line appeared to be high when idle, and went low in two short pulses per quarter credited over the line. From the above information, we were able to reverse engineer the protocol used on the cable to be a singledirection serial protocol, similar to 12V RS-232, at 1000 baud. Two characters, 0xFF and 0x00 are sent per quarter to the machine, in 549ms intervals. Figures 3 and 4 show two levels of detail of the analyzed signal. 4 Implementation Our goal was to build a device capable of recreating the sequence of pulses above. By definition, this is not a complicated or expensive device to build. The only necessary parts are a microcontroller connected to a high-side switch (which will, if hooked into the data line from the card reader itself and Vdd, automatically regulate the pulses to the acceptable voltage range). The microcontroller then needs to be programmed to set a pin high or low at the given intervals, as specified by the reversed protocol. Our first attempt used an Atmel ATTiny44 processor (see Fig 6) connected to a 12V high side switch (printed onto a PCB board). This device was sound in principle, and additionally, at under 4 in length, would be practically unnoticeable if made as an underground commercial product. However, the design was a failure due to the Atmel s reliance on an internal RC oscillator, which has a temperature dependent frequency, resulting in unacceptable levels of clock drift. Our second attempt fixes these problems by replacing the Atmel microprocessor with a PIC microcontroller. The PIC relies on a ceramic resonator instead of an RC oscillator, which gives us the necessary accuracy to recreate these pulses. We chose to develop this design on a breadboard, as we wanted the ability to swap out components easily. However, our final design could be placed onto the same PCB as in the first attempt, and concealed more easily. Figure 7 shows the final breadboard circuit we used for our attack. The device successfully credits quarters to either the washing machine or the dryer. With this device, which is extremely easy to reproduce, in both hardware and software. We feel this device sufficiently defeats any previously considered security mechanisms, as we can demonstrate with a high-degree of reliability the ability to obtain free laundry. Additionally, this vulnerability can be extended by wiring the connector ports together and then into the device, so one device could credit all of the machines in the laundromat at once. Furthermore, even without our device, an attacker could disable all the units in the

4 Figure 5: Male-pin connector cable, shown removed from the machine. building by preventing the 5V line from being pulled low. Finally, an attacker could simply cause chaos by switching all the connectors to machines they are not supposed to be hooked into. 5 Defenses All attacks on the system are made possible by two compounded vulnerabilities: 1. The connection between the card reader and the 2. laundry machine is not physically secured in any way. The communication protocol between the card reader and the laundry machine provides no security whatsoever. We present two methods for addresses these problems individually. A robust solution would employ a combination of both methods, however either one by itself would serve to disrupt the attack presented in this paper. 5.1 Physical Security Physically securing the laundry machine, and most importantly, the connecting cable is a straightforward way to secure the device. If the connector was difficult to remove, or inaccessible to the attacker, it would be significantly harder to attack the system. Additionally, physically securing the connector would serve as a deterrent to many of the people who would be interested in using our theoretical commercial device to get free laundry. One simple idea is to place a lockbox around the connector; one that would allow the wire through but would have to be unlocked to remove the connector. This would still allow full functionality of the device, but would prevent physical access to the connector without a key. However, locks can always be Figure 6: First prototype PCB. This device is pocket sized. While this particular device did not generate the correct pulses, it would be trivial to adapt our working prototype onto a form factor of this size. picked, and a sufficiently determined attacker could destroy the lock itself, allowing everyone unlimited access until the lockbox was repaired. Therefore, a more complete security implementation would include some kind of electronic security as well. 5.2 Electronic Security A second way to secure the device is to add security to the transmission itself. Currently, there is no security involved with the transmission at all. The washing machine will accept the correct pulse from anyone at anytime, and what s worse, the signal never changes, so as demonstrated, a simple replay attack can successfully attack the system. In lieu of physical security, a secure transmission protocol could be created on top of the existing protocol. The protocol would need to provide authentication (on both part of the card-reader and the laundry machine), as well as some kind of nonce to prevent a replay attack. Following the model of the SSL private key portion of the handshake [2], we can set up a similar handshake between the laundry machine and the card reader (note that this assumes public/private keys for both parties) When the user swipes his/her card, the card reader generates a nonce, and will encrypt with its private key and send to the laundry machine. The laundry machine will decrypt, mark the nonce as seen, and encrypt a message containing the nonce with its own private key. It will then wait for the decrement message. The card reader will then decrypt the user s response, and if the nonces match, will send an encrypted message with the decrement pulses, and the nonce.

5 Acknowledgements We would like to thank Prof. J. Alex Halderman for allowing us to perform this security analysis as part of the EECS 588 course project. We would also like to acknowledge Prof. Prabal Dutta, for his invaluable advice in circuits while building our device. References Figure 7: Prototype attack device. On the right, is a PIC12F609 microprocessor, and in the center, a 12V high-side switch. We powered this device off a 5V USB supply from a laptop, though it could be modified to take power from the 12V line. 4. The laundry machine can now match the nonce one final time, make sure it is unacceptable for use again (if it matches), and decrement the counter. Of course, this type of security needs fairly powerful hardware. It is not apparent that either the card reader or the laundry machines (which apparently only have enough processing power to decrement the counter a certain amount when a given pulse comes in) have the necessary hardware to support this, so upgrading everything might result in a significant cost. 6 Conclusion Any system that deals with money in any form, generally brings with it a larger incentive for attack. This requires these systems to carefully weigh the cost of implementing security against the potential cost of lost revenue in the event of a successful attack. In the case of the University of Michigan laundry machines, it appears to us that these machines were designed to be mounted in a wall, such that the back of the machine remains inaccessible without physically tearing the machine out of the wall. However, the simple protocol used by the Maytag machines and Diebold card reader was also inadequate to prevent even simple attacks from being widely successful. Our recommendation for the short term is that the University physically secure the back of the laundry devices, as well as the length of cable all the way to the card reader. While these do not prevent a determined attacker, we feel that requiring physical destruction of University property may deter the majority of attackers attempting to gain free laundry. [1] Maytag Appliance and Part Diagrams and Manual ehouse.com/maytag-parts/model _view_cats.cfm?model=mah20pdaww [2] Introduction to SSL, Sun Microsystems /contents.htm