hacking voice assistants using ultrasonic waves
Researchers have discovered a new means to target voice-controlled devices by propagating ultrasonic waves through solid materials in order to interact with and compromise them using inaudible voice commands without the victims' knowledge.

Called "SurfingAttack," the attack leverages the unique properties of acoustic transmission in solid materials — such as tables — to "enable multiple rounds of interactions between the voice-controlled device and the attacker over a longer distance and without the need to be in line-of-sight."

In doing so, it's possible for an attacker to interact with the devices using the voice assistants, hijack SMS two-factor authentication codes, and even place fraudulent calls, the researchers outlined in the paper, thus controlling the victim device inconspicuously.

The research was published by a group of academics from Michigan State University, Washington University in St. Louis, Chinese Academy of Sciences, and the University of Nebraska-Lincoin.

The results were presented at the Network Distributed System Security Symposium (NDSS) on February 24 in San Diego.

How Does the SurfingAttack Work?

MEMS microphones, which are a standard in most voice assistant controlled devices, contain a small, built-in plate called the diaphragm, which when hit with sound or light waves, translates the input into an electrical signal that is then decoded into the actual commands.

The novel attack exploits the nonlinear nature of MEMS microphone circuits to transmit malicious ultrasonic signals — high-frequency sound waves that are inaudible to the human ear — using a $5 piezoelectric transducer that's attached to a table surface. What's more, the attacks can be executed from as far as 30 feet.

To conceal the attack from the victim, the researchers then issued a guided ultrasonic wave to adjust the volume of the device low enough to make the voice responses unnoticeable, while still be able to record the voice responses from the assistant via a hidden tapping device closer to the victim's device underneath the table.

Once set up, an interloper can not only activate the voice assistants (e.g., using "OK Google" or "Hey Siri" as wake words), but also generate attack commands (e.g. "read my messages," or "call Sam with speakerphone") using text-to-speech (TTS) systems — all of which are transmitted in the form of ultrasonic guided wave signals that can propagate along the table to control the devices.

SurfingAttack was tested with a variety of devices that use voice assistants, such as the Google Pixel, Apple iPhone, Samsung Galaxy S9, and Xiaomi Mi 8, and each of them were found to be vulnerable to ultrasonic wave attacks. It was also found to work despite using different table surfaces (e.g., metal, glass, wood) and phone configurations.

The experiments, however, come with two failure cases, including Huawei Mate 9 and Samsung Galaxy Note 10+, the former of which becomes vulnerable upon installing LineageOS. Observing that the recorded sounds of the ultrasound commands from Galaxy Note 10+ were very weak, the researchers attributed the failure "to the structures and materials of the phone body."

In what's a big consolation, smart speakers from Amazon and Google — Amazon Echo and Google Home — were not found to be impacted by this attack.

Voice-based Attacks on the Rise

While there are no indications so far that it has been maliciously exploited in the wild, this is not the first time injection attacks of this kind have been uncovered.

Indeed, the research builds upon a recent string of studies — BackDoor, LipRead, and DolphinAttack — that shows it's possible to exploit the nonlinearity in microphones to deliver inaudible commands to the system via ultrasound signals.

Furthermore, a study by researchers from Tokyo-based University of Electro-Communications and the University of Michigan found late-year a series of attacks — called Light Commands — that employed lasers to inject inaudible commands into smartphones and speakers, and surreptitiously cause them to unlock doors, shop on e-commerce websites, and even start vehicles.

While this attack required the laser beam to be in direct line of sight to the target device in question, SurfingAttack's unique propagation capabilities eliminate this need, thereby allowing a potential attacker to remotely interact with a voice-activated device and execute unauthorized commands to access sensitive information without the victim's knowledge.

If anything, the latest research presents a new attack vector that would require device makers to erect new security defenses and safeguard devices from voice-based attacks that are increasingly becoming an entry point for everything smart home.

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