Hear no evil: Ultrasound attacks on voice assistants

How your voice assistant could do the bidding of a hacker – without you ever hearing a thing

Regular WeLiveSecurity readers won’t be stunned to read that cyberattacks and their methods keep evolving as bad actors continue to enhance their repertoire. It’s also become a common refrain that as security vulnerabilities are found and patched (alas, sometimes after being exploited), malicious actors find new chinks in the software armor.

Sometimes, however, it is not “just” a(nother) security loophole that makes the headlines, but a new form of attack. This was also the case recently with a rather unconventional attack method dubbed NUIT. The good news? NUIT was unearthed by academics and there are no reports of anybody exploiting it for pranks or outright cybercrime. That said, it doesn’t hurt to be aware of another way your privacy and security could be at risk – as well as about the fact that NUIT can actually come in two forms.

How NUIT saw the light of day

NUIT, or Near-Ultrasound Inaudible Trojan, is a class of attack that could be deployed to launch silent and remote takeovers of devices that use or are powered by voice assistants such as Siri, Google Assistant, Cortana, and Amazon Alexa. As a result, any device accepting voice commands – think your phone, tablet or smart speaker – could be open season. Ultimately, the attack could have some dire consequences, ranging from a breach of privacy and loss of trust to even the compromise of a company’s infrastructure, which could, in turn, result in hefty monetary losses. 

Described by a team of researchers at the University of Texas in San Antonio (UTSA) and the University of Colorado Colorado Springs (UCCS), NUIT is possible because microphones in digital assistants can respond to near-ultrasound waves played from a speaker. While inaudible to you, this sound command would prompt the always-on voice assistant to perform an action – let’s say, turn off an alarm, or open the front door secured by a smart lock. 

To be sure, NUIT isn’t the first acoustic attack to have made waves over the years. Previously, attacks with similarly intriguing names have been described – think SurfingAttackDolphinAttackLipRead and SlickLogin, including some other inaudible attacks that that, too, targeted smart-home assistants.

Night, night

As mentioned, NUIT comes in two forms: They are:

  • NUIT 1 – This is when the device is both a source and the target of an attack. In such cases, all it takes is a user playing an audio file on their phone that causes the device to perform an action, like sending a text message with its location.

VIDEO: NUIT 1 End to End no response Attack

  • NUIT 2 – This attack is launched by a device with a speaker to another device with a microphone, like from your PC to a smart speaker.


As an example, let’s say you are watching a webinar on Teams or Zoom. A user could unmute themselves and play a sound, which would then be picked up by your phone, prompting it to visit a dangerous website and compromising the device with malware.

Alternatively, you could be playing YouTube videos on your phone with your loudspeakers, and the phone would then perform an unwarranted action. From the user’s perspective, this attack does not require any specific interaction, which makes it all the worse.

Should NUIT keep you up at night?

What does it take to perform such an attack? Not much, as for NUIT to work, the speaker from which it is launched needs to be set to above a certain level of volume, with the command lasting less than a second (0.77s).

Moreover, obviously you need to have your voice assistant enabled. According to the researchers, out of the 17 devices tested, only Apple Siri-enabled devices were harder to crack. This was because a hacker would need to steal your unique voice fingerprint first to get the phone to accept commands.

Which is why everyone should set up their assistants to only work with their own voice. Alternatively, consider switching your voice assistant off when it’s not needed; indeed, keep your cyber-wits about you when using any IoT devices, as all sorts of smart gizmos can be easy prey for cybercriminals.

The doctor’s orders

The researchers, who will also present their NUIT research at the 32nd USENIX Security Symposium, also recommend that users scan their devices for random microphone activations. Both Android and iOS devices display microphone activation, usually with a green dot on Android, and with a brown dot on iOS in the upper part of the screen. In this case, also consider reviewing your app permissions for microphone access, as not every app needs to hear your surroundings.

Likewise, listen to audio using earphones or headsets, as that way, you are less likely to share sound with your surroundings, protecting against an attack of this nature.

This is also a good time to make sure you have the cybersecurity basics covered –  keep all your devices and software updated, enable two-factor authentication on all of your online accounts, and use reputable security software across all your devices. 


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