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Privacy and Security Aspects of the Ultrasound Ecosystem

Gimme some context!

Nowadays users often possess a variety of electronic devices for communication and entertainment. In particular, smartphones are playing an increasingly central role in users' lives: Users carry them everywhere they go and often use them to control other devices. This trend provides incentives for the industry to tackle new challenges, such as cross-device authentication, and to develop new monetization schemes. A new technology based on ultrasounds has recently emerged to meet these demands. Ultrasound technology has a number of desirable features: it is easy to deploy, flexible, and inaudible by humans. This technology is already utilized in a number of different real-world applications, such as device pairing, proximity detection, and cross-device tracking.

What's the problem?

For the first time, we examine the different facets of ultrasound-based technology. Initially, we discuss how it is already used in the real world, and subsequently examine this emerging technology from the privacy and security perspectives. In particular, we first observe that the lack of OS features results in violations of the principle of least privilege: an app that wants to use this technology currently needs to require full access to the device microphone. We then analyse real-world Android apps and find that tracking techniques based on ultrasounds suffer from a number of vulnerabilities and are susceptible to various attacks. For example, we show that ultrasound cross-device tracking deployments can be abused to perform stealthy deanonymization attacks (e.g., to unmask users who browse the Internet through anonymity networks such as Tor), to inject fake or spoofed audio beacons, and to leak a user's private information.

Where do we go from here?

Based on our findings, we introduce several defense mechanisms. We first propose and implement immediately deployable defenses that empower practitioners, researchers, and everyday users to protect their privacy. In particular, we introduce a browser extension and an Android permission that enable the user to selectively suppress frequencies falling within the ultrasonic spectrum. We then argue for the standardization of ultrasound beacons, and we envision a flexible OS-level API that addresses both the effortless deployment of ultrasound-enabled applications, and the prevention of existing privacy and security problems.

Frequently Asked Questions

Am I affected?
Likely not, unless you installed an Android app that uses an ultrasound-based framework and requests access to your microphone.
Aren't ultrasounds bad for my health?
We're not experts in this matter. Please refer to proper resources.
How widespread is all this?
We haven't performed large-scale measurements, although some of the apps that embed ultrasound-based frameworks were downloaded by hundreds of thousands users, according to the metadata published on the Google Play Store.
Can this be fixed?
Yes, but it'll take a long, long time. This is not a software vulnerability that can be fixed by applying a simple patch. Although we have created a proof-of-concept patch for the Android Open Source Project (AOSP) and a "personal firewall" to prevent your browser's Web API to emit ultrasounds, a holistic action is needed. Decision and policy makers should agree on what's the next step in terms of regulations and standardization, OS vendors and developers should integrate support for ultrasound beacons to provide a transparent API (e.g., like for other physical and data layers such as Bluetooth), and finally developers should adopt such API.
Is every mobile operating system capable of capturing uBeacons, or just Android?
It depends more on the hardware of the device (i.e., the microphone) and less on the operating system. The great majority of commercial microphones found in mobile phone can capture uBeacons. Nevertheless, the operating system plays a role as it determines what an application can and can't do. For our research, we worked with Android and we can confirm that it is possible to listen for ultrasounds in the background. We haven't checked iOS, but we cannot exclude either possibilities.
Do you have information on the actual frequencies these beacons operate on?
We have seen frameworks listening for beacons starting from 18.000Hz and higher. For instance, a relevant patent can be found here. However, the exact implementation of ultrasound beacons varies between companies.
What is the situation now?
The awesome people at City Frequencies maintain an up to date page on the current status of the ecosystem, as well as present some additional findings: Here!

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Who we are

Vasilios Mavroudis, @mavroudisv
PhD student, University College London (UCL)
Shuang Hao
Postdoc researcher, University of California, Santa Barbara (UCSB)
Yanick Fratantonio, @reyammer
PhD student, University of California, Santa Barbara (UCSB)
Federico Maggi, @phretor
Professor, Politecnico di Milano (POLIMI)
Visiting Researcher at Univeristy of California, Santa Barbara (UCSB)
Giovanni Vigna
Professor, University of California, Santa Barbara (UCSB)
Christopher Kruegel,
Professor, University of California, Santa Barbara (UCSB)

Publications and Talks

Vasilios Mavroudis, Shuang Hao, Yanick Fratantonio, Federico Maggi, Giovanni Vigna, and Christopher Kruegel. The Good, the Bad and the Ugly of the Ultrasonic Communications Ecosystem. RSA Conference 2018, San Fransisco US, 17 April 2018. [Video]

Vasilios Mavroudis, Shuang Hao, Yanick Fratantonio, Federico Maggi, Giovanni Vigna, and Christopher Kruegel. Talking Behind Your Back: On the Security of the Ultrasound Tracking Ecosystem. Chaos Communication Congress, Hamburg, Germany, 27-30 December 2016. [Video (En)] [Video (De)]

Vasilios Mavroudis, Shuang Hao, Yanick Fratantonio, Federico Maggi, Giovanni Vigna, and Christopher Kruegel. The Ultrasound Tracking Ecosystem. Report. November 2016. [PDF]

Vasilios Mavroudis, Shuang Hao, Yanick Fratantonio, Federico Maggi, Giovanni Vigna, and Christopher Kruegel. Talking Behind Your Back: Attacks and Countermeasures of Ultrasonic Cross-device Tracking. Blackhat Europe, London, UK, 3–4 November 2016. [Slides]

Vasilios Mavroudis, Shuang Hao, Yanick Fratantonio, Federico Maggi, Giovanni Vigna, and Christopher Kruegel. On the Privacy and Security of the Ultrasound Ecosystem. 17th Privacy Enhancing Technologies Symposium, Minneapolis, MN, July 2017. [PDF]

Please use the following bibtex entry to cite our work:

  title={On the Privacy and Security of the Ultrasound Ecosystem},
  author={Mavroudis, Vasilios and Hao, Shuang and Fratantonio, Yanick and Maggi, Federico and Kruegel, Christopher and Vigna, Giovanni},
  journal={Proceedings on Privacy Enhancing Technologies},


This section includes our proof-of-concept countermeasures (all released under the Apache 2.0 license):

  • SilverDog: your sound firewall! A chrome extension that we developed to filter ultrasound frequencies. [Source Code]
  • Set of AOSP patches to implement a new permission to filter ultrasound spectrum. The patch should apply cleanly against AOSP android-5.0.0_r3. Note: it is just a research prototype! [Download]

Feedback, ideas and source code contributions are very welcome!

Press Coverage

Articles covering our work (in reverse chronological order, non-exhaustive):

Want to write about this research? The best starting point is our report, as it provides a detailed but easy-to-understand explanation of the essential points.