Cross-device tracking refers to technology which enables tracking of users across multiple devices, such as smartphones, television sets, smart TVs, and personal computers. Tracking users across multiple devices is possible using inaudible sounds (ultrasound) emitted by one device and detected and recognized by the microphone of another device. Distinct inaudible signals are called "audio beacons".
Audio "beacons" can be embedded into television advertisements. In a similar manner to radio beacons, these can be picked up by mobile apps. This allows the behavior of users to be tracked, including which ads were seen by the user and how long they watched an ad before changing the channel.
In March 2016, the FTC issued warning letters to 12 app developers using cross-device tracking in their apps. The FTC warned these developers that they may be violating the FTC Act if they state or imply that their apps are not tracking television viewing habits when they in fact are.
Cross-device tracking has privacy implications and allows for more detailed tracking of users than traditional tracking methods. Data can be collected from multiple devices used by a single user and correlated to form a more accurate picture of the person being tracked. Moreover, malicious actors may use variants of the technology to deanonymize anonymity network users.
- "Your Phone Is Listening—Literally Listening—to Your TV". The Atlantic. Retrieved 2 April 2016.
- "Silverpush Quits Creeping World Out, Ceases Tracking TV Habits Via Inaudible 'Beacons'". Forbes. Retrieved 1 April 2016.
- "Re: Comments for November 2015 Workshop on Cross – Device Tracking" (PDF). Center for Democracy and Technology. Retrieved 1 April 2016.
- "How TV ads silently ping commands to phones: Sneaky SilverPush code reverse-engineered". The Register. Retrieved 1 April 2016.
- "FTC letter to app developers" (PDF). Retrieved 1 April 2016.
- "Beware of ads that use inaudible sound to link your phone, TV, tablet, and PC". Ars Technica. Retrieved 31 March 2016.
- "FTC Issues Warning Letters to App Developers Using 'Silverpush' Code". Retrieved 1 April 2016.
- Vasilios Mavroudis; et al. "On the Privacy and Security of the Ultrasound Ecosystem" (PDF). ubeacsec.org. Proceedings on Privacy Enhancing Technologies. Retrieved 30 November 2017.