reposted from Schneier on Security.
Ad networks using ultrasonic communications to jump from one device to another. The idea is for devices like televisions to play ultrasonic codes in advertisements and for nearby smartphones to detect them. This way the two devices can be linked.
Creepy, yes. And also increasingly common, as this research demonstrates: http://christian.wressnegger.info/content/projects/sidechannels/2017-eurosp.pdf
Privacy Threats through Ultrasonic Side Channels on Mobile Devices
by Daniel Arp, Erwin Quiring, Christian Wressnegger and Konrad Rieck
Abstract: Device tracking is a serious threat to the privacy of users, as it enables spying on their habits and activities. A recent practice embeds ultrasonic beacons in audio and tracks them using the microphone of mobile devices. This side channel allows an adversary to identify a user's current location, spy on her TV viewing habits or link together her different mobile devices. In this paper, we explore the capabilities, the current prevalence and technical limitations of this new tracking technique based on three commercial tracking solutions. To this end, we develop detection approaches for ultrasonic beacons and Android applications capable of processing these. Our findings confirm our privacy concerns: We spot ultrasonic beacons in various web media content and detect signals in 4 of 35 stores in two European cities that are used for location tracking. While we do not find ultrasonic beacons in TV streams from 7 countries, we spot 234 Android applications that are constantly listening for ultrasonic beacons in the background without the user's knowledge.