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234 Android Applications Are Currently Using Ultrasonic Beacons to Track Users

A team of researchers from the Brunswick Technical University in Germany has discovered an alarming number of Android applications that employ ultrasonic tracking beacons to track users and their nearby environment.

Their research paper focused on the technology of ultrasound cross-device tracking (uXDT) that became very popular in the last three years.

uXDT is the practice of advertisers hiding ultrasounds in their ads. When the ad plays on a TV or radio, or some ad code runs on a mobile or computer, it emits ultrasounds that are picked up by the microphone of nearby laptops, desktops, tablets or smartphones.

SDKs embedded in apps installed on those devices relay the beacon back to the online advertiser, who then knows that the user of TV "x" is also the owner of smartphone "Y" and links their two previous advertising profiles together, creating a broader picture of the user's interests, device portfolio, home, and even family members.

Earlier this year, researchers showcased a method of tracking and unmasking Tor users using uXDT ultrasonic beacons.

Location Tracking. An ultrasonic signal also enables an adversary
to track the user’s movement indoor without requiring
GPS. A location, for example a drug store, emits an ultrasonic
signal with a location identifier. This information reveals
where and when an individual usually stays. Furthermore,
the adversary can learn when people are meeting or are in
close proximity to each other.

De-Anonymization. The side channel through ultrasonic
codes makes the de-pseudonymization of Bitcoin and
de-anonymization of Tor users possible. As an example, a
malicious web service can disclose the relation between a
Bitcoin address and a user’s real-world identity. Whenever
the service shows an uniquely generated address to which
the user has to pay, it also transmits an ultrasonic signal to
the payer’s mobile device. This in turn enables the service
to link the user’s Bitcoin address to her mobile device. A
similar attack strategy against Tor users