Why is large-scale location-trace data (as studied in "Big broker is tracking you!") considered among the most sensitive personal data?
Location traces reveal daily routines, relationships, health, and political affiliations — and data brokers collect them at a scale that enables deep inference far beyond what users consent to.
The research by De Boeck, Verdonck, Willocx & Naessens ("Big broker is tracking you!", ACSAC '25) underscores how the large-scale collection of location traces by data brokers allows inference of highly sensitive information about individuals — well beyond what people expect from seemingly benign apps.
Why location is so revealing:
- It's near-unique. A handful of timestamped locations uniquely identifies almost anyone — location is effectively a biometric.
- It exposes the sensitive by association. Visits to clinics, places of worship, protests, or a particular home reveal health, religion, politics, and relationships without any explicit "sensitive" field.
- Brokers aggregate it. Many apps quietly ship coordinates to brokers, who pool everyone into one dataset — multiplying both inference power and breach impact (cf. the Gravy Analytics case).
Tip: You can refuse to state your religion or politics, but your movements state them for you — which is exactly why location data resists anonymization.
Go deeper:
Big broker is tracking you! (ACSAC '25) — the cited DistriNet/KU Leuven study.- Unique in the Crowd (de Montjoye, 2013) — four points uniquely identify ~95% of people.