A newly uncovered document confirms what privacy advocates have warned about for years: Customs and Border Protection has been using location data from the internet advertising ecosystem to track phones without warrants. The 404 Media report reveals CBP's pilot program from 2019-2021 relied on data harvested through real-time bidding auctions that power targeted ads.

This surveillance method exploits a fundamental vulnerability in how websites and apps auction ad space. When you visit a site, your device broadcasts detailed information including GPS coordinates, advertising IDs, and browsing data to thousands of potential advertisers in milliseconds. Data brokers harvest this bidstream data at massive scale, with companies like Mobilewalla collecting information on over a billion people.

Beyond CBP, Immigration and Customs Enforcement has purchased location tracking tools like Webloc that can filter millions of phones by geographic area and advertising IDs. The report shows how federal agencies bypass warrant requirements by buying data that would otherwise need court approval. Privacy advocates recommend disabling mobile advertising IDs and reviewing app location permissions as basic protections against this pervasive tracking.