A bidstream is the flow of information exchanged during real-time advertising auctions. It includes bid requests and related auction data, which may contain signals such as device characteristics or approximate location, depending on how the system is configured.
Bidstream data helps advertisers target ads, but repeated or linkable records can also reveal behavioral or geographic patterns and create privacy risks. In attacks or surveillance, an actor may analyze auction data without breaching the ad platform itself. Defenses include minimizing sensitive fields in bid requests, limiting access and retention, and monitoring bidders for activity inconsistent with legitimate advertising.