Evaluating Google's Protected Audience Protocol

While third-party cookies have been a key component of the digital marketing ecosystem for years, they allow users to be tracked across web sites in ways that raise serious privacy concerns. Google has proposed the Privacy Sandbox initiative to enable ad targeting without third-party cookies. While there have been several studies focused on other aspects of this initiative, there has been little analysis to date as to how well the system achieves the intended goal of preventing request linking. This work focuses on analyzing linkage privacy risks for the reporting mechanisms proposed in the Protected Audience (PrAu) proposal (previously known as FLEDGE), which is intended to enable online remarketing without using third-party cookies. We summarize the overall workflow of PrAu and highlight potential privacy risks associated with its proposed design, focusing on scenarios in which adversaries attempt to link requests to different sites to the same user. We show how a realistic adversary would be still able to use the privacy-protected reporting mechanisms to link user requests and conduct mass surveillance, even with correct implementations of all the currently proposed privacy mechanisms.