  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.
