Beyond the Cookie Jar: Uncovering Privacy Flaws in Google’s New Ad Tech
The book From Day Zero to Zero Day, published by No Starch Press, has been released, offering a comprehensive introduction to the fundamentals of vulnerability research. The author presented it at DEF CON 33 and the Crypto & Privacy Village conference, where he highlighted flaws in the implementation of Google’s Privacy Sandbox—a project intended to safeguard user data, yet one that in certain cases inadvertently enables de-anonymization.
The Privacy Sandbox is Google’s initiative to replace third-party cookies while preserving essential advertising functions such as conversion tracking and audience segmentation, all without exposing users’ personal information. To achieve this, Chrome incorporates a suite of new APIs. However, the research revealed that errors in the implementation of these interfaces can not only nullify the promised privacy protections but also grant adversaries the ability to harvest user data.
One striking example involved a vulnerability in the Attribution Reporting API, a mechanism designed to determine whether a specific advertisement led to a purchase or another action. Built-in safeguards—such as randomized noise and limits on report frequency—were meant to obscure the behavior of individuals. In practice, however, loopholes emerged: from bypassing Referrer Policy via debug reports, to exploiting “dead” domains for fabricated attributions, and even reconstructing a user’s browsing history.
Another weakness was identified in the Shared Storage API, a tool intended to store cross-site data. Conceived as a means for ad networks to deliver relevant advertising without directly exposing a user’s interests, it nevertheless proved flawed. Due to configuration errors, researchers were able to bypass restrictions and extract values that should have remained hidden.
The author stresses that the technologies underpinning the Privacy Sandbox are intricate and far from perfected. As advertising platforms continue to experiment with their deployment, the risk persists that these very mechanisms may evolve into new vectors for privacy attacks. Researchers are convinced that these APIs require stricter auditing and refinement; without it, they will fail to deliver on their central promise—to make advertising both safe and truly private.
