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Today, Google launched an “origin trial” of Federated Learning of Cohorts (aka FLoC), its experimental new technology for targeting ads. A switch has silently been flipped in millions of instances of Google Chrome: those browsers will begin sorting their users into groups based on behavior, then sharing group labels with third-party trackers and advertisers around the web. A random set of users have been selected for the trial, and they can currently only opt-out by disabling third-party cookies. Although Google announced this was coming, the company has been sparse with details about the trial until now. We’ve pored over blog posts, mailing lists, draft web standards, and Chromium’s source code to figure out exactly what’s going on. EFF has already written that FLoC is a terrible idea.  Google’s launch of this trial—without notice to the individuals who will be part of the test, much less their consent—is a concrete breach of user trust in the service of a technology that should not exist. Below we describe how this trial will work, and some of the most important technical details we’ve learned so far. FLoC is supposed to replace cookies. In the trial, it will supplement them. Google designed FLoC to help advertisers target ads once third-party cookies go away. During the trial, trackers will be able to collect FLoC IDs in addition to third-party cookies. That means all the trackers who currently monitor your behavior across a fraction of the web using cookies will now receive your FLoC cohort ID as well. The cohort ID is a direct reflection of your behavior across the web. This could supplement the behavioral profiles that many trackers already maintain. As described above, a random portion of Chrome users will be enrolled in the trial without notice, much less consent. Those users will not be asked to opt-in. In the current version of Chrome, users can only opt-out of the trial by turning off all third-party cookies. Future versions of Chrome will add dedicated controls for Google’s “privacy sandbox,” including FLoC. But it’s not clear when these settings will go live, and in the meantime, users wishing to turn off FLoC must turn off third-party cookies as well. Turning off third-party cookies is not a bad idea in general. After all, cookies are at the heart of the privacy problems that Google says it wants to address. But turning them off altogether is a crude countermeasure, and it breaks many conveniences (like single sign-on) that web users rely on. Many privacy-conscious users of Chrome employ more targeted tools, including extensions like Privacy Badger, to prevent cookie-based tracking. Unfortunately, Chrome extensions cannot yet control whether a user exposes a FLoC ID. FLoC calculates a label based on your browsing history. For the trial, Google will default to using every website that serves ads — which is the majority of sites on the web. Sites can opt-out of being included in FLoC calculations by sending an HTTP header, but some hosting providers don’t give their customers direct control of headers. Many site owners may not be aware of the trial at all. This is an issue because it means that sites lose some control over how their visitors’ data is processed. Right now, a site administrator has to make a conscious decision to include code from an advertiser on their page. Sites can, at least in theory, choose to partner with advertisers based on their privacy policies. But now, information about a user’s visit to that site will be wrapped up in their FLoC ID, which will be made widely available (more on that in the next section). Even if a website has a strong privacy policy and relationships with responsible advertisers, a visit there may affect how trackers see you in other contexts. For complete details visit OUR FORUM.