In a blog post titled "A step in the wrong direction", senior privacy researcher Peter Snyder and chief executive Brendan Eich said FLoC, as it is known, would make any browser that implemented support share a user's browsing behaviour and interests with every site and advertiser on sites which were visited.
"Brave opposes FLoC, along with any other feature designed to share information about you and your interests without your fully informed consent," the duo said.
"To protect Brave users, Brave has removed FLoC in the Nightly version of both Brave for desktop and Android. These changes will reach all Brave users in the next stable release this week. Brave is also disabling FLoC on our websites, to protect Chrome users learning about Brave."
A blog post by EFF staff technologist Bennett Cyphers said the only way people could opt out at the moment was by disabling third-party cookies. "We’ve been told that the trial is currently deployed to 0.5% of Chrome users in some regions – for now, that means Australia, Brazil, Canada, India, Indonesia, Japan, Mexico, New Zealand, the Philippines, and the US," he wrote.
Snyder and Eich said companies were finally beginning to respect user privacy, due to increased user education, the success of privacy-first tools and laws, including the CCPA [California Consumer Privacy Act] and the GDPR [General Data Protection Regulation].
"In the face of these trends, it is disappointing to see Google, instead of taking the present opportunity to help design and build a user-first, privacy-first Web, proposing and immediately shipping in Chrome a set of smaller, ad-tech-conserving changes, which explicitly prioritise maintaining the structure of the Web advertising ecosystem as Google sees it," the two said.
"For the Web to be trusted and to flourish, we hold that much more is needed than the complex, yet conservative, chair-shuffling embodied by FLoC and Privacy Sandbox.
"Deeper changes to how creators pay their bills via ads are not only possible, but necessary. The success of Brave’s privacy-respecting, performance-maintaining, and site-supporting advertising system shows that more radical approaches work. We invite Google to join us in fixing the fundamentals, undoing the harm that ad-tech has caused, and building a Web that serves users first."
They detailed three aspects of FLoC that they claimed were particularly harmful and concerning.
First, they said, FLoC informed sites about a user's browsing history. "FLoC shares information about your browsing behaviour with sites and advertisers that otherwise wouldn’t have access to that information. Unambiguously, FLoC tells sites about your browsing history in a new way that browsers categorically do not today."
Eich and Snyder knocked back Google's claims that FLoC was preserving privacy in comparison to third-party cookies and because so-called interest cohorts were designed not to be unique to a user.
"Many browsers don’t send third-party cookies at all; Brave hasn’t ever," the two said. "Saying a new Chrome feature is privacy-improving only when compared to status-quo Chrome (the most privacy-harming popular browser on the market), is misleading, self-serving, and a further reason for users to run away from Chrome.
"Many things about a person are i) not unique, but still ii) personal and important, and shouldn’t be shared without consent," they said about the second Google claim.
"Whether I prefer to wear 'men’s' or 'women’s' clothes, whether I live according to my professed religion, whether I believe vaccines are a scam, or whether I am a gun owner, or a Brony-fan, or a million other things, are all aspects of our lives that we might like to share with some people, but not others, and under our terms and control.
The other two aspects of FLoC which Eich and Snyder cited as being against user interests were that it made tracking users across the Web easier and promoting a false notion of privacy and why it was important.
"Overall, FLoC, along with many other elements of Google’s 'Privacy Sandbox' proposal, are a step backward from more fundamental, privacy-and-user focused changes the Web needs," the two Brave officials said.
"Instead of deep change to enforce real privacy and to eliminate conflicts of interest, Google is proposing Titanic-level deckchair-shuffling that largely maintains the current, harmful, inefficient system the Web has evolved into, a system that has been disastrous for the Web, users and publishers."