EFF's Privacy Badger blocked trackers by behavior, so no blocklist went stale

By 2015, Do Not Track was a request that most of the ad industry ignored, while existing blockers relied on human-curated lists that needed maintenance and made deals about which ads to allow. EFF wanted one extension it could recommend without reservation: something that automatically blocked any tracker or ad violating the principle of user consent, worked with no settings or knowledge on the user's part, used algorithmic methods to decide what was tracking, and came from an organization unambiguously working for users rather than for profit.

Privacy Badger, launched in August 2015, is that answer. It defines what tracking looks like and then blocks or restricts the domains it observes tracking in the wild, so what counts as a tracker is based entirely on how a specific domain acts rather than on human judgment. If the same third-party host is seen setting unique identifiers on three separate sites, Privacy Badger tells the browser to stop loading content from that source — and a source that cannot load can no longer track.

The design deliberately blunts the usual objections. Privacy Badger is not an ad blocker: nothing in its code targets ads, and it only blocks the ones that happen to track, because one of its stated goals is to incentivize advertisers to adopt better privacy practices. Trackers can comply by honoring EFF's Do Not Track policy, and domains that appear necessary for site functionality sit on a yellowlist where their cookies and referrers are screened instead of the whole domain being cut off.

The tool also goes beyond cookies: it detects canvas fingerprinting, the tracking technique ProPublica documented as virtually impossible to block, and replaces social media widgets with click-to-activate placeholders so a Like button cannot watch a reader who never clicks it. The launch discussion on Hacker News drew 322 points and 109 comments on August 6, 2015, where an EFF developer noted that behavioral detection lets Privacy Badger catch new trackers faster than list-based blockers.

Stop curating blocklists: define tracking as repeated cross-site behavior and block whichever domain does it

No curator and no conflict of interest: decisions follow observed behavior, so the tool needs no blocklist vendor, no acceptable-ads deal and no manual updates from the user

It enforces consent instead of asking for it: the extension first sends DNT and GPC signals, then acts only after a domain ignores them, making the block a consequence of the tracker's own conduct

The three-site rule is a simple, auditable definition of tracking that catches a new tracker before any hand-maintained list knows it exists

Compliance is rewarded: a tracker can escape blocking by publishing EFF's DNT policy, turning the tool into an incentive for better behavior rather than a blanket ad ban

A consent signal without enforcement is a wish: put the rule in the client, judge domains by observed behavior, and make compliance the cheapest route to staying unblocked.

Privacy Badger remains EFF's maintained extension, offered for Chrome, Firefox, Firefox for Android, Edge, Opera and Brave; it sends Global Privacy Control by default alongside Do Not Track, blocks canvas fingerprinting, and replaces social widgets with click-to-activate placeholders. Each installation still learns locally, now augmented by periodic Badger Sett training data drawn from thousands of popular sites. The August 2015 launch drew 322 points and 109 comments on Hacker News, where EFF's developer noted algorithmic detection finds trackers faster than list-based tools.