A privacy researcher has revealed the evil genius behind a for-profit web analytics service capable of following users across more than 500 sites, even when all cookie storage was disabled and sites were viewed using a browser's privacy mode.

The technique, which worked with sites including Hulu, Spotify and GigaOm, is controversial because it allowed analytics startup KISSmetrics to construct detailed browsing histories even when users went through considerable trouble to prevent tracking of the websites they viewed. It had the ability to resurrect cookies that were deleted, and could also compile a user's browsing history across two or more different browsers. It came to light only after academic researchers published a paper late last month.

KISSmetrics CEO responded with a post on its website claiming the research “significantly distorts our technology and business practices.” The company also responded by adding a “consumer-level opt-out for those who wish to be entirely removed from all KISSmetrics tracking, going well beyond the options that other analytics companies provide.”

Ashkan Soltani, one of the researchers, stands by the findings and said KISSmetrics' recently updated privacy policy doesn't make it clear how users go about opting out of tracking.

At the heart of the technique is the practice of storing a unique identifier, known as an ETag value, in a browser's cache and metadata folders. A piece of JavaScript hosted on kissmetrics.com accesses the serial number each time one of the KISSmetrics websites is viewed.

“It's effectively acting like a cookie because with every connection to KISSmetrics, it will send a referrer header and the ETag value,” Soltani told The Register. “The ETag is effectively acting as a cookie. It has the same exact value of the cookie as well.”

KISSmetrics analytics combined the the ETag technique with several other controversial technologies that use cookies based on Adobe Flash and HTML5 to reproduce tracking cookies even after a user had specifically deleted them. Soltani and his colleagues first documented the sneaky move in 2009 and dubbed it cookie “respawning.”

Adobe responded by building an application interface that made it easy for users to delete Flash cookies using standard features in a browser's menu. The advent of server-based scripts that pull up ETag data means that it's once again trivial for analytics services to defy the wishes of visitors who don't want to be tracked.

“The more accurately they can represent the number of uniques that have visited their sites the more value they can provide for their analytics customers,” Soltani explained. “That might mean you as a person who doesn't want to be tracked uniquely trying to opt out. They're incentivized to circumvent that opt-out.”

Soltani said the only way to block the tracking using the technique is to block all cookies and to clear the browser cache after each site visited. He has published a detailed technical description of the new technique here. ®