Flash Cookies and Privacy II: Now with HTML5 and ETag Respawning

In August 2009, we demonstrated that popular websites were using “Flash cookies” to track users. Some advertisers had adopted this technology because it allowed persistent tracking even where users had taken steps to avoid web profiling. We also demonstrated “respawning” on top sites with Flash technology. This allowed sites to reinstantiate HTTP cookies deleted by a user, making tracking more resistant to users’ privacy-seeking behaviors.

In this followup study, we reassess the Flash cookies landscape and examine a new tracking vector, HTML5 local storage and Cache-Cookies via ETags.

We found over 5,600 standard HTTP cookies on popular sites, over 4,900 were from third parties. Google-controlled cookies were present on 97 of the top 100 sites, including popular government websites. Seventeen sites were using HTML5, and seven of those sites had HTML5 local storage and HTTP cookies with matching values. Flash cookies were present on 37 of the top 100 sites.

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.