Flash Cookies and Privacy II

A detailed technical followup to Flash Cookies and Privacy II, describing the mechanisms behind Hulu/KISSmetrics’ respawning practices

cookiemonsterdeleteI thought I’d take the time to elaborate a bit further regarding the technical mechanisms described in our Flash Cookies and Privacy II paper that generated a bit of buzz recently. For a bit of background, I, along with Chris Hoofnagle and Nathan Good, had the honor of supervising Mika Ayenson and Dietrich J. Wambach in replicating our previous 2009 study which found that websites were circumventing user choice by deliberately restoring previously deleted HTTP cookies using persistent storage outside of the control of the browser (a practice we dubbed ‘respawning’).

In our follow up study, we found that Hulu was still respawning deleted user cookies using homegrown Flash and Javascript code present on the Hulu.com site. Additionally, Hulu, Spotify, and many others were also respawning using code provided by analytics firm KISSmetrics.* Hitten Shah, the founder of KISSmetrics, initially confirmed that the research surrounding respawning was correct in an interview with Ryan Singel although he later criticized the findings after a lawsuit was filed.

(*Hulu and KISSmetrics have both ceased respawning as of July 29th 2011)

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


In August 2009, the research team published Flash Cookies and Privacy, a paper that demonstrated that popular websites were using Flash cookies to track users.  Some advertisers has adopted this technology because it allowed persistent tracking, even where users had taken steps to avoid web profiling. This allowed sites to reinstantiate HTTP cookies deleted by a user, making tracking more resistant to users’ privacy-setting 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. [Read more…]