In August 2009, I and other graduate students at the University of California, Berkeley – School of Law, Berkeley Center for Law & Technology published Flash Cookies and Privacy, a paper that examined of the use of ‘Flash cookies’ by popular websites.

Websites and Cookies

Advertisers are increasingly concerned about unique tracking of users online. Several studies have found that over 30% of users delete first party HTTP cookies once a month, thus leading to overestimation of the number of true unique visitors to websites, and attendant overpayment for advertising impressions.

Mindful of this problem, online advertising companies have attempted to increase the reliability of tracking methods. In 2005, United Virtualities (UV), an online advertising company, exclaimed, “All advertisers, websites and networks use [HTTP] cookies for targeted advertising, but cookies are under attack.” The company announced that it had, “developed a backup ID system for cookies set by web sites, ad networks and advertisers, but increasingly deleted by users. UV’s ‘Persistent Identification Element’ (PIE) is tagged to the user’s browser, providing each with a unique ID just like traditional cookie coding. However, PIEs cannot be deleted by any commercially available antispyware, mal-ware, or adware removal program. They will even function at the default security setting for Internet Explorer.”

United Virtualities’ PIE leveraged a feature in Adobe’s Flash MX: the “local shared object,” also known as the “Flash cookie.” [Read more…]