Privacy / Data Protection

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

Author(s): Chris Jay Hoofnagle
Year: 2011

Abstract: 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. We found two sites that were respawning cookies, including one site – – where both Flash and cache cookies were employed to make identifiers more persistent. The cache cookie method used ETags, and is capable of unique tracking even where all cookies are blocked by the user and “Private Browsing Mode” is enabled.

Our 2009 study is also available at SSRN:

Keywords: privacy, tracking, flash, cookies, local shared object, local stored object, online advertising, behavioral targeting, self-help, persistent identification elemen