Tracking Using ETags
ETags can be used to track unique users, as HTTP cookies are increasingly deleted by privacy-aware users. As of July 2011, researchers report that a number of Websites, including Hulu.com, are using ETags for tracking purposes. Hulu and KISSmetrics have both ceased "respawning" as of 29 July 2011, as KISSmetrics and over 20 of its clients are facing a class-action lawsuit over the use of "undeletable" tracking cookies partially involving the use of ETags.
Because ETags are cached by the browser, and returned with subsequent requests for the same resource, a tracking server can simply repeat any ETag received from the browser to ensure an assigned ETag persists indefinitely (in a similar way to persistent cookies).
ETags may be flushable by clearing the browser cache (but browser implementations may vary).
In 2007, two Mozilla Firefox add-ons were made to prevent the usage of ETags for tracking, though as of 14 September 2011, they don't work with the latest version of Firefox.
Also Privoxy and Proxomitron may block e-tags if properly configured.
Read more about this topic: HTTP ETag
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