supercookie

A supercookie is a tracking cookie that is not deleted when a consumer clears cookies from a browser.

For example, in 2011 Direct Marketing News reported: “According to research from the Stanford University Security Laboratory, Microsoft Corp. ran a “supercookie” on several of its domains that was able to access a browser's history regardless of whether the browser's cookies had been cleared. The Stanford researchers found that Microsoft actually had two supercookies in place, one of which was able to respawn cleared identifier cookies. The other supercookie featured a mechanism that contained the contents of a defunct identifier cookie. A Microsoft spokesperson said via email that the company disabled the “legacy code” that enabled the cookie behavior observed by the researchers. Data collected through the supercookie was not shared outside of Microsoft, said the spokesperson.”

Shortly thereafter, PC Pro reported: "Microsoft will stop using so-called "supercookies" on its own sites. The move comes after a researcher highlighted Microsoft's use of supercookies, which are tracking systems that don't actually use cookies, but hold enough information to recreate cookies on a users' machine after they've been deleted. Microsoft suggested it wasn't aware of the use of supercookies, and promptly investigated. A Microsoft spokesperson said they determined that the cookie behavior observed was occurring under certain circumstances as a result of older code that was used only on Microsoft sites, and was already scheduled to be discontinued. Microsoft also said they have no plans to "develop or deploy" more supercookies."