The supercookies (‘Flash cookies‘ and ‘zombie cookies‘) are computer tracking files which are stored in places that are different from regular cookies but serve the same purpose as regular cookies by tracking user preferences and browsing histories.
But this breed is difficult to detect and subsequently remove, unlike the regular cookies. These cookies secretly collect user data beyond the limitations of common industry practice, and thus raise serious privacy concerns.
Supercookies can be used to ‘respawn’ cookies that users delete, and have been found on major websites such as MSN and Hulu and hundreds of smaller sites as well.
[advt]Supercookies are stored in different locations on a user’s machine, for example, in a file used by a plug-in (Flash is the most common). This makes them harder to find and delete, especially since a browser’s built-in cookie detection process won’t remove them either. Furthermore, some supercookies have additional capabilities, like regenerating regular cookies to prevent their removal by the user.
Removing master supercookies is much more difficult. It requires the user to dig through the file system and delete them manually, an inconvenient task even for advanced users. The novice, on the other hand, likely won’t even realize supercookies exist, let alone be able to find them.
The kind of data supercookies track isn’t typical cookie material. A browser limits the typical cookie to be written, read and ultimately removed by the site that created it. The supercookie, on the other hand can track and record user behavior across multiple sites. While it’s easy to understand that a site would want to track a user’s activity while she navigates its turf, it’s ethically questionable that site operators are able to record a user’s actions beyond site parameters.
While many companies that had been challenged on their use of supercookies were quick to cease, some choose to continue the practice. Many web marketing firms, advertisers and websites still refuse to follow industry best practices — they continue to practice knowingly which will ultimately end up in losing the brand’s reputation. And many more sites don’t even realize they’re utilizing supercookies in the first place. [source]