…if United Virtualities has their way, some “deleted” cookies may come back to spy on you. Seems they have developed something they call a Persistent Identification Element (PIE). It’s technology that lets a web site know if a cookie is missing, and it will look for the missing cookie in Macromedia’s Flash Player local shared objects. If the cookie is there, PIE will put it back in your browser. While this will happen mostly with advertising cookies, it opens the door to more insidious uses.
In his CNet article, Keep deleted cookies from sneaking back and spying on you, Tom Merritt says:
Be afraid. If the ad industry finds ways to get around you[r] deleting cookies, what’s to stop it from getting around ways to prevent your personal information from being tagged specifically to you instead of aggregated anonymously? It’s another cautionary tale that the Internet is still in its Wild West stage, and you need to keep your eyes open.
The good news is that, for now, you are still in control.
Thankfully, as usual in the arms race of Internet privacy, there’s a way around this. Macromedia has posted notes on how to disable local shared objects. You can delete data on a per-site basis, similarly to the way Firefox handles cookies, or you can wipe out all the data stored locally.