You likely have heard of "cookies," the small bits of code stored by websites in your browser. Cookies allow sites to look up your preferences when you return, or present different content to new visitors. Cookies can improve your internet experience, but they can also be used to track your movements across websites. Generally speaking, this is often accomplished through "third-party cookies," which belong to an advertising site, and not the website you are visiting. When you visit subsequent sites that have relationships with an ad network, that network can ask your browser if it is storing one of its cookies. If it is, the network can assemble a profile of your internet activity.
Many people know that browser settings can be adjusted to reject third-party cookies. Or, you can clear out all cookies stored in your browser. Don't you feel safe now that you've cleared all those cookies?
Turns out that clearing cookies isn't enough anymore. Adobe's flash technology allows sites providing flash video of any sort to store a code snippet on a user's computer when the user views that flash content. These "Flash cookies" are larger in size than html cookies (100 kilobytes of information in the case of flash cookies, or 25 times what a browser html cookie can hold). This added size allows for the storage of far more information about a user's internet activities. Moreover, flash cookies can be used for one particularly despicable purpose - they can be used to restore deleted html cookies. In other words, you think you actively protected your privacy by deleting all "cookies" (really, just the html cookies), and when a site that can read a flash cookie sees that you used to have a related html cookie on your browser, it can restore that deleted cookie. Thus the recent trend to refer to these cookies as "zombie cookies," the cookies that cannot die.
This sneaky new form of visitor tracking has resulted in a number of class action lawsuit. The New York Times reported on several such lawsuits in an article earlier today. Wired reported on other suits in July of this year. These days you need your own home IT support just to know whether you are being spied on by the websites you visit.