Evercookie was developed by developer and security hacker Samy Kamkar and it uses fairly simple and straightforward web and browser technologies to make it possible for sites, advertising networks and more nefarious people to add a tracking cookie to your browser that will last, essentially, forever.
Evercookie does this by placing cookie data in several places within the browser, including the standard cookie cache, browser history, even within a specialized image file. Evercookie also utilizes new data storage features in HTML 5.
Even worse, the whole system is self-repairing, meaning that if you fail to delete all of the areas where an evercookie is hiding, the hidden cookie will be able to restore all of its deleted brethren.
One can easily think of many ways where this technology could be abused, and I don't just mean by pushy ad networks that want to track every site you go to. Think about the recent Twitter problem, where many people were sent unwittingly to porn sites. What if these sites used an evercookie? And of course this technology could prove to be very popular to phishing sites.
This technology bothers me for several reasons, above and beyond the abuse of privacy.
First, it resists the will of the user. If someone takes the time to go into their browser settings and delete the cookie for Scumbag Networks, then those cookies should disappear. By explicitly ignoring the user's request, evercookies are no better than an Uninstall program that not only doesn't uninstall the app but also adds new applications (including spyware).
Also, this technology breaks the compromise between privacy geeks and vendors and advertising and search networks.
For the most part, privacy geeks are happy if they have the tools to protect their privacy and the vendors and tracking networks will be happy in the knowledge that the vast majority of users will never bother to use privacy tools.
Of course, it's only a matter of time until some of the popular privacy tools and hopefully the browsers themselves, are able to fully remove evercookies. And while it's not totally clear right now, there's good evidence that browser-extensions like NoScript should be able to stop evercookies.
Right now this is new. But since the code has been fully posted and is open source, it's only a matter of time until evercookies start to pop-up on the web. And privately, just between me and you, I think that stinks.