A zombie cookie is a piece of data that could be stored in multiple locations -- since failure of removing all copies of the zombie cookie will make the removal reversible, zombie cookies can be difficult to remove. Since they do not entirely rely on normal cookie protocols, the visitor's web browser may continue to recreate deleted cookies even though the user has opted not to receive cookies. The term was used by Attorney Joseph H. Malley, who initiated the Super-Cookie class actions in 2010. Purpose Web analytics collecting companies use cookies to track Internet usage and pages visited for marketing research. Sites that want to collect user statistics will install a cookie from a traffic tracking site that will collect data on the user. As that user surfs around the web the cookie will add more information for each site that uses the traffic tracking cookie and sends it back to the main tracking server. Zombie cookies allow the web traffic tracking companies to retrieve information such as previous unique user ID and continue tracking personal browsing habits. When the user ID is stored outside of a single browser's cookie storage, such as in a header injected by the network into HTTP requests, zombie cookies can track users across browsers on the same machine. Zombie cookies are also used to remember unique IDs used for logging into websites. This means that for a user who deletes all their cookies regularly, a site using this would still be able to personalize to that specific user. Implications A user who does not want to be tracked may choose to decline or block third party cookies or delete cookies after each browsing session. Deleting all cookies will prevent some sites from tracking a user but it may also interfere with sites that users want to remember them. Removing tracking cookies is not the same as declining cookies. If cookies are deleted, the data collected by tracking companies becomes fragmented. For example, counting the same person as two separate unique users would falsely increase this particular site's unique user statistic. This is why some tracking companies use a type of zombie cookie. Implementation According to TRUSTe: "You can get valuable marketing insight by tracking individual users' movements on your site. But you must disclose your use of all personally identifiable information in order to comply with the Fair Information Practices guidelines". Possible places