Zombie cookies

A zombie cookie is an HTTP cookie that is recreated after deletion. Cookies are recreated from backups stored outside the web browser's dedicated cookie storage. It may be stored online or directly onto the visitor's computer, in a breach of browser security. This makes them very difficult to remove. These cookies may be installed on a web browser that has opted to not receive cookies since they do not completely rely on traditional cookies.

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 that deletes all his cookies regularly, a site using this would still be able to personalize to that specific user. This helps the site appear more consistent and professional to its users. For a site that wishes to ban a certain user a zombie cookie may be installed. This prevents the user from being able to simply delete the cookie and create a new login.

A user that doesn't want to be tracked may choose to decline 3rd 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 this causes the data collected by tracking companies to become 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.

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".

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Wikipedia