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A zombie cookie is an HTTP cookie that returns to life automatically after being deleted by the user. Zombie cookies are recreated using a technology called Quantcast, which creates flash cookies to trace users on the Internet. The flash cookies are then used to recreate browser cookies, becoming zombie cookies that never die.
Websites may use flash cookies for tasks, such as setting volume levels and tracking users with a unique ID. When a user tries to delete cookies after visiting a website with Quantcast technology, the user ID is stored in the Adobe Flash player storage bin. The Quantcast program retrieves the user ID and reapplies it to keep track of the user’s browsing history.
The main purpose of zombie cookies is to store the personal information of Web users for online marketing activities. Quantcast technology is used by many websites to measure website traffic and gather personal profiles of website visitors. Websites that used Quantcast were sued in 2010 on the grounds that they had violated federal computer intrusion laws. The court order required that all prosecuted website companies erase the users' personal information and stop collecting that information in the future.
Adobe Flash is used in about 98% of all PC systems, which means that potentially all of these computers are planted with zombie cookies. The problem with zombie cookies is that they invade user privacy and are hard to be delete using normal techniques.
Modern browsers include control settings that allow users to decide whether they want to accept the cookies or to opt out. Browsers like Firefox provide an option to delete flash cookies as well as zombie cookies through the BetterPrivacy add-on setting.