What is zombie cookie?
A zombie cookie is an HTTP cookie that is automatically restored after being deleted by the user. Zombie cookies are recreated using a technology called Quantcast, which creates Flash cookies to track users across the Internet. The flash cookies are then used to recreate browser cookies and become zombie cookies that never crash.
Websites can use Flash cookies for tasks such as: For setting volume levels and tracking users with a unique ID. If a user tries to delete cookies after visiting a website with Quantcast technology, the user ID is stored in the Adobe Flash Player storage compartment. The Quantcast program retrieves and reapplies the user ID 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 collect personal profiles from website visitors. Sites that used Quantcast were sued in 2010 on the grounds that they violated federal computer break-in laws. The court order required that all tracked website companies delete users' personal data and stop collecting this information in the future.
Adobe Flash is used in approximately 98% of all personal computer systems, which means that all of those computers may have zombie cookies. The problem with zombie cookies is that they violate user privacy and are difficult to delete using normal techniques.
Modern browsers contain control settings with which users can decide whether to accept cookies or to log out. Browsers like Firefox offer an option to delete flash cookies as well as zombie cookies via the BetterPrivacy add-on setting.