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.