A zombie cookie is any HTTP cookie that is recreated after deletion 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.
Zombie cookies allow the web traffic tracking companies to retrieve information such as previous unique user ID and continue tracking personal browsing habits. Zombie cookies work across browsers on the same machine since the data is kept in folders that are common to all browsers.
Zombie cookies are also used to remember unique ID's used for logging in to 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.”.
The following is a list, presented for the informational purpose of showing readers the numerous possible places in which zombie cookies may be hidden, of available storage mechanisms:
- Standard HTTP cookies
- Storing cookies in and reading out web history
- Storing cookies in HTTP ETags
- Internet Explorer userData storage (starting IE9, userData is no longer supported)
- HTML5 Session Storage
- HTML5 Local Storage
- HTML5 Global Storage
- HTML5 Database Storage via SQLite
- Storing cookies in RGB values of auto-generated, force-cached PNGs using HTML5 Canvas tag to read pixels (cookies) back out
- Local Shared Objects (Flash cookies)
- Silverlight Isolated Storage
- Cookie syncing scripts that function as a cache cookie and respawn the MUID cookie
If a user is not able to remove the cookie from every one of these data stores then the cookie will be recreated to all of these stores on the next visit to the site that uses that particular cookie. Every company has their own implementation of zombie cookies and those are kept proprietary. An open-source implementation of zombie cookies, called Evercookie, is available.
An influential academic study of zombie cookies was completed in 2009 by a team of researchers at UC Berkeley, where they noticed that cookies kept coming back after they were deleted over and over again. They cited this as a serious privacy breach. Since most users are barely aware of these storage methods, it's unlikely that users will ever delete all of them. From the Berkeley report, “few websites disclose their use of Flash in privacy policies, and many companies using Flash are privacy certified by TRUSTe”.
Ringleader Digital made an effort to keep a persistent user ID even when the user deleted cookies and their HTML5 databases. The only way to opt out of the tracking was to use the company's opt-out link which gives no confirmation. This resulted in a lawsuit against Ringleader Digital filed by Fears | Nachawati Law Firm and Wilson Trosclair & Lovins.
A lawsuit was filed in the United States District Court for the Central District of California against Clearspring and affiliated sites owned by Walt Disney Internet Group, Warner Bros and others. According to the charges Adobe Flash cookies are planted to "track Plaintiffs and Class Members that visited non-Clearspring Flash Cookie Affiliates websites by having their online transmissions intercepted, without notice or consent".
- Mayer, Jonathan. "Tracking the Trackers: Microsoft Advertising". The Center for Internet and Society. Retrieved 28 September 2011.
- Soltani, Ashkan, Canty, Shannon, Mayo, Quentin, Thomas, Lauren and Hoofnagle, Chris Jay, Flash Cookies and Privacy (August 10, 2009). Available at SSRN: http://ssrn.com/abstract=1446862
- Burt, David. "Update on the issue of ‘supercookies’ used on MSN". Retrieved 28 September 2011.
- Device Fingerprint - Site that demonstrates the way zombie cookies are restored