What are Zombie Cookies?

They are cookies that rise from the dead come back to life after you kill or delete them. It was first identified by UC Berkley. They noticed that they were deleting cookies, but they kept coming back over and over again. No amount of deleting them would kill them. Many people have absolutely no idea what a zombie cookie is, or that they even exist. Until a massive lawsuit in 2009, which targeted some of the biggest names on the web. Because of breach of privacy.

Differences between Zombie Cookies & regular cookies.

Regular Cookies Zombie Cookies
Stored in Browser Stored in Flash or Silverlight
Easily blocked and deleted from browser Blocking and deleting them is not easy
Size=4kb Size=upto 100kb
Work with only one browser Work across all browsers on the same machine

How do they work exactly?

What you think happens: You visit a website, They plant browser Cookies.

You visit the website again, and they retrieve those cookies.

You can block them or delete them, and that’s that.

How do they work exactly?

What REALLY happens (in some cases): You visit a website, They plant browser Cookies AND Adobe Flash cookies.

You block or delete regular cookies. Doesn’t matter!

You visit the website again, they check for regular cookies - No luck?

They check for Adobe Flash cookies, which are EXACTLY the same, if not even more detailed (remember 4kb vs. 100kb). So in a sense, you deleting or blocking browser cookies doesn’t matter. Because Zombie cookies are there.

Who used them?

ESPN, MTV, HULU, ABC, MySpace, NBC, YouTube, Scribid, and that is just for starters. It isn’t even the tip of the iceberg when it comes to who is hiding zombie cookies on your computer.

Purposes of using them.

Marketing Research or Tracking personal browsing habits.

Since Zombie cookies have a bigger size, they store more, detailed information about users’ behavior. Remember unique visitors. Preferences, such as video volume or page layout, are saved, no matter what browser you use. When a site bans a user, he/she will still be banned even if you delete browser cookies or use different browsers.

Privacy Risk and Breach

Fact: almost 98% of computers have Adobe Flash, which means almost everyone is exposed to Zombie Cookies. Some people feel that if you delete or block a cookie, it should stay deleted. But since Zombie cookies are not affected by regular deletion of cookies, people consider sites that use them to be breaching their privacy. A huge lawsuit was filed against Clearspring and affiliated sites owned by Walt Disney Internet Group, Warner Bros and others. It said that Adobe Flash cookies, a type of zombie cookie, 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".

How to kill them?

Before: You had to uninstall Adobe Flash, and re-install it.

Now: Go to Adobe’s webpage and set controls on the Global Privacy Settings page (Google this for more details). If you use Firefox you can get rid of Flash cookies - including zombie cookies- by using theBetterPrivacy add-on.

The Cookie List


Sometimes known as a transient cookie, stored in temporary memory and remains available for the duration of your active “session” within the browser.

session cookie...


Also known as a stored cookie, it stores a file on your hard drive. The cookie would remain on the hard drive until it reaches its expiration date.

persistent cookie...

Secure & HttpOnly

A secure cookie is just like a regular cookie, except it contains a special ‘HttpOnly’ flag that instructs the browser to restrict access to cookie data.

secure httponly cookie...


Visit a web site, but have a cookie created by a completely different domain. This allows the third-party domain to track you i.e. Tracking Cookies

Third-Party Cookie...


Dangerous: Uses various techniques to resists deletion even when you clear your entire history they can remain hidden and reappear like a virus!

Super Cookie...


Dangerous: This is a cookie that can come back to life, hence the name Zombie. After it has been deleted it recreates itself.

Zombie Cookie...


This is an example of a VERY persistent cookie. A cross between Super and Zombie types of cookie.

Ever Cookie...