A zombie cookie is a piece of data that could be stored in multiple locations -- since failure of removing all copies of the zombie cookie will make the removal reversible, zombie cookies can be difficult to remove. Since they do not entirely rely on normal cookie protocols, the visitor's web browser may continue to recreate deleted cookies even though the user has opted not to receive cookies.
The term was used by Attorney Joseph H. Malley, who initiated the Super-Cookie class actions in 2010.
Purpose
Web analytics
Web analytics is the measurement, collection, analysis, and reporting of web data to understand and optimize web usage. Web analytics is not just a process for measuring web traffic but can be used as a tool for business and market research and ...
collecting companies use cookies to track Internet usage and pages visited for
marketing research
Marketing research is the systematic gathering, recording, and analysis of qualitative and quantitative data about issues relating to marketing products and services. The goal is to identify and assess how changing elements of the marketing mix i ...
.
Sites that want to collect user statistics will install a cookie from a traffic tracking site that will collect data on the user. As that user surfs around the web the cookie will add more information for each site that uses the traffic tracking cookie and sends it back to the main tracking server.
Zombie cookies allow the
web traffic
Web traffic is the data sent and received by visitors to a website. Since the mid-1990s, web traffic has been the largest portion of Internet traffic. Sites monitor the incoming and outgoing traffic to see which parts or pages of their site are ...
tracking companies to retrieve information such as previous
unique user
Website popularity is commonly determined using the number of unique users, and the metric is often quoted to potential advertisers or investors. A website's number of unique users is usually measured over a standard period of time, typically a m ...
ID and continue tracking personal browsing habits. When the user ID is stored outside of a single browser's cookie storage, such as in a header injected by the network into HTTP requests, zombie cookies can track users across browsers on the same machine.
Zombie cookies are also used to remember unique IDs used for logging into websites. This means that for a user who deletes all their cookies regularly, a site using this would still be able to personalize to that specific user.
Implications
A user who does not want to be tracked may choose to decline or block third 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, the data collected by tracking companies becomes 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.
Implementation
According to
TRUSTe TrustArc (formerly TRUSTe) is a privacy compliance technology company based in San Francisco, California. The company provides software and services to help corporations update their privacy management processes so they comply with government laws a ...
: "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".
Possible places in which zombie cookies may be hidden include:
* Standard HTTP cookies
* Storing cookies in and reading out web history
* Storing cookies in
HTTP ETag
The ETag or entity tag is part of HTTP, the protocol for the World Wide Web. It is one of several mechanisms that HTTP provides for Web cache validation, which allows a client to make conditional requests. This mechanism allows caches to be more ...
s
* Internet Explorer userData storage (starting
IE9, userData is no longer supported)
*
HTML5
HTML5 is a markup language used for structuring and presenting content on the World Wide Web. It is the fifth and final major HTML version that is a World Wide Web Consortium (W3C) recommendation. The current specification is known as the HTML ...
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 object
A local shared object (LSO), commonly called a Flash cookie (due to its similarity with an HTTP cookie), is a piece of data that websites that use Adobe Flash may store on a user's computer. Local shared objects have been used by all versions of ...
s (Flash cookies)
* Silverlight Isolated Storage
* Cookie syncing scripts that function as a cache cookie and respawn the MUID cookie
*
TCP Fast Open
In computer networking, TCP Fast Open (TFO) is an extension to speed up the opening of successive Transmission Control Protocol (TCP) connections between two endpoints. It works by using a ''TFO cookie'' (a TCP option), which is a cryptographic co ...
*
TLS's Session ID
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
Evercookie (also known as supercookie) is a JavaScript application programming interface (API) that identifies and reproduces intentionally deleted cookies on the clients' browser storage. It was created by Samy Kamkar in 2010 to demonstrate th ...
,
is available.
Controversies
In 2015, TURN, an online advertising clearinghouse,
["Zombie Cookie: The Tracking Cookie That You Can't Kill"](_blank)
/ref> introduced zombie cookies based on Flash Local Shared objects. Privacy advocates quickly denounced the technology.
An academic study of zombie cookies was completed in 2009, by a team of researchers at UC Berkeley
The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California) is a public land-grant research university in Berkeley, California. Established in 1868 as the University of California, it is the state's first land-grant uni ...
, where they noticed that cookies which had been deleted, kept coming back, over and over again. They cited this as a serious privacy breach. Since most users are barely aware of the storage methods used, it's unlikely that users will ever delete them all. 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.
The term ''"zombie cookie"'' was created by Attorney Joseph H. Malley who initiated the Super-Cookie Class Actions in 2010. The etiology of the phrase was derived from his prior research into Apple's third-party iPhone applications. Some of these which had been criticized as being ''"zombie-like"'' applications such as the ''"super-cookies"'' which ''"re-spawned"'' when deleted. Attorney Malley envisioned a cookie that seemed to come back from the ''"dead"''. Blending the two ideas, he first coined the phrase Zombie Cookies within his filed Class Actions, as a means to enable the court, jury, and public understand the basis of the litigation.
The Zombie Cookie lawsuits were filed suit in the against Quantcast
Quantcast is an American technology company, founded in 2006, that specializes in AI-driven real-time advertising, audience insights and measurement. It has offices in the United States, Canada, Australia, Singapore, United Kingdom, Ireland, Fran ...
, Clearspring, VideoEgg
Say Media (formerly VideoEgg) is a technology and advertising firm. The company provides a publishing platform (Tempest) to professional publishers and sells advertising across that platform and extended network of sites. Say Media has offices ...
, and affiliated sites owned by Walt Disney Internet Group
Disney Interactive is an American video game and internet company that oversaw various websites and interactive media owned by The Walt Disney Company.
History
1995–1996: Formation and beginnings
In December 1994, Disney announced that it wa ...
, Warner Bros.
Warner Bros. Entertainment Inc. (commonly known as Warner Bros. or abbreviated as WB) is an American film and entertainment studio headquartered at the Warner Bros. Studios complex in Burbank, California, and a subsidiary of Warner Bros. D ...
and others. According to the charges, Adobe Flash cookies
A local shared object (LSO), commonly called a Flash cookie (due to its similarity with an HTTP cookie), is a piece of data that websites that use Adobe Flash may store on a user's computer. Local shared objects have been used by all versions of ...
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".
Two " supercookie" mechanisms were found on Microsoft websites in 2011, including cookie syncing that respawned MUID cookies.[ Due to media attention, Microsoft later disabled this code.
Consumer outrage related to Flash cookies and violation of consumers' privacy caused U.S. Congressional Hearings, led by Senators ]Al Franken
Alan Stuart Franken (born May 21, 1951) is an American comedian, politician, media personality, and author who served as a United States senator from Minnesota from 2009 to 2018. He gained fame as a writer and performer on the television comed ...
and John Rockefeller. Reportedly, the "Zombie Cookie", aka Flash Cookie filings, forced Adobe Systems Inc. to stop processing flash cookies on 98% of all consumers' computing devices.
The online advertising clearinghouse TURN implemented zombie cookies on Verizon
Verizon Communications Inc., commonly known as Verizon, is an American multinational telecommunications conglomerate and a corporate component of the Dow Jones Industrial Average. The company is headquartered at 1095 Avenue of the Americas ...
mobile phones, using a hidden, unremovable number by which Verizon could track customers. After an article by ProPublica
ProPublica (), legally Pro Publica, Inc., is a nonprofit organization based in New York City. In 2010, it became the first online news source to win a Pulitzer Prize, for a piece written by one of its journalists''The Guardian'', April 13, 2010P ...
revealed this fact in January 2015, TURN claimed it had suspended usage of their zombie cookies.
References
{{Reflist, 30em
External links
Device Fingerprint
- Site that demonstrates the way zombie cookies are restored
Internet privacy