Supercookie - Privacy and Third-party Cookies

Privacy and Third-party Cookies

See also: Do Not Track and Web analytics: Problems with cookies

Cookies have some important implications on the privacy and anonymity of Web users. While cookies are sent only to the server setting them or the server in the same Internet domain, a Web page may contain images or other components stored on servers in other domains. Cookies that are set during retrieval of these components are called third-party cookies. The older standards for cookies, RFC 2109 and RFC 2965, specify that browsers should protect user privacy and not allow sharing of cookies between servers by default; however, the newer standard, RFC 6265, explicitly allows user agents to implement whichever third-party cookie policy they wish. Most browsers, such as Mozilla Firefox, Internet Explorer, Opera and Google Chrome do allow third-party cookies by default, as long as the third-party website has Compact Privacy Policy published. Newer versions of Safari block third-party cookies, as will Firefox 22.

Advertising companies use third-party cookies to track a user across multiple sites. In particular, an advertising company can track a user across all pages where it has placed advertising images or web bugs. Knowledge of the pages visited by a user allows the advertising company to target advertisements to the user's presumed preferences.

Website operators who do not disclose third-party cookie use to consumers run the risk of harming consumer trust if cookie use is discovered. Having clear disclosure (such as in a privacy policy) tends to eliminate any negative effects of such cookie discovery.

The possibility of building a profile of users is considered by some a potential privacy threat, especially when tracking is done across multiple domains using third-party cookies. For this reason, some countries have legislation about cookies.

The United States government has set strict rules on setting cookies in 2000 after it was disclosed that the White House drug policy office used cookies to track computer users viewing its online anti-drug advertising. In 2002, privacy activist Daniel Brandt found that the CIA had been leaving persistent cookies on computers which had visited its website. When notified it was violating policy, CIA stated that these cookies were not intentionally set and stopped setting them. On December 25, 2005, Brandt discovered that the National Security Agency (NSA) had been leaving two persistent cookies on visitors' computers due to a software upgrade. After being informed, the National Security Agency immediately disabled the cookies.