Third-party cookies

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Third-party cookies are HTTP cookies which are used principally for web tracking as part of the web advertising ecosystem.

While HTTP cookies are normally sent only to the server setting them or a server in the same Internet domain, a web page may contain images or other components stored on servers in other domains. Third-party cookies are the cookies that are set during retrieval of these components.

A third-party cookie thus can belong to a domain different from the one shown in the address bar, yet can still potentially be correlated to the content of the main web page, allowing the tracking of user visits across multiple websites.

This sort of cookie typically appears when web pages feature content from external websites, such as banner advertisements. Although not originally intended for this purpose, the existence of third party cookies opened up the potential for web tracking of a user's browsing history and is used by advertisers to serve relevant advertisements to each user. Third-party cookies are widely viewed as a threat to the privacy and anonymity of web users.

As of 2024^[update], all major web browser vendors had plans to phase out third-party cookies.^[1] This decision was reversed for Google Chrome in July of 2024.^[2]