Third-Party Cookie.

A cookie set by a domain different from the one in the browser's address bar, historically used to track users across websites. It belongs to the Privacy & Data Protection category of cybersecurity.

A cookie set by a domain different from the one in the browser's address bar, historically used to track users across websites.

A third-party cookie is stored by a domain other than the page being visited, typically because the page embeds scripts, pixels, or iframes from another origin such as an ad network. Because the same identifier follows the user across every site that loads the third party, third-party cookies were the foundation of cross-site advertising tracking. Safari and Firefox now block them by default and Chrome is phasing them out via the Privacy Sandbox initiative, which proposes alternatives like Topics, FedCM, Attribution Reporting, and Storage Partitioning. Defences include browser blocking, anti-tracking extensions, and partitioned storage that scopes cookies to the top-level site.

Defences for Third-Party Cookie typically combine technical controls and operational practices, as detailed in the full definition above.

Common alternative names include: 3P cookie, Cross-site cookie.