Third-party cookie

Definition

A third-party cookie is set by a different domain than the one being visited. For example, when you visit site-a.com and a pixel from facebook.com sets a cookie, that cookie is third-party. These cookies enable tracking a user across different websites to build a browsing profile and personalize advertising.

Planned phase-out

Third-party cookies are being phased out. Safari blocks them by default since 2020 (ITP), Firefox since 2021 (ETP), and Chrome has announced their progressive restriction. This evolution profoundly transforms web marketing: retargeting, behavioral targeting and cross-site conversion measurement are directly impacted.

Difference from first-party cookies

The difference is both technical and legal. A first-party cookie is set by the visited site’s domain and generally serves useful functions (session, preferences). A third-party cookie is set by an external domain and primarily serves advertising tracking. GDPR requires consent for both, but third-party cookies are particularly targeted by regulators.

Alternatives

Facing this phase-out, advertisers are migrating to server-side tracking solutions, Enhanced Conversions and Google’s Privacy Sandbox APIs. Measuring marketing performance must adapt to a cookieless world, which often requires rethinking the tracking plan.