Conversion Tracking & Data
Third-Party Cookies
By the AdFlint research team · Last reviewed July 2026
Written by a domain other than the one being visited, historically the backbone of cross-site retargeting and frequency capping.
An embedded script from an ad network writes a cookie under its own domain, letting it recognize the same browser across unrelated sites. Safari and Firefox have blocked them by default for years. Chrome's phase-out was repeatedly delayed and then abandoned in 2025, so they still function there, but building new measurement on them is a mistake, since the practical coverage they offer has already collapsed.
Third-Party Cookies compared with
The settings this gets confused with, and how to tell them apart.
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