Third-party cookies are cookies set not by the website you are visiting but by another domain, typically an ad network. For years they were the backbone of cross-site tracking and retargeting.
How do third-party cookies work?
When a website embeds a script or image from another domain, that domain can set its own cookie in the browser. When the same user later visits a different site with the same script, the third-party domain reads its cookie again. Repeat that across thousands of sites and you get a cross-site profile.
A first-party cookie, by contrast, belongs to the domain being visited and only works there.
What do third-party cookies mean for advertisers?
Third-party cookies are in decline:
- Safari has fully blocked them by default since 2020 through Intelligent Tracking Prevention.
- Firefox blocks known tracker cookies with Enhanced Tracking Protection.
- Chrome postponed deprecation several times and now leans on user choice. Do not build your measurement on it.
- An ad blocker often stops the scripts from loading in the first place.
On top of that, under the GDPR, third-party cookies for advertising need consent through a cookie banner.
For conversion tracking, the takeaway is simple: the more you depend on third-party cookies, the bigger your signal loss. More resilient options are first-party data, click IDs from the URL and server-side tracking, where events travel directly from server to server.
Example
A user clicks an ad in Safari, reads your blog, comes back directly three days later and buys. The ad platform's third-party cookie was never set in Safari. The conversion can only be attributed if the click ID was stored as first-party information on the first visit and sent along with the purchase.
Third-Party Cookies with LeadSignal
LeadSignal does not rely on third-party cookies. Its script captures click IDs such as gclid, fbclid and ttclid plus UTM parameters on the first visit and works with a first-party identity after consent. For Meta, LinkedIn and TikTok, events can also be sent server-side through each platform's Conversions API. Read why that matters in Server-Side Tracking: Why Your Ad Campaigns Are Flying Blind.