Third-party cookie: definition, scheduled phase-out and alternatives
Updated on February 22, 2026
Quick definition
A third-party cookie is a text file placed on a user's browser by a domain different from the site being visited, enabling cross-site behavior tracking and the building of cross-site audience profiles. Third-party cookies sit at the heart of retargeting and ad targeting, but their progressive removal by browsers is forcing the industry to find GDPR-compliant alternatives.
How it works
A first-party cookie is set by the domain of the site you are visiting and can only be read by that site. A third-party cookie is set by an external script loaded from another domain (Facebook pixel, Google Analytics script, retargeting tag). This cookie allows the third-party platform to identify the user across all sites that load that script and build a cross-site behavioral profile.
The phasing out of third-party cookies has been underway for several years:
- Safari (Apple): blocked by default since 2017 via ITP (Intelligent Tracking Prevention)
- Firefox: blocked since 2019
- Chrome (Google): Google ultimately backed away from a general removal in 2024 but continues developing alternatives via the Privacy Sandbox
For analytics, the consequences are significant: tools that rely on cookies for session and conversion measurement show incomplete data on Safari and Firefox. Sublim is built to operate without any cookies, making it immune to these blocks.
Why it matters
The progressive end of third-party cookies is one of the most significant disruptions in the digital ecosystem since the rise of mobile. It forces a complete rethinking of data collection, targeting, and measurement models:
- For advertisers: reduces the precision of retargeting and lookalike audiences
- For publishers: weakens programmatic ad revenue
- For analytics teams: makes data from certain tools less and less representative of actual traffic
How to improve or use it
- 1Run a complete audit of tools that depend on third-party cookies on your site.
- 2Migrate to first-party or cookieless analytics solutions.
- 3Develop your first-party data strategy: sign-ups, loyalty programs, authenticated logins.
- 4Explore technical alternatives: server-side tracking, Privacy Sandbox, universal identifiers based on hashed emails.
- 5Prioritize direct relationships with your customers to reduce dependence on third-party platforms.
With Sublim
Sublim is built natively cookieless — neither first-party nor third-party. It places no tracker on the user's browser and measures audiences via lightweight fingerprinting and anonymous server-side hashes. This architecture guarantees complete and reliable data collection regardless of browser configuration, and full GDPR compliance with no consent banner.
Frequently asked questions
What is the difference between a first-party and a third-party cookie?
A first-party cookie is set by the domain of the site you are visiting and can only be read by that site. A third-party cookie is set by an external script loaded from another domain (Google, Facebook, etc.) and can be read on every site that loads the script, enabling cross-site tracking. First-party cookies are less affected by browser restrictions.
Did Google ultimately remove third-party cookies in Chrome?
In July 2024, Google announced that it would not proceed with a general removal of third-party cookies in Chrome, abandoning its initial timeline. Instead, it is developing the Privacy Sandbox to provide alternatives such as the Topics API. Third-party cookies therefore remain in Chrome, but they are already blocked on Safari and Firefox.
How can I measure the impact of cookie blockers on my analytics?
Compare the session volume reported by your analytics tool with the raw traffic visible in your server logs. A significant gap (>20%) indicates a major data loss. You can also compare data by browser: if Safari and Firefox show numbers significantly lower than Chrome, that reveals a tracking issue tied to cookie restrictions.
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