# Third-party cookies and the construction of cross-site browsing histories | Dead Internet Theory Dead Internet Theory: Third-party cookies historically let the same embedded company recognize a browser across unrelated sites, helping construct cross-site browsing histories even when the user never intentionally visited the tracking company. 2026-05-27 :: Switchboard Tech Services ![Third-party cookies and the construction of cross-site browsing histories | Dead Internet Theory](https://switchboardtechservices.com/static/images/defaults/articles.webp) Part of the [Dead Internet Theory Series](https://switchboardtechservices.com/articles/dead-internet-theory-series/). You visit a shoe store. Then a newspaper. Then a recipe site. Three unrelated websites. One advertising company may have been present on all three. That is the basic trick that made third-party cookies one of the classic technologies of cross-site tracking. ## The cookie is ordinary; the context makes it third-party There is no magical file format called a **third-party cookie**. WebKit's tracking documentation explains the distinction clearly: if `news.example` is in the address bar while the page loads a resource from `adtech.example`, the news site is the first party and the ad-tech domain is a third party. If the browser allows that third party to read and write its cookies in this context, those cookies can carry an identifier across sites. See [WebKit's Tracking Prevention documentation](https://webkit.org/tracking-prevention/). Imagine the browser receives this identifier from the tracker: `user=847219` Later, another unrelated site loads the same tracker. If the browser sends `user=847219` again, the tracker can recognize the same browser. Now the tracker can associate both visits with one identifier. Repeat that across hundreds of participating sites and the result can become a browsing history. ## A browsing history reveals patterns, not just pages One visit says little. A sequence can say much more. Travel research followed by mortgage calculators, moving companies, and school-district pages may suggest a relocation. Repeated visits to automotive sites can reveal purchase interest. Visits to hobby stores, political publications, technical forums, financial pages, or health-related sites can contribute additional categories and inferences. Mozilla's current Firefox documentation describes cross-site cookies in similar terms: when the same tracker appears on multiple sites, it can use cookies to build a more complete profile of browsing activity over time. See [Mozilla's explanation of third-party trackers](https://support.mozilla.org/en-US/kb/third-party-trackers). That profile does not require the person to intentionally visit the tracking company's own website. The tracker rides inside other pages. ## Browsers have sharply restricted this technique The old third-party-cookie model no longer works uniformly across browsers. Safari blocks third-party cookies by default as part of Intelligent Tracking Prevention. Firefox enables Total Cookie Protection by default, isolating third-party cookies into separate per-site cookie jars so the same cookie cannot simply follow the browser across unrelated sites. Chrome took a different path. Google announced in April 2025 that it would maintain user choice for third-party cookies rather than roll out a new standalone prompt or complete a universal deprecation. Chrome blocks third-party cookies by default in Incognito mode, while regular browsing exposes controls that let users allow or block them. See [Google's April 2025 Privacy Sandbox update](https://privacysandbox.google.com/blog/privacy-sandbox-next-steps) and [Chrome's cookie controls](https://support.google.com/chrome/answer/95647). So in 2026, saying **browsers killed third-party cookies** is too broad. The reality depends on the browser and settings. ## Blocking cookies does not end tracking Trackers can also use link decoration, browser fingerprinting, first-party storage, account logins, server-side data sharing, IP-derived signals, and other techniques. WebKit's own tracking-prevention work discusses defenses against several of those methods because restricting one identifier creates incentives to find another. That is why third-party cookies belong at the beginning of **Surveillance Economy — The Internet Watching You Back**, not the end. They are the easiest version of the idea to see. You thought you were visiting three websites. The interesting question is whether a fourth company quietly recognized you at all three. Part of the [Dead Internet Theory Series](https://switchboardtechservices.com/articles/dead-internet-theory-series/). - Categories: [Dead Internet Theory](https://switchboardtechservices.com/articles/categories/dead-internet-theory/) - Tags: [#Tracking](https://switchboardtechservices.com/articles/tags/tracking/), [#Dead Internet Theory](https://switchboardtechservices.com/articles/tags/dead-internet-theory/), [#Third-Party Cookies](https://switchboardtechservices.com/articles/tags/third-party-cookies/), [#Browsing Histories](https://switchboardtechservices.com/articles/tags/browsing-histories/), [#Surveillance Economy](https://switchboardtechservices.com/articles/tags/surveillance-economy/) [Articles RSS](https://switchboardtechservices.com/feeds/articles.xml "Subscribe using an RSS reader") --- Canonical URL: https://switchboardtechservices.com/articles/dit-third-party-cookies-and-the-construction-of-cross-site-browsing-histories/ Language: en-us Content type: article Author: Switchboard Tech Services Published: 2026-05-28T04:00:07+00:00