How do cookies let websites follow you around?
Cookies were invented in 1994 to make online shopping carts work. Advertisers found a second use.
A cookie is a small block of data that a creates while you browse a site and that your web browser stores on your device. Websites use cookies to remember things about you, and advertisers use a particular kind, third-party cookies, to recognise you on many different sites.
The idea came from a practical problem. In June 1994, programmer Lou Montulli had the idea of using magic cookies in web communications, a trick already known in computing. Cookies provided a solution to the problem of reliably implementing a virtual shopping cart. They let a server store information such as the items you added to a basket, and cookies let a site know you are logged in, and with which account.
The same tool can follow you. A cookie is normally sent only to the server that set it, or one in the same Internet : that is a . But a web page may contain images or other parts stored on servers in other domains. A third-party cookie belongs to a domain different from the one shown in the address bar. It typically appears when a page features content from an external site, such as a banner advertisement. That opens up the potential for tracking a user's browsing history, and advertisers use it to serve relevant advertisements. If the same advertiser's ads sit on many websites, it can use its cookies to build up a browsing history of the user across all of them.
Step 1: Site A carries an advertiser's banner
Loading it sets a cookie from the advertiser's
Step 2: Site B carries the same advertiser's banner
Your browser sends the same cookie back
Step 3: The advertiser links the visits
A browsing history builds up across both sites
Most people did not know any of this at first. The public learned about cookies after the Financial Times published an article about them on 12 February 1996.
Today browsers differ. Since 2020, Apple Safari, Firefox and Brave block all third-party cookies by default. In July 2024, Google announced a plan to avoid blocking them by default in Chrome and to prompt users to allow them instead. And in Europe, the law requires websites targeting EU member states to gain before storing non-essential cookies, which is why so many sites show a banner. Cookies that are strictly needed for a service you asked for, such as remembering what is in your shopping basket, do not need that consent.
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Recap
The same cookie that remembers your basket can, when it belongs to an advertiser's domain, help track you across sites.
๐ก A trick to remember it ยท Picture a coat-check ticket: it lets one cloakroom find your coat, but a ticket handed out by an advertiser on every street is a trail of crumbs.
Surprising fact ยท Cookies began as a way to make shopping carts work, in 1994.
Sources (1)
No source, no claim. Every fact in this lesson (18 claims) cites at least one of these.