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Third-party cookies

The tracker that follows you across sites.

HOW IT WORKSA browser opens awebsite that carriesan ad company's codeThe ad companyasks the browser tostore its cookie,holding an IDThe browser opensanother site with thesame ad company's codeThe browser sends backthe same cookie, sothe ad company linksthe two visitsThe ad company buildsa record of the sitesvisited and aims adswith itOne outside company can recognise the same browser on thousands of sites, which is why browsers andprivacy laws now restrict it.HOW IT WORKSA browseropens awebsite thatcarries an adcompany'scodeThe adcompany asksthe browserto store itscookie,holding an IDThe browseropens anothersite with thesame adcompany'scodeThe browsersends backthe samecookie,so the adcompanylinks thetwo visitsThe adcompanybuilds arecord of thesites visitedand aims adswith itOne outside company can recognise the same browser on thousands of sites, which is why browsers andprivacy laws now restrict it.HOW IT WORKSA browser opens a website that carries anad company's code↓The ad company asks the browser to storeits cookie, holding an ID↓The browser opens another site with thesame ad company's code↓The browser sends back the same cookie,so the ad company links the two visits↓The ad company builds a record of thesites visited and aims ads with itOne outside company can recognise the same browser onthousands of sites, which is why browsers and privacy lawsnow restrict it.
An ad company's cookie, stored while one site loads, is sent back from every other site that carries the same company's code, so the ad company can link one browser's visits.
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Third-party cookies are small files that a web browser stores on behalf of a company other than the website being visited, usually an advertising company. Because the same advertising company's code appears on thousands of websites, its cookie lets it recognise one browser on all of them, and build a record of where that person has been. Browser makers and privacy laws now restrict this.

Here is how it works in general. A cookie is a small text file that a website asks the browser to store. It holds an identifier, such as a random number. When the browser visits that website again, it sends the cookie back, so the site recognises the visitor. A first-party cookie belongs to the site in the address bar. It keeps you logged in, or remembers your basket. A third-party cookie is set by another company whose code runs on that page, such as an ad network. Imagine that an ad company's code runs on a shoe shop's site and on a cricket site. When you visit both, your browser sends the same ad company's cookie each time. The ad company now knows that one browser looked at shoes and then read about cricket.

Why are they being restricted? People did not know they were being followed, and could not see or delete the records. The Safari browser blocks all third-party cookies by default. The Firefox browser blocks the cookies of known tracking companies by default. It also keeps every other third-party cookie separate for each website, so the cookie cannot follow a person to other sites. Chrome, the most used browser in India, planned to remove third-party cookies, but Google has decided to keep them. Under India's Digital Personal Data Protection Act, companies must get a person's consent for this kind of tracking from 13 May 2027. In the European Union, the General Data Protection Regulation (GDPR) and the ePrivacy Directive already require it.

Take, for example, a person who looks at a sofa on a furniture website. Retargeting uses a third-party cookie to show that sofa in ads on other websites the next day.

Consider a news publisher whose readers' cookies are blocked. Its contextual ad network chooses ads by what the page is about, such as a recipe, instead of by who the reader is.

Say a brand wants to find its own customers online without cookies. Data onboarding matches the brand's customer list to people's online identities through hashed email addresses and phone numbers, inside a secure system.

How are third-party cookies different from first-party data? A third-party cookie lets an outside company follow a person across sites, even though the person never chose to share data with that company. First-party data is collected by the company that the person actually dealt with, on its own site or app. That is why first-party data is becoming more valuable as third-party cookies fade.

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