What are third-party cookies?

Definition

A third-party cookie is a cookie set by a domain other than the website you are visiting, usually through embedded content such as ads. It can let the company behind it recognise you on other sites that use the same service.

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Privacy
Also called
third party cookies, 3rd party cookies
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When you open a website, it can store small text files called cookies in your browser. A first-party cookie is set by the site you are on. A third-party cookie is set by a different domain, which typically happens when the page embeds something from another company: an ad, an image, a social media plugin.

The Office of the Privacy Commissioner of Canada describes the mechanism: an advertiser's cookie can be placed while you load a page, and when you visit that site again, or another site that uses the same advertising company, the advertiser can read the cookie.

Why that matters

If the cookie holds a unique identifier, the company can link your visits to different websites together. The UK ICO notes that third-party cookies can link a person's activity across different sites and devices. That linked history is what advertisers use to build interest profiles and show you targeted ads.

A typical case: you look at a pair of boots on one site, and ads for those boots follow you across unrelated sites for days. The ad company's cookie on both sites is the thread.

First-party vs. third-party cookies

First-party cookies are usually what keep you logged in or remember items in your basket. Third-party cookies mostly serve the company that placed them, not the site you chose to visit. Blocking third-party cookies often leaves a site working normally.

Cookies are only one tracker

Blocking them reduces tracking but does not end it. Sites and ad networks can also link activity with other identifiers, such as your account, device details or email address.

How to protect yourself

  1. 1

    Block third-party cookies in your browser

    Open your browser's privacy settings and set third-party cookies to blocked. Most sites keep working.

  2. 2

    Clear cookies now and then

    Delete stored cookies regularly, or set the browser to clear them when you close it.

  3. 3

    Choose carefully at cookie banners

    If a banner offers a reject option, use it. Rejecting non-essential cookies stops most third-party ones from being set.

  4. 4

    Check what is listed about you elsewhere

    Cookie-based profiles are separate from data brokers, but brokers also sell profiles of you. Serus finds where your address and phone number are listed and removes them. See our data broker guides.

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Frequently asked

Are third-party cookies dangerous?

They are a tracking tool rather than malware. The privacy concern is that they can link your activity across sites into a profile you did not choose to build.

How do I block third-party cookies?

In your browser's privacy or cookie settings, choose the option to block third-party cookies. The exact menu name varies by browser.

What is the difference between first-party and third-party cookies?

A first-party cookie is set by the site you are visiting. A third-party cookie is set by another domain, such as an ad network, through content embedded on that site.

Sources

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