So many places, including sites you’ve never “been” to!
It’s more than possible to find cookies from websites you’ve never been to. I’d say it’s almost a certainty.
However, I can’t think of a way of telling third-party cookies apart from those sites you actually visited.
It gets surprisingly complex.
Let’s look at where cookies come from.
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Where Cookies Come From
Cookies can come from the sites you visit, sites used by the sites you visit, sites that advertise on the sites you visit, sites linked to by the sites you visit, and probably more. In short, they come from all sorts of places whether or not you actually look at pages from those specific sites.
The sites you visit
The most obvious source of cookies are those created when you visit a website.
Resources used by the sites you visit
A less-obvious way to accumulate cookies is from sites that pull resources from more than one web server.
For example, a site like https://reallybigbookstore.com might create and leave cookies under its own name, as above.
That’s one source of unexpected cookies: when you visit https://reallybigbookstore.com, you get cookies for https://reallybigbookstore.com, but you also get cookies for https://somerandomservice.com.
That’s a third-party cookie. You’re the first party, the site you visit is the second party, and any additional sites involved are third parties.
Advertisers on the sites you visit
This is what most people think of when they think of third-party cookies.
Much like the image resources I just talked about, ads are also typically served up by another server. That server, too, has an opportunity to leave cookies on your machine.
Once again, Ask Leo! is a fine example. I have advertising on this site. As a result, you may find cookies produced from a wide variety of domains used to provide those ads.
If you have third-party cookies enabled in your browser settings, you can expect to find cookies from advertising sites. (If you aren’t sure, Google “[your browser name] third party cookie setting”.)
Cookies from pop-ups
Many people block or hide pop-up windows. Even if you do, cookies can still result from them.
I know that this sounds a little odd, but the net effect is that a cookie gets left for a webpage you never visited.
It depends on how your browser detects pop-ups, what kind of pop-up technology is being used, how quickly they’re detected, and exactly how they’re blocked, closed, or hidden.
In some cases, the pop-up can exist just long enough to leave a cookie on your machine.
Cookies from sites you haven’t been to… yet
This might be the oddest of all.
Some web browsers pre-fetch pages linked on the current page you’re viewing.
For example, this page may have links to other Ask Leo! articles. Once your browser has completed displaying this page, it may decide to fetch those other pages just in case you decide to go to one of them.
Why? In a word, speed. If you click one of those links, having already been downloaded, the page will come up faster.
Now, the question is: if there are links to other sites on a page you’re viewing, does pre-loading those sites also allow them to leave cookies? I believe the answer is: maybe. It depends on the site, the browser, and several other technical details.
But if that happens, you’ll find cookies for sites you might never have visited.
Cookies are interesting, but they’re also a fundamental way the web works.
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