Helping people with computers... one answer at a time.
Cookies are a fact of life when browsing the web. But, if you look at the cookies stored on your machine, you might be surprised at the number.
I have cookies on my computer from websites that no one in my household said they had visited. Is this possible? Is there a way to tell if a cookie was an actual site visited or a third-party cookie?
Yes, it's very possible. Likely, even.
But 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.
Many sites pull resources from more than one actual web server.
For example, a site http://reallybigbookstore.com might use and leave cookies under its own name, as above.
That's one source of unexpected cookies: you visited http://reallybigbookstore.com, but you got cookies for http://somerandomservice.com because that's where some of the images (or other content) were retrieved.
Technically, that's a third-party cookie.
This is what most people think of when they think of third-party cookies. Much like the image resources that I just talked about, ads are no different. These are typically served up from another server, so that it too has an opportunity to leave cookies on your machine.
Once again, http://ask-leo.com is a fine example. I have advertising on this site provided by Google's AdSense service. As a result, you may find cookies from http://googleads.g.doubleclick.net, the server from which Google serves ads.
This is expected with advertising and when you have third-party cookies enabled.
Much like browsing history entries, cookies can be left for pop-up windows that are successfully blocked.
I know that this sounds a little odd.
The problem is that it depends on how pop-ups are detected, how quickly they're detected, and how they're blocked or closed.
In some cases, the pop-up can actually exist long enough to leave a cookie on your machine.
This is the oddest of all.
Some web browsers will pre-fetch the pages that are linked to the current page you're viewing.
For example, this page will have some links to other Ask Leo! articles. Once your browser has completed displaying this page, it may decide to go and fetch those other pages just in case you decide to go to one of them.
Why? Well, there's noticeable speed degradation as it does this while you're presumably reading the contents of this page. If you do elect to click one of those links, the resulting page will come up that much faster, having already been pre-loaded.
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'm betting that it very well may.
The net result?
Cookies for sites that you've never been to might be on your machine.
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