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.

The Sites You Visit

This is perhaps the most obvious: when you visit a website and it happens to use cookies, you'll find cookies directly associated with that website on your machine.

That makes sense and it's probably what most folks immediately think of when they hear that a site uses cookies.

"Cookies for sites you've never been to might be on your machine."

http://ask-leo.com is a fine example. The comment form that you see on articles uses cookies for the Remember Me option.

Resources Used by the Sites You Visit

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.

However, http://reallybigbookstore.com might load images from another server entirely, say http://somerandomservice.com. If http://somerandomservice.com uses cookies, it now has the opportunity to place a cookie on your machine which will be associated with its domain.

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.

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 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.

Cookies from Pop-Ups

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.

Cookies from Sites You Haven't Been to ... Yet

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.

Article C4745 - February 18, 2011 « »

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Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

Not what you needed?

8 Comments
Alex
February 22, 2011 8:22 AM

really clear and useful answer. it is amazing if you browse with the cookies set to the setting that they must pop up one and by one and ask for permission before settling in your browser.

Dozens can jump at one web page and if you block all of them google refuses to work properly.

Lester
February 22, 2011 8:32 AM

I use a utility called ccleaner to periodically clean up my cookies. I have set it to leave the cookies that I need and wipe the rest. It gets rid of clutter and could also avoid misunderstandings down the road.

Rod
February 22, 2011 11:32 AM

256 byte Cookies are easily erased and do not add up all that fast. What most people are unaware of is the up-to 100,000 byte flash cookies, more correctly referred to as Local Shared Objects or .LSO's, put onto your computer by companies using Adobe Flash. Since I installed Better Privacy an a Firefox add-on last summer, 3,838 LSO's have been deleted from my computer. 264 have been removed since Feb 1st. Now if each LSO was 100k in size that would mean over 380 megabytes of unwanted information stored on my computer.

See: http://windowssecrets.com/2008/10/23/06-Flash-cookies-are-putting-your-privacy-at-risk .

Also see https://addons.mozilla.org/en-US/firefox/addon/betterprivacy/ for more information.

Richard Deem
February 22, 2011 2:29 PM

For most (maybe all) browsers you can block third-party cookies. This is what I recommend. You don't really need them and they are just used for marketing.

Snert
February 23, 2011 8:57 AM

CCleaner is nice. I recommend it! I use it to clean cookies and other junk I don't want using real estate. It's fast and efficient.

Rob
February 24, 2011 12:50 PM

I guess what it really means is: stay off the porn sites if you don't want the family to know you have been there!!!

Patrick
February 24, 2011 2:22 PM

Why doesn't Norton et al ask if you want to accept the cookie?

Because you'd spend all day answering the question for every cookie that gets left on your machine and would get nothing done.
Leo
24-Feb-2011

Joe Doran
February 28, 2011 12:59 PM

I use IE tools to delete my temp files and used to also delete cookies. I was told to leave the cookies. When I was deleting the along with the temp files , I never noticed a problem. What do you recommend. Thanks ,Joe

I rarely, if ever, delete cookies. If I remember, I turn off "accept third party cookies" but that's about as far as I go.
Leo
28-Feb-2011

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