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When I visit a site, It connects as it should, however I notice it also connects to unknown third party sites automatically. In the Mozilla Firefox Browser, an add-on called Lightbeam shows the third party sites which I was connected to without my intention. I visited 15 sites, and it automatically connects me to nearly 50 other third party sites.

What is the reason behind this?

Can anybody explain?

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Where did you see that the browser connects to other websites? –  tumchaaditya Jan 15 at 5:00
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While 3rd party requests are not necessarily dangerous, they are rarely useful to you, often relating to advertising and/or for tracking your surfing habits across multiple websites. If this worries you (as it does me), then you should consider using the Ghostery phugin to block them. addons.mozilla.org/en-US/firefox/addon/ghostery –  Mike Chamberlain Jan 15 at 8:42
    
Note: most browsers have setting that prevent third party website to set cookies. –  jokoon Jan 15 at 13:07
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Fix that easily. –  smonff Jan 21 at 2:05

3 Answers 3

up vote 30 down vote accepted

Many different reasons.

  1. Almost all ads you see come from 3rd parties.
  2. The developers of the site may use 3rd party libraries to acheive their goals, such as jQuery and others.
  3. The site may depend on data from another site, such as an API.

The list is giant, however these 3 are the main reasons. Most are harmless and nothing to worry about, however, one should always educate themselves of the dangers.

Edit: According to pizza boy, you should worry about it

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Good answer. To clarify your second point, use of a 3rd party library would only elicit a request to another site if the library is being served from a CDN, which is often a good idea for performance. –  Mike Chamberlain Jan 15 at 8:38
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In addition, if one of the connections is encrypted, the certificate presented will (well, should) contain an URL to check for certificate revocation information. –  Simon Richter Jan 15 at 12:35
    
I don't agree with the nothing to worry about statement. Some of these third parties are tracking you, so, you should worry about it. –  smonff Jan 21 at 1:58
    
@smonff Just go eat your pizza –  Oxymoron Jan 21 at 15:24

I found the Firefox extension Request Policy to be extremely helpful in showing who grabs what from where.

You can decide whether to allow certain queries (e.g. to CDN websites and general APIs, I found out that font.googleapis.com and ajax.googleapis.com are widely used) or to block them.

It took me a while to get proficient with the most harmless queries/sites, but I really got to see the internals of everything a website is requesting to work. It has the charm that you manually control what happens, compared to plugins like Ghostery (that might forward your surfing habits to others).

Happy surfing!

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Oxymoron has given 3 common reasons, but in addition (or rather additional clarification) many if not most websites will call scripts from domains such as google-analytics. Some of these scripts attempt to gather information about individual users, while many are more interested in users' interactions with particular sites, or correlations across multiple sites.

Ghostery has already been mentioned as one way of avoiding some of these. You might also consider adblock plus (particularly useful if you are on a slow connection, or come across those ads that expand on mouseover).

Another powerful option is to install noscript. This will block all scripts from any source until you tell it to allow them. It's quite annoying at least at first, before you instruct it about sites you regularly visit. Even with most scripts allowed it still offers some protection against more esoteric threats (though that may be starting to veer off topic) but doesn't strip 3rd party images (for example) unless being called from a script.

These blocking tools aren't mutually exclusive - some combination may be helpful.

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Ghostery is interesting but it has been acquired by Evidon, it tracks behavior of people who don't want to be tracked –  smonff Jan 21 at 2:03
    
@smonff, interesting, I hadn't used Ghostery for some time, after some difficulties with it (a bug I think). Most of what it does is covered by the combination of privacy/security addons I have for other things: Noscript + Adblockplus + Sharemenot + BetterPrivacy + Unsocialize + RefControl (in approximate descending order of how much they "block") –  Chris H Jan 21 at 12:57

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