I am trying to determine if the information shown on www.whatsmyip.org is the absolute maximum amount of information that a webserver can obtain from a web visitor. Are there other sites that will be able to get more information from the user passively like this?

I'm not talking about port-sniffing or any kind of interaction from the user, just the information that a server can get from a 'dumb' visit.

This question was a Super User Question of the Week.
Read the blog entry for more details or contribute to the blog yourself

7 Answers 7


There is more: the Electronic Frontier Foundation (EFF) brought out a tool called Panopticlick which shows mostly the same information but additionally scans your installed fonts.

Installed fonts are probably the most identifying piece of information as soon as you start adding one or two. Just because of the amount of fonts out there, it is unlikely to have the same set of fonts on two different computers. (As long as they are used by different persons)

Edit (from comments): A countermeasure to this is either disabling JavaScript (through an addon like NoScript for example) or to disable both Java and Flash plugins in the browser, as at least one of them is needed to extract the information.

  • 2
    This requires Java in order to extract some of its information (and it gets very little if you decline the prompt to allow Java on the site) - the test OP linked gathers far more using passive means.
    – PhonicUK
    Sep 5, 2012 at 14:10
  • 1
    It does not require Java, it requires JavaScript. Most people don't have an addon like NoScript installed in their browser, thus in most cases all the information can be extracted. The sites doing this kind of scans will normally not ask the user if they are allowed to do.
    – Baarn
    Sep 5, 2012 at 14:13
  • 2
    Yes it does use java, it has a java applet that does the font check. Chrome even prompts you when you visit the page for whether or not you wish to allow the applet to run. Do an inspect element on the page and you see <applet codebase="java" code="fonts.class" id="javafontshelper" name="javafontshelper" mayscript="true" width="1" height="1"></applet>
    – PhonicUK
    Sep 5, 2012 at 14:37
  • 1
    @Indrek I can confirm this, as soon as you have both Java and Flash disabled, no fonts can be extracted.
    – Baarn
    Sep 5, 2012 at 14:59
  • 2
    If it can't do it via Java it uses Flash instead. If you disable both flash and java it just gives "No Flash or Java fonts detected". You can't get the list of fonts just using Javascript. Granted it's passive in so far that it doesn't require any interaction from the user but extras are still required to do it.
    – PhonicUK
    Sep 5, 2012 at 14:59

How do they get it?

Passive identifiable information is mostly collected from headers of the communication packets.

When a browser requests a URL, this requests undergoes through several layers of OSI model and several network protocols. The upper level protocols such as HTTP and TCP/IP probably provide most of the information displayed on that web site. This information is usually stored in a packet header and was originally embedded there to help servers understand: what is the best representation of the information for your environment.

A user-friendly list of current HTTP headers is available from Wikipedia. A more technical reference is RFC 2616 Header Field Definitions or RFC 2616 itself, see section 14.

How to protect your privacy?

Another very popular technique to track a user is via specific cookie - this is how ad providers know which ad to show you (which makes me very wary). See answers for my question: How to remove tracking cookies. Answers actually cover a lot more possible defences against other tracking techniques.

Perhaps a more secure way to stay anonymous online is to use some dedicated security projects, one of which is TOR.


In terms of information you can obtain passively without using Java/Flash - that's pretty exhaustive.

You could perhaps do things like estimate PC performance using a JavaScript benchmark, but you're really pushing at that point.


That page doesn’t really show much if you simply deny the browser prompts to run plugins, allow location detection, etc.

The hostname, IP address, etc. can be easily hidden via a proxy, and browser/OS information can easily be spoofed via extensions and such.

In the end, unless you install and allow third-party plugins, web-sites cannot gather much information because browsers are specifically designed to limit how much access they have to a system. The most common tool that sites use to collect data is cookies, but there’s limits to how much they can report as well.

The only real way for a site to get unfettered access to your system is to try to exploit a vulnerability in the browser or one of its plugins, but you can mitigate even that by installing as little as possible and keeping them updated.


There is something extra what the previous answers don't list:

A website can track which other websites you have visited (before the last time you erased your browsing history).

How is it done?

Your browser colors links differently, based on whether you visited them before, or not. A website can make a big list of a lot of well-known websites (of which the site wants to know if you visited them), and display that list in a way the user cannot see it (hidden behind an image, with a font size of 1 pixel, with the same color as the background, etc.) Now a script scans how the list is "displayed" by the browser, and can know which of them were visited.

  • 1
    I heard about this before, but I think it is no longer possible.
    – Baarn
    Sep 11, 2012 at 20:46
  • These days (2012), when a modern browser allows this, it's treated as a serious security vulnerability. For example, a beta (?) release of Firefox 16 was pulled recently when the developers realised they were vulnerable to this exploit. This was considered a serious enough near-miss to be a news story: bbc.co.uk/news/technology-19909106 Oct 16, 2012 at 23:05

Just found this site, didn't see it mentioned above: http://browserspy.dk Quite interesting, to say the least!

BrowserSpy.dk is the place where you can see just how much information your browser reveals about you and your system.

Did you know that all websites that you visit can find out which fonts you have installed?

It's also possible to find out if you have a range of programs installed. These include Adobe Reader, OpenOffice.org, Google Chrome and Microsoft Silverlight. Perhaps even which sites you have visited lately can be detected!

When you surf around the internet your browser leaves behind a trail of digital footprints. Websites can use these footprints to check your system.

BrowserSpy.dk is a service where you can check just what information it's possible to gather from your system, just by visiting a website.


You can also detect installed fonts using JavaScript/CSS. No Java or Flash usage is required to accomplish this task.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .