Peter Eckersley of EFF writes:
Whenever you visit a web page, your browser sends a “User Agent” header to the website saying precisely which operating system and web browser you are using. This information could help distinguish Internet users from one another because these versions differ, often considerably, from person to person. We recently ran an experiment to see to what extent this information could be used to track people (for instance, if someone deletes their browser cookies, would the User Agent, alone or in combination with some other detail, be unique enough to let a site recognize them and re-create their old cookie?).
Our experiment to date has shown that the browser User Agent string usually carries 5-15 bits of identifying information (about 10.5 bits on average). That means that on average, only one person in about 1,500 (210.5) will have the same User Agent as you. On its own, that isn’t enough to recreate cookies and track people perfectly, but in combination with another detail like geolocation to a particular ZIP code or having an uncommon browser plugin installed, the User Agent string becomes a real privacy problem.
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