  Browser fingerprinting is a relatively new method of uniquely identifying
browsers that can be used to track web users. In some ways it is more
privacy-threatening than tracking via cookies, as users have no direct control
over it. A number of authors have considered the wide variety of techniques
that can be used to fingerprint browsers; however, relatively little
information is available on how widespread browser fingerprinting is, and what
information is collected to create these fingerprints in the real world. To
help address this gap, we crawled the 10,000 most popular websites; this gave
insights into the number of websites that are using the technique, which
websites are collecting fingerprinting information, and exactly what
information is being retrieved. We found that approximately 69\% of websites
are, potentially, involved in first-party or third-party browser
fingerprinting. We further found that third-party browser fingerprinting, which
is potentially more privacy-damaging, appears to be predominant in practice. We
also describe \textit{FingerprintAlert}, a freely available browser extension
we developed that detects and, optionally, blocks fingerprinting attempts by
visited websites.
