  How easy is it to uniquely identify a person based solely on their web
browsing behavior? Here we show that when people navigate the Web, their online
traces produce fingerprints that identify them. Merely the four most visited
web domains are enough to identify 95% of the individuals. These digital
fingerprints are stable and render high re-identifiability. We demonstrate that
we can re-identify 80% of the individuals in separate time slices of data. Such
a privacy threat persists even with limited information about individuals'
browsing behavior, reinforcing existing concerns around online privacy.
