One afternoon late last month, security researcher Hristo Bojinov placed his Galaxy Nexus phone face up on the table in a cramped Palo Alto conference room. Then he flipped it over and waited another beat. And that was it. In a matter of seconds, the device had given up its “fingerprints.” Code running on the website in the device’s mobile browser measured the tiniest defects in the device’s accelerometer — the sensor that detects movement — producing a unique set of numbers that advertisers could exploit to identify and track most smartphones. It turns out every accelerometer is predictably imperfect, and slight differences in the readings can be used to produce a fingerprint. Marketers could use the ID the same way they use cookies — the small files that download from websites to desktops — to identify a particular user, monitor their online actions and target ads accordingly.