The majority of the most popular applications available for iPhones use the devices' serial numbers to track users, a professor at Bucknell University says in a new research paper.

For the report, assistant director of information security and networking Eric Smith examined 57 of the most popular iPhone apps and found that 68% of them transmitted the devices' unique serial numbers to remote servers owned by either the developer or an advertiser. Furthermore, many applications -- including Amazon, Facebook and Twitter -- also collected users' log-in data...