Kashmir Hill writes:
The generally accepted trade-off on the Internet is that you give up your privacy to get free stuff. It’s summed up by a frequently repeated adage, “If you’re not paying for it, you’re the product.” But sometimes you’re paying for it, and you’re still the product. Verizon and AT&T customers are paying an (often steep) monthly bill, but the payment doesn’t ensure privacy. Researchers say the carriers are inserting a unique code into customers’ browser requests to help serve up personalized ads. The way they are doing it makes you trackable by the sites you visit, third party ad networks, or, of course, the NSA, even if you take measures to protect your privacy, such as clearing your cookies.
Wired reports that it was first discovered by digital rights group EFF. Kenn White, a security consultant, created a site where mobile users can find out whether their phone is broadcasting the tracking code.
Read more on Forbes.