Earlier this year AT&T and Verizon were caught modifying wireless user traffic to inject unique identifier headers (UIDH). This allowed the carriers to ignore a user's privacy preferences on the browser level and track all online behavior. In Verizon's case, the practice wasn't discovered for two years after implementation, and the carrier only integrated a working opt out mechanism only after another six months of public criticism. Verizon and AT&T of course denied that these headers could be abused by third parties. Shortly thereafter it was illustrated that it was relatively easy for these headers to be abused by third parties.
While the fracas over these "stealth" or "zombie" cookies has quieted down since, a new study suggests use of such stealth tracking is increasing around the world as carriers push to nab their share of the advertising pie. Consumer advocacy group Access has been running a website called AmiBeingTracked.com, which analyzes user traffic to determine whether or not carriers are fiddling with their packets to track online behavior. According to a new study from the group (pdf) examining around 200,000 such tests, about 15% of site visitors were being tracked by the carriers in this fashion all over the globe: