The iPod Sport kit is not secure and may put joggers at risk of being tracked by thieves and stalkers, say researchers.

The Nike iPod Sport kits uses a receiver plugged into the iPod Nano to read signals from a sensor in the user's shoe. It then measures distance travelled, calories burned, and other statistics.

However, researchers from the University of Washington have found that it “fails to offer even the most basic level of user privacy”.

Those who target the joggers can use a receiver to detect the transmitter in the shoe, and as there's no encryption on the signal, it's easy to keep an eye on a user's whereabouts.

The team carried out two different experiments to see how it could be done. The first hardware setup involved an easily-made receiver that logged when a sensor/transmitter was nearby, and then, via Wi-Fi, transmitted the information to a server that showed the location on Google Maps.

The second involved simply plugging the receiver into a laptop to log the user's movements.

Professor Tadayoshi Kohno from the university said, “This situation begs the broader question: as manufacturers continue to introduce other new, sophisticated technological personal gadgets, will these new gadgets erode our privacy even further?”.

The researchers suggest linking each sensor to its receiver, as well as encrypting the information transmitted.