Whenever radio frequencies are involved, there's a potential security risk. Could it be possible for an unscrupulous person to eavesdrop on communications between NFC devices? The answer is a resounding yes. With the right antenna, hardware and software, it's possible to snoop on transactions.
Even though NFC transmissions must take place over very short ranges -- 10 centimeters is the maximum distance, with many applications requiring even shorter ranges -- it's possible to pick up transmissions from much further away. Defining exactly how far away an eavesdropper can be isn't easy. It relies on several factors, including whether the information is being sent in active or passive mode, the type of antenna and receiver the eavesdropper is using and how much power the active component pours into the transmission. It's possible that someone trying to listen in on an active component could get a signal as far away as 10 meters [source: Haselsteiner and Breitfuß].
It's harder to detect transmissions from passive components. Even so, an eavesdropper could detect signals from about a meter away with the right equipment. To prevent someone from getting valuable data -- including your financial information -- hardware and software manufacturers use encryption to keep valuable information away from prying eyes. With encryption, both components need a specific type of key to decrypt information into something useful. An outsider without access to the key would only see gibberish.
Another way NFC devices could prevent eavesdroppers from stealing information is if both devices involved information simultaneously. Here's how it works: Both devices begin to transmit a random series of bits, which are either 0s or 1s. An eavesdropper would be able to tell if both devices transmitted a 0 or a 1 at the same time. But what if one device transmits a 1 and the other a 0? The two devices know which is which, but an eavesdropper would be unable to tell. At that point, the two devices could simultaneously communicate and mask communications so that the eavesdropper can't make out what is being transmitted -- there's no way to know who's sending which bit.
Another potential problem with NFC is that someone could attempt to disrupt communications by broadcasting radio signals in the NFC spectrum during transactions. While this isn't the same as eavesdropping, it could be a source of annoyance.
Before we have to worry about these kinds of problems, we'll have to adopt NFC technology on a wider basis. It remains to be seen if NFC will take off. If it does, you may be able to ditch most of your gear and rely primarily on your smartphone. The utility belt industry could be headed for some lean years pretty soon.