Tuesday, January 08, 2013


I just watched Hacking Cisco Phones: Just because you are paranoid doesn't mean your phone isn't listening to everything you say, an excellent presentation by Ang Cui and Michael Costello at 29C3. I particularly liked that they coined the term "funtenna" to describe the potential capability of malware using the off-hook switch in a VoIP phone as an antenna to transmit data over RF.

I appreciate that they credited me with the idea, but I would like to set the record straight. I met Ang and Michael at a Cyber Fast Track event a couple months ago, and they approached me with the idea of exfiltrating data from the phone by toggling a GPIO pin on the embedded CPU at radio frequencies. My only contribution was looking at the hardware and suggesting that the wire extending to the off-hook switch was probably the best candidate antenna for the hack.

Although it hasn't been implemented yet, I think the idea has merit. I don't know how fast a GPIO pin can be toggled on the platform, but the CPU operates at something like 800 MHz. That makes it very likely that the maximum GPIO toggle rate is at least in the tens of MHz, maybe even over 100 MHz. I don't know the resonant frequency of the wire extending to the off-hook switch, but it is probably a few hundred MHz. If my guesses are close, then it is likely that the funtenna could be used to transmit data a short distance, perhaps through a wall or two. It isn't a very good radio, but it should work to some extent. Even a short range wireless transmission is very interesting when it originates from unmodified hardware not intended for wireless operation.

With Ang and Michael's approval, I would like to formalize the definition of "funtenna" a bit: A funtenna is an antenna that was not intended by the designer of the system to be an antenna, particularly when used as an antenna by an attacker. In the case of the Cisco phone, the funtenna could be used to transmit data from the phone. In certain systems, it may be possible to use a funtenna to receive radio signals as well. (I even know of some people working on a way to inject data into an untouched device using nothing but a high power radio signal; it is a very limited capability but theoretically possible.) The field of emission security studies unintentional radio emissions that leak data, and I would call any radiating element (a cable with poor shielding, for example) that leaks useful or sensitive information a funtenna.

Whenever I crack open an electronic device for the first time, I now look for potential funtennas. Maybe you will too. :-)