...Like any magic trick, the most plausible method of eavesdropping through a switched-off phone starts with an illusion. Security researchers posit that if an attacker has a chance to install malware before you shut down your phone, that software could make the phone look like it’s shutting down—complete with a fake “slide to power off” screen. Instead of powering down, it enters a low-power mode that leaves its baseband chip—which controls communication with the carrier—on.
This “playing dead” state would allow the phone to receive commands, including one to activate its microphone, says Eric McDonald, a hardware engineer in Los Angeles. McDonald is also a member of the Evad3rs, a team of iPhone hackers who created jailbreaks for the two previous iPhone operating systems. If the NSA used an exploit like those McDonald’s worked on to infect phone with malware that fakes a shutdown, “the screen would look black and nothing would happen if you pressed buttons,” he says. “But it’s conceivable that the baseband is still on, or turns on periodically. And it would be very difficult to know whether the phone has been compromised.”
The Solution
McDonald suggests users turn off their iPhones by putting them into device firmware upgrade (DFU) mode, a kind of “panic” state designed to let the phone reinstall its firmware or recover from repeated operating system crashes. In DFU mode, says McDonald, all elements of the phone are entirely shut down except its USB port, which is designed to wait for a signal from iTunes to install new firmware. (more)
P.S. If you do this, be sure to watch the tutorial about getting your iPhone out of DFU mode.
