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How to stop the NSA from bugging your phone even when it's off
Device firmware upgrade (DFU) mode acts kind of like a system reset - it installs new firmware and recovers the virus-free operating system. In other words, it gets rid of the malware that allows the NSA to listen in on your conversations or bug your phone.

This assumes that NSA hasn't hacked the DFU software.
Posted by: Squinty 2014-06-13
http://www.rantburg.com/poparticle.php?ID=393133