They do physically intercept some hardware, so that's one vector: http://www.theguardian.com/books/2014/may/12/glenn-greenwald-nsa-tampers-us-internet-routers-

They do physically intercept some hardware, so that's one vector: http://www.theguardian.com/books/2014/may/12/glenn-greenwald-nsa-tampers-us-internet-routers-snowden http://www.theguardian.com/books/2014/may/12/glenn-greenwald... Alternatively could they use software exploits in an OS to execute arbitrary code and rewrite the firmware?