According to revelations about the QUANTUM program, the NSA can “shoot” (their words) an exploit at any target it desires as his or her traffic passes across the backbone. It appears that the NSA and GCHQ were the first to turn the internet backbone into a weapon; absent Snowdens of their own, other countries may do the same and then say, “It wasn’t us. And even if it was, you started it.”

via Our Government Has Weaponized the Internet. Here's How They Did It | Wired Opinion | Wired.com.

This includes a fairly detailed list of the methodologies employed.