“An NSA unit known as the Transgression Branch specializes in this kind of track-the-hacker work and takes things one step further. The branch watches a hacker break into another country’s computer system, then follows him inside. In a 2010 operation called Ironavenger, the Transgression Branch saw e-mails containing malware being sent to a government office in a hostile country—one that the NSA wanted to know more about. Upon further inspection, the branch discovered that the malware was coming from a US ally, whose own intelligence service was trying to break in. The Americans let their allies do the hard work and watched silently as they scooped up passwords and sensitive documents from the adversary’s system. The Americans saw everything the allies saw and got some inside knowledge about how they spied.”