Known as Quantum Insert, the  man-on-the-side hacking technique has been used to great effect since 2005 by the NSA and its partner spy agency, Britain's GCHQ, to hack into high-value, hard-to-reach systems and implant malware.

Quantum Insert is useful for getting at machines that can't be reached through phishing attacks. It works by hijacking a browser as it's trying to access web pages and forcing it to visit a malicious web page, rather than the page the target intend to visit. The attackers can then surreptitiously download malware onto the target's machine from the rogue web page.

Quantum Insert has been used to hack the machines of terrorist suspects in the Middle East, but it was also used in a controversial GCHQ/NSA operation against employees of the Belgian telecom Belgacom and against workers at OPEC, the Organization of Petroleum Exporting Countries. The "highly successful" technique allowed the NSA to place 300 malicious implants on computers around the world in 2010, according to the spy agency's own internal documents—all while remaining undetected.