From Cnet.com:

Earlier this week, Techdirt picked up on a passing mention in a Brazilian news story and a Slate article to point out that the US National Security Agency had apparently impersonated Google on at least one occasion to gather data on people. (Mother Jones subsequently pointed out Techdirt’s point-out.)

A technique commonly used by hackers, a MITM attack involves using a fake security certificate to pose as a legitimate Web service, bypass browser security settings, and then intercept data that an unsuspecting person is sending to that service. Hackers could, for example, pose as a banking Web site and steal passwords.