Brazilian site Fantastico obtained and published a document leaked by Edward Snowden, which diagrams how a “man in the middle attack” involving Google was apparently carried 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.

The technique is particularly sly because the hackers then use the password to log in to the real banking site and then serve as a “man in the middle,” receiving requests from the banking customer, passing them on to the bank site, and then returning requested info to the customer — all the while collecting data for themselves, with neither the customer nor the bank realizing what’s happening. Such attacks can be used against e-mail providers too.