According to the article, the NSA impersonated none other than Internet giant Google, using a well-known "man in the middle" attack. This type of attack works the way it sounds. The hacker inserts himself in the middle of two victims, and relays messages back and forth between the two, all the while grabbing data.
Brazilian site Fantastico obtained and published a document from by NSA leaker Edward Snowden, which diagrams how the "man in the middle attack" involving Google was apparently carried out. Both Techdirt and Mother Jones also reported on the MITM attack.
There have been rumors of the NSA and others using those kinds of MITM attacks, but to have it confirmed that they're doing them against the likes of Google, Yahoo and Microsoft is a big deal -- and something I would imagine does not make any of those three companies particularly happy.
In response to a query by a Mother Jones reporter, Google provided the following short statement:
As for recent reports that the U.S. government has found ways to circumvent our security systems, we have no evidence of any such thing ever occurring. We provide our user data to governments only in accordance with the law.
None of this should prove surprising, as the depth and breadth of NSA spying into the private lives and communications of the public has been starkly revealed. The New York Times reported last week that the NSA has managed to work around common Internet encryption methods in a number of ways, including stealing encryption keys, collaborating with tech companies to build in backdoors, and secretly introducing weaknesses into encryption standards.
A Scribd document outlining the Google MITM attack is available here.