Thanks to Bob Brown's hint and an article on Ars Technica he is probably referring to, we get to know at least some details on the hack rather than just gossip.
From the article:
And it still remains unclear how the malware was implanted on Sony Pictures’ network in the first place—or how multiple terabytes of data from corporate systems could have been hauled out of the network within just a few days of the wiper attack.
According to the article, one malware used in the attack is Destover or a variant thereof, also known as (a) Wiper, which has also been subsequently code-signed with a Sony certificate and was used to remove/destroy data on the victim's computers, e.g. to cover up and remove traces of an intrusion.
According to Security Week, attackers used a worm for Windows networks (referred to as SMB Worm Tool) to propagate their malware with brute-force authentication code and tools for file transfer, system survey, process manipulation, file time matching and proxy capability, an arbitrary code execution mechanism and aforementioned features to erase and destroy data.
A related Security Week article also refers to the US CERT alert detailing some Indicators of Compromise (IOC), i.e. hashes of the malware that can be added to security solutions / intrusion detection systems and malware detection engines.