Here you can read some details about the hack.

1,600 physical and virtual Linux and Unix servers, and 811 Windows servers. Additionally, a spreadsheet in the leak included the location, IP address, MAC address, Windows computer name, and assigned username of over 3,000 individual PCs in North America and over 7,700 more worldwide on Sony Pictures’ network. These details allowed the attackers to pick out Sony Pictures’ most vulnerable servers and infrastructure.

Hackers promise “Christmas present” Sony Pictures won’t like
http://arstechnica.com/security/2014/12/hackers-promise-christmas-present-sony-pictures-wont-like/

State-sponsored or not, Sony Pictures malware “bomb” used slapdash code
http://arstechnica.com/security/2014/12/state-sponsored-or-not-sony-pictures-malware-bomb-used-slapdash-code/

http://marcrogers.org/2014/12/18/why-the-sony-hack-is-unlikely-to-be-the-work-of-north-korea/

https://www.riskbasedsecurity.com/2014/12/a-breakdown-and-analysis-of-the-december-2014-sony-hack/