They have not been able to establish how exactly the hackers broke into the system, but believe it may have been through a so-called spear-phishing attack, where an employee clicked on an email or link containing malicious code.

BBC: New York Times 'hit by hackers from China'

The NYT suspects a so-called "spear phishing" attack that sent targeted, booby-trapped messages to a few key individuals.

BBC: How the New York Times cleaned house after its hack attack

The rise of social networks, for example, has aided an increasingly common kind of attack known as spear-phishing, says Greg Bell, global service leader for KPMG's information protection practice.

WSJ: Tech Companies Struggle to Keep Hackers at Bay

The White House would not say if the attack originated in China, describing it as a "spear-phishing" attempt.

BBC: The White House as seen on 17 March 2010