IMF officers were notified of the attack last Wednesday, though it had been going on several months, according to the New York Times.
IMF officials didn't say whether they thought the attack came from criminals interested in using inside information to move or anticipate stock markets, or from a member country interested in using the information for a different benefit of its own.
It also declined to offer details on the exploit except to say some form of malware was involved and that its source is unclear.
The FBI is investigating (though it's not clear why the Pentagon felt the need to tell us this and the FBI didn't).
CIA director Leon Panetta told Congress June 9 the country faces a "real possibility" of a crippling attack on utilities, government facilities, financial and security systems.
Most experts theorize the malware infection resulted from a spear-phishing attack – a combination exploit China has used successfully in frequent attacks on U.S. government and financial institutions according the U.S. security officials.
They don't know for sure; or at least, if they do, they're not being allowed to give details that prove their information is more accurate than the rest of the hearsay.
Former World Bank security exec tom Kellerman is quoted all over the place as saying the attack was closely targeted and that it used sophisticated malware created to create a digital data-gathering presence inside secure networks.
He runs his own security company now, rather than the World Bank's security, so his specific knowledge of the malware involved is probably only one or two friend-of-a-friend-type sources closer to the real information than anyone else.