He said it could inspire attacks on other large institutions. "If they can't catch them, I'm afraid it might embolden others to try," said Moss, who is chief security officer for ICANN.
But cyber security experts cautioned it might be difficult for investigators to prove which nation was behind the attack.
"Even developing nations are able to leverage the Internet in order to change their standing and ability to influence," said Jeffrey Carr, author of the book, "Inside Cyber Warfare."
"It's something they never could have done before without gold or without military might," Carr said.
Experts say cyber threats are increasing worldwide. CIA Director Leon Panetta told the US Congress this week the United States faced the "real possibility" of a crippling cyber attack.
"The next Pearl Harbor that we confront," he said, could be a cyber attack that "cripples our power systems, our grid, our security systems, our financial systems, our governmental systems."
"This is a real possibility in today's world," Panetta told a June 9 confirmation hearing in his bid to become the next US defense secretary.
'Suspicious file transfers'
Bloomberg News reported the attack occurred before the May 14 arrest of former IMF Managing Director Dominique Strauss-Kahn on sexual assault charges. It resulted in the loss of e-mails and other documents, Bloomberg said.
The New York Times cited computer experts as saying the IMF's board of directors was told of the attack on Wednesday, though the assault had lasted several months.
An Internal IMF memo issued on Wednesday warned employees to be on their guard.
"Last week we detected some suspicious file transfers, and the subsequent investigation established that a Fund desktop computer had been compromised and used to access some Fund systems," said a June 8 email to employees from Chief Information Officer Jonathan Palmer.
Details of the email were first reported by Bloomberg. Reuters' sources confirmed the wording of the email.