On the afternoon of Oct. 24, a Unix engineer was told he was being fired because of a scripting error he'd made earlier in the month, but he was allowed to work through the end of the day. Five days later, another Unix engineer at the data center discovered the malicious code hidden inside a legitimate script that ran automatically every morning at 9:00 a.m.

Had it not been found, the FBI says the code would have executed a series of other scripts designed to block the company's monitoring system, disable access to the server on which it was running, then systematically wipe out all 4,000 Fannie Mae servers, overwriting all their data with zeroes.

The U.S. housing market lost $3.3 trillion in value last year and almost one in six owners with mortgages owed more than their homes were worth as the economy went into recession. The median estimated home price declined 11.6 percent in 2008 to $192,119 and homeowners lost $1.4 trillion in value in the fourth quarter alone. The U.S. economy shrank the most in the fourth quarter since 1982, contracting at a 3.8 percent annual pace, the Commerce Department said. Record foreclosures have pushed down prices as unemployment rose. More than 2.3 million properties got a default or auction notice or were seized by lenders last year.