Another Hacker Targets PCs JON SWARTZ San Francisco Chronicle In a possible slap at Microsoft Corp., a mysterious hacker unleashed a virtual assault on thousands of Windows-based machines nationwide late Monday and early Tuesday. The attack happened just hours before Microsoft CEO Bill Gates testified before the Senate on the company's business practices, authorities said Wednesday. Based on the timing and the fact that it targeted Windows machines, authorities suspect the hacker aimed the assault at the software firm. The wide-scale cyberattack disabled computers at nine of NASA's 10 field offices, the Navy, the University of California at Berkeley, UCLA, UC San Diego, UC Irvine, the Massachusetts Institute of Technology and several other colleges. Computer experts called the break- in a ``denial of service'' attack that shut down systems without altering or erasing data. [A Denial of Service (DoS) attack is not a "break-in". The attack is designed to deny service to the remote machine.] ``Our systems locked up and crashed,'' said Mike Mewhinney, a spokesman for the NASA Ames Research Center in Mountain View, which was affected. ``A computer user would see a blue screen with an error message displayed.'' Computer experts said the attack exploited a bug in the Windows 95 and NT operating systems known as ``Teardrop II,'' which causes data to overload and lock up computer systems. Macintosh machines were not affected by the assault. Mewhinney said the attack, which lasted until Tuesday morning, forced computer users to restart their machines. Microsoft has issued a free software patch on its Web site (http://www.microsoft.com/security) to fix disabled machines. Jesus Mena, a spokesman for UC Berkeley, said the university quickly built a safety border around its computer network within an hour of the attack. UC Berkeley and NASA officials did not disclose the number of machines shut down in the online raid. Peter Neumann, a computer security expert at SRI International in Menlo Park, and others said they were not surprised by the attack. They claim Microsoft and America Online are common targets for hackers because of their ``mass marketing approach to technology.'' The software giant has become a pinata of sorts for a growing number of high-tech and government critics. On Tuesday, Gates was grilled by a hostile Senate Judiciary Committee hearing on the company's business methods. And the government is suing Microsoft in an antitrust case. The identity of the online intruder is not known, sources said. The hacker attack is the second high-profile case in less than a week. Two teenage hackers from Cloverdale are suspected of repeatedly breaking into the Pentagon's computer system to view unclassified information.