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New Worm Overwrites Master Boot Records

Win32/Zimuse could make it difficult for users to access or restore their data, researchers say
Researchers are reporting a new worm in the wild that overwrites master boot records (MBRs) of all available drives with its own data, making the data stored on a user's computer inaccessible.

Restoring the corrupted data is complicated, requiring specialized software or a third-party service provider, the researchers say.

Win32/Zimuse A and Win32/Zimuse B has been spotted on hundreds of computers, according to security service provider ESET. Immediately after the outbreak, only users in Slovakia were affected, accounting for more than 90 percent of all infections. Now, however, the greatest number of infected computers is in the United States, followed by Slovakia, Thailand, and Spain, ESET says.

The worm spreads in two ways, according to ESET. It could be embedded in legitimate Websites in the form of a self-unpacking ZIP file or an IQ test program, or it could be spread via portable media, such as a USB device. Its ability to spread via portable media could allow the worm to spread faster, the researchers say.

The worm's two variants -- Win32/Zimuse.A and Win32/Zimuse.B -- differ in the way they spread and the timing of activation. While the A variant needs 10 days to start spreading via USB devices, the B variant needs only seven days since infiltration, ESET says. Moreover, the time needed for the execution of the destructive routine is shortened in the B variant from the original 40 days to 20.

If the right removal method is not used, then the worm shifts to its destructive mode, ESET warns. This is similar to making the right choice aboutwhich wire to cut -- and in what sequence -- in a bomb-defusing operation, the researchers say.

Some observers believe the worm was intended to infect the computers of fans of a motorcycle club in the central Slovakian Liptov region. Once it began to penetrate company networks, the attack spread beyond this target group, ESET says.

ESET has published the Zimuse Removal Tool, a security application that can eliminate the worm, according to the vendor.

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