"We started seeing it yesterday," said David Wray, a spokesman with the US Department of Homeland Security, which monitors glitches in the information technology sector as part of its mandate to safeguard US domestic security.

"It's being called alternately the "MS Blast" or "Blaster" or "Lovesan," he said.

"We've been asking people to update their Microsoft operating systems to patch for the vulnerability that this worm is exploiting," Wray said.

Officials were unable to say how many computers have fallen victim to date.

Among the Microsoft programs targeted by the bug are the highly popular Windows 2000 and Windows XT, used in millions of computers around the world.

"They are most prevalent operating systems on home computers and in most corporations," he said.

He said the fix can be made by logging on to the Microsoft home page and downloading the appropriate software.

"That will prevent their system from being compromised and will help minimize the spread," he said.

The worm does not appear to erase files or create major damage other than duplicating itself to other e-mail addresses in a computer user's address book.

But security officials fear that hackers will devise copycat versions of the worm that can wreak far greater havoc if the remedy is not applied soon.

In the meantime, the presence of the bug can make for sluggish computer response, Wray said.

"Anytime there's excess traffic on the Internet, there's a noticeable degradation of access and service," he said.

"It could be a lot worse, but it's not a good thing."

SPACE.WIRE