A variety of worms that exploit a Windows vulnerability disclosed last week are causing major problems across the world.

The worms affect Windows 2000 systems, as well as earlier versions of Windows XP, and the fast-paced attack is reported to have sent Microsoft into ‘emergency response’ mode, according to CNN.

The attacks are reported to have shut down Windows PCs in the US, Germany and Asia. Computers in government departments, at CNN, ABC and the New York Times are all reportedly affected.

Microsoft tells users - ‘update now’

"The next twelve hours will tell us, is this going to be big, or is it just going to go away in the next couple of hours?" said Joe Hartmann, director of antivirus research at Trend Micro, in Cupertino, California.

Microsoft’s director of its security response centre, Debbie Fry, told CNN: “The key thing I want to stress for customers is making sure that they install security updates as quickly as possible."

Hartmann warned that the worms could spread as fast and wide as Code Red or Nimda. Like those worms, they are designed to seek out all vulnerable systems on a network once they get onto one computer.

Technical specs

It's not clear exactly what worms are involved in the attacks now or what effect they ultimately will have on the systems they infect, according to Hartmann. Trend Micro has received reports of many attacks, some involving variants of the Zotob worm and others involving the Rbot worm. It has received reports of systems being shut down and restarted repeatedly.

The worms spread using the TCP/IP (Transmission Control Protocol/Internet Protocol) port 445, which is associated with Windows file sharing, and take advantage of the Plug and Play system bug to seize control of the operating system. Infected computers are then told to await further instructions on an IRC (Internet Relay Chat) channel, meaning that they could then be used to attack other systems, according to Johannes Ullrich, chief research officer at The SANS Institute, a security training company in Bethesda, Maryland.

Limiting the danger is the fact that the worms can only effectively attack Windows NT and Windows 2000 systems, according to Hartmann. A patch for the vulnerability was released by Microsoft last Tuesday.

Microsoft claims 200 million users to have downloaded the patch.

A patch too far?

Many enterprises don't apply patches immediately for fear of crippling custom software, Ullrich explained. Microsoft can test a patch against all its products but not against an individual company's special applications.

"More often than not, a patch will actually do more damage than good if you roll it out too quickly without testing it first," Ullrich said.