A variety of worms that exploit a Windows vulnerability disclosed last week are hitting many systems worldwide, reportedly including some at cable network Cable News Network (CNN), and could reach critical mass in the next several hours, according to anti-virus vendor Trend Micro.

"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, Trend Micro's director of anti-virus research.

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

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 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 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.

Judging from reports to SANS Institute from users who monitor attempted attacks against their firewalls, this is not a particularly widespread attack, according to Ullrich.

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.