One day after Microsoft issued a rare emergency Windows security patch, the bad guys have a few new ways to take advantage of the bug.

By Friday, security researchers had identified a new worm, called Gimmiv, which exploited the vulnerability, and a hacker had posted an early sample of code that could be used to exploit the flaw on the Web.

Microsoft issued the patch more than two weeks ahead of its next security updates because the bug could be used to create an Internet worm attack and Microsoft had already seen a small number of attacks that exploited the flaw.

This vulnerability lies in the Windows Server service used to connect with other devices on networks. Although the firewall software that ships with Windows will block the worm from spreading, security experts are worried that the flaw could be used to spread infections between machines on a local area network, which are not typically protected by firewalls.

And that's exactly what the Gimmiv worm is designed to do, according to Ben Greenbaum, a senior research manager with Symantec. "It is downloaded onto a target machine via social engineering and then proceeds to scan and exploit machines on the same network, using this newly disclosed vulnerability in the Server service," he said.

The worm then loads software that steals passwords, security experts say.

Both Symantec and McAfee said Friday that they had seen only a very small number of attacks based on this exploit, but Symantec says that, starting Thursday evening, they found a 25 percent jump in network scans looking for potentially vulnerable machines. That could be a sign that more attacks are coming.

That scenario becomes more likely, too, as more tools that exploit the flaw are released to the public. On Friday, sample exploit code was posted to the Milw0rm.com hacker site, and over the next few days hackers are expected to move that code into attack tools that are easy to use.

Greenbaum predicted that the attack code will soon be used to build botnet networks of infected computers. "What we are going to see is this attack being added to the arsenal of botcode," he said.

"Once it evolves to the point where people really don't have to know much about the exploit ... those are the situations where people write the worms that do a lot of [damage]," said McAfee researcher Craig Schmugar.

Does he expect a damaging worm to emerge from this latest bug? "If history is a lesson, then yes," he said.