The Windows attack used by a recently discovered worm is being picked up by other virus writers and will soon become much more widespread, according to security vendor Eset.

Eset reported Thursday that two new families of malicious software have popped up, both of which exploit a vulnerability in the way Windows processes .link files, used to provide shortcuts to other files on the system.

The vulnerability was first exploited by the Stuxnet worm, discovered on computer systems in Iran last month. Highly sophisticated, Stuxnet targets systems running Siemens industrial control system management software. The worm steals SCADA (supervisory control and data acquisition) project files from Siemens' computer systems.

Siemens issued a Security Update for its customers on Thursday, but Microsoft has yet to patch the Windows bug that permits the worm to spread.

The newly discovered malware is "far less sophisticated" than Stuxnet and "suggests bottom feeders seizing on techniques developed by others," said Eset researcher Pierre-Marc Bureau, writing in a blog post.

One of the new samples installs a keystroke logger, a tool hackers use to steal passwords and other data, on the victim's computer. "The server used to deliver the components used in this attack is presently located in the US, but the IP is assigned to a customer in China," Bureau said.

The other variant could be used to install one of several different pieces of malicious software.

As each new variant of the attack pops up, it adds pressure on Microsoft to patch the underlying vulnerability. Microsoft's next set of security patches is due Aug. 10, but if enough customers get infected, the company may be forced to rush out an emergency patch for the issue.

Microsoft has already posted a temporary workaround to the problem and says it is working on a patch.

Right now, the Stuxnet worm makes up a very tiny volume -- less than 1/100th of a percent -- of the malware that Eset is seeing on the Internet, said Randy Abrams, Eset's director of technical education, in an interview.

However that's likely to change. "It's likely to become one of the most prevalent attack vectors," he said. "I expect that within a few months, we'll see hundreds if not thousands of pieces of malware using the link vulnerability."

Robert McMillan covers computer security and general technology breaking news for The IDG News Service. Follow Robert on Twitter at @bobmcmillan. Robert's e-mail address is robert_mcmillan@idg.com