Hackers have released attack code that exploits a critical vulnerability in Microsoft's Word software – the third such bug to be disclosed in the past week.

The proof of concept code was posted Tuesday on the Milw0rm.com Web site, making it widely available to the hacking community. It exploits a previously unreported bug in Word.

Like the other recent Word bugs disclosed this past week, it could possibly be used by attackers to run unauthorised software on a victim's computer, said David Marcus, security research and communications manager with McAfee's Avert Labs

Attackers have been using these Word exploits in extremely targeted attacks, where a small number of victims are sent an email with a maliciously encoded Word document attached. The hackers use social engineering techniques to try to trick the victim into opening the dangerous attachment.

For example, in a recent Word attack, first reported Sunday, the malicious email "was sent to a very high-profile company, directly to three people at the company," Marcus said.

Microsoft is investigating reports of this latest Word bug, a spokesman for the company's public relations agency said.

Though they are not being widely exploited, the unpatched Word vulnerabilities are causing some enterprises concern.

Attacks on Microsoft's Office software have been on the rise for months now, said Marc Maiffret, chief technology officer with security vendor eEye Digital Security. Office vulnerabilities were once released "on a monthly basis," he said. "Now we're at the point were it's almost daily."

Still, publishing attack code ultimately works contrary to the interests of the bad guys, he added. "It's kind of disruptive, and it creates a panic," he said. "But all it does is make the industry focus and come up with a resolution."