With attackers finding more ways to exploit a critical flaw in its Windows operating system, Microsoft has published an emergency software patch.

The update fixes seven separate Windows vulnerabilities, but security experts are most concerned about a bug in the way Windows processes .ani Animated Cursor files. Online criminals have been exploiting this bug since late last week.

This is the only one of the seven vulnerabilities rated "critical" by Microsoft.

Microsoft was forced to release the early update a week ahead of schedule because attacks had become too widespread, director of malicious code intelligence with iDefense, Ken Dunham, said. "We have over 400 different URLs identified and related to attacks, and multiple emails have been sent out that direct people back there," he said. "We have proof that organised groups are now launching attacks."

The .ani attack vector would probably be one of the most prevalent and persistent types of attacks over the next months and years, he said.

Exploit code for the flaw had now been added to the widely used Metasploit hacking tool, and there were automated malicious website generation tools available, he said.

This is the third such "out-of-band" patch release Microsoft has made since January 2006. While attacks based on this .ani flaw are still considered limited, exploitation of the bug is following trends similar to the Windows Metafile (WMF) and Vector Markup Language (VML) vulnerabilities that were patched in the other two updates, according to director of Microsoft's Security Response Center, Mark Miller.

Microsoft had seen only Web-based exploitation of the .ani flaw, Miller said. "There have been some indications that email has been used, but we haven't seen anything on that front."

It was first notified of the flaw in December 2006 by security vendor, Determina.

Earlier this week, a Determina executive said Microsoft would have been better off issuing a patch for the .ani flaw sooner, rather than waiting for the April update and forcing customers to rush an emergency fix. "The customers are now going to incur the same cost as they would before, except that they are going to have to do this in panic," Determina's vice-president of marketing, Nand Mulchandani, said. "I have no idea why they didn't do this earlier."

Miller defended Microsoft's decision. He said that because the .ani flaw could affect other applications it required a great deal of testing. "The amount of time taken to patch was appropriate given the level of quality we were trying to release with," Miller said.

Windows users are strongly encouraged to install the patch, because the .ani flaw can be used to exploit computers running virtually any version of Windows, including Vista, even if they are running non-Microsoft browsers like Firefox and Opera, Mulchandani said.