Microsoft has published an emergency software patch after hackers found a way to exploit a critical flaw in Windows.

The update, released as expected on Tuesday morning, actually 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, said Ken Dunham, director of malicious code intelligence with iDefense. "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 will probably be one of the "most prevalent and persistent types of attacks we will see in the next months and years," he said.

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

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 WMF (Windows Metafile) and VML (Vector Markup Language) vulnerabilities that were patched in the other two updates, according to Mark Miller, director of Microsoft's Security Response Center.

Microsoft has 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."

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

On Tuesday, a Determina executive said that 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," said Nand Mulchandani, Determina's vice president of marketing. "I have no idea why they didn't do this earlier."

Miller defended Microsoft's decision, saying 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," he 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.