Microsoft has finally released a patch for a critical hole in its Internet Explorer browser, ahead of its next scheduled round of security updates.

The patch fixes a vulnerability in the way Explorer renders VML graphics. Hackers had been exploiting the flaw, which also affects some versions of Outlook, for more than a week, and in recent days malicious activity had been on the upswing. The out-of-cycle release is unusual, but not unprecedented.

Microsoft generally releases its security updates on the second Tuesday of every month, giving sysadmins a predictable way to set aside time to test the new software. Occasionally, the company will release patches ahead of time if a flaw is being widely exploited by attackers. In January, it patched a critical flaw in the Microsoft Windows Metafile (WMF) image-rendering engine after it became a widespread problem.

With attack code that works on the latest version of Windows XP now publicly available, the VML bug is shaping up as a very serious concern for administrators, said Ken Dunham, the director of Verisign's iDefense Rapid Response Team. VML attacks have now "dwarfed the WMF activity in the same period of time compared to last year," he said.

By Tuesday, more than 3,000 websites were already infecting users with malware that exploited the VML bug, according to Dunham. One week into the WMF outbreak last January, iDefense saw about 600 sites exploiting the problem.

Security experts also warn that there are many variants of the VML malware, some of which may be missed by security software. Researchers at iDefense are now looking at a dozen possible variations of the VML exploit code and have confirmed the existence of seven variants, Dunham said. "With WMF there wasn't nearly as much modification. We see a lot of different permutations and obfuscation techniques being utilise with VML attacks."

A group of security researchers released a patch for the VML flaw late last week, independent of Microsoft. But scammers have even found a way to make patching seem dangerous. In the past few days they have been circulating phoney emails, claiming to be a patch for the VML problem. If downloaded, the fake patch installs malicious software on the victim's system, Dunham said.

Microsoft's next regularly scheduled security update is on 10 October.