Hackers are intensifying their attacks on Internet Explorer users, increasing the chances that Microsoft will patch a critical flaw in the software ahead of its regularly scheduled 10 October security update.

On Sunday, hackers released sample code showing how to exploit the IE flaw on a fully patched version of Windows XP, a move that security experts believe will step up the attacks. Criminals have been taking advantage of a previously unknown vulnerability in cPanel software, widely used by web hosting providers, to install the IE malware on servers.

The Microsoft bug has to do with the way IE processes web-based graphics code written in the Vector Markup Language (VML). It was first reported Monday 18 September by researchers at Sunbelt Software, who found that attackers were exploiting this vulnerability on a number of pornographic websites.

Late last week, attackers started spreading their attack code by taking advantage of a second flaw in cPanel's Web hosting administration software. This product is used by web hosting providers to give their customers an easy way to administer sites.

Hackers were able to exploit this problem to gain access to servers at hosting provider HostGator.com and began adding malicious VML exploit code to the pages of HostGator's clients, starting late on Thursday.

In fact, the attackers had used the cPanel flaw to gain control of HostGator's servers weeks before the VML vulnerability was disclosed, according to Brent Oxley, CEO of HostGator.com.

"They took control of as many servers as they could and they were building an army," Oxley said. "They waited idly for a month and when the Microsoft exploit came out that's when they launched the attack."

HostGator servers are used to host about 500,000 sites, but not all were compromised. Oxley estimated that "thousands" were, however, and said that a number of other hosting providers have also fallen victim to the cPanel attack.

The cPanel flaw has now been patched and HostGator.com customers are no longer serving up the VML exploits, but security experts wonder how widespread the cPanel compromise has been.

"It's a little worrisome that these hackers appeared to be targeting that piece of software," said Rich Miller, an analyst with Netcraft. "It's very common among large hosting companies."

Whether or not these latest developments are worrisome to Microsoft is unclear. The company declined to comment on the matter on Monday, referring reporters to a Friday blog posting by Microsoft security response centre operations manager Scott Deacon in which he said that his company was increasingly confident on its ability to release an early patch.

"There's been some confusion about that, that somehow attacks are dramatic and widespread," Deacon wrote Friday. "We're just not seeing that from our data, and our Microsoft Security Response Alliance partners aren't seeing that at all either," he said.

"Of course, that could change at any moment," Deacon added.

Sunbelt Software agreed that the VML bug has not yet been widely exploited, but Sunbelt vice president Eric Sites predicted that Microsoft would release an early patch. "I think this week will be a lot more active because of the new proof-of-concept codes coming out," he said.

As a temporary workaround, Microsoft's has advised users to unregister the dll used to render VML images.