As Microsoft readies its latest set of security updates, online attackers have begun exploiting a new flaw in the company's Internet Explorer.

The flaw was made public in Chinese language discussion forums two days ago by a security group called the Knownsec team. In tests, it worked on IE 7 running on Windows XP, Service Pack 2.

It has already been used by attackers who have hosted it on hacked websites to attack unsuspecting visitors, said Wayne Huang, CEO of security vendor Amorize Technologies. Now that the bug has been publicly disclosed, he expects attacks based on the flaw to become much more widespread.

The code exploits a bug in the way IE handles XML and works on the browser about "one in three times," Huang said in an instant message interview. For the attack to work, a victim must first visit a website that serves the malicious JavaScript code that takes advantage of the flaw.

A Chinese language statement on the flaw can be found online.

In attacks, the code drops a malicious program on the victim's PC which then goes to download malicious software from various locations.

Microsoft is expected to release six critical patches on Tuesday, including a fix for IE. In a statement, the company said it was investigating the flaw made public by Knownsec, but did not say whether it expected to patch the bug on Tuesday.