Earlier in the day, the company said it was investigating the incident which emerged over the weekend when someone published the exploit code to the Bugtraq mailing list. By Monday night, Microsoft switched gears and issued the advisory. There have not been any active exploits of the vulnerability reported so far.
Microsoft released Security Advisory 977981, which includes workarounds for an issue that exposes a flaw in Cascading Style Sheets that could allow for remote code execution. Vulnerabilities that allow remote-code execution generally result in patches rated as critical by Microsoft.
Microsoft said IE 5.01 Service Pack 4 and IE 8 on all supported versions of Windows are not affected.
For an attack to work, the hacker would first have to get his victim to visit a Web site that hosted the exploit code. This could be a malicious Web site set up by the hacker himself or it could be a site that allows users to upload content.
Another way cybercriminals have launched this type of attack, however, is by hacking into legitimate Web sites. Earlier this week, for example citizen's band radio vendor Cobra Electronics disclosed that it had been hacked in June, most likely by a professional hacker who had used the site to download malware to customers.
Microsoft did not say whether it would patch the flaw during its next regularly scheduled set of security updates, due Dec. 8.
(Robert McMillan of the IDG News Service contributed to this report.)