San Francisco- Microsoft issued an emergency patch Wednesday for its Internet Explorer browser, aiming to fix a critical flaw that allowed hackers to take over computers merely by steering them to infected websites.

Microsoft took the rare step of issuing the patch after numerous security experts advised people to switch to rival browsers until Microsoft fixed the problem. Microsoft usually releases patches on a set schedule and this is only the third emergency patch in the last three years.

The flaw has been in circulation since the first week of December. So far, more than 2 million computers are believed to have been infected, according to PC Magazine.

The vulnerabilities are found on copies of Internet Explorer 7, as well as IE6 and IE5. The patch is designed to prevent attackers from downloading malware onto users' computers if they visit a malicious Web site, or a legitimate Web site that has been infected. (dpa)