Microsoft has raised the severity rating of an Outlook flaw to "critical," the highest level, after its initial analysis was challenged by the researcher who found the security hole.

The vulnerability in Outlook 2002, first publicized on Tuesday, when Microsoft released a patch, could allow an attacker to use a malicious Web site to cause an affected PC to download and execute a program.

When Microsoft released its fix, it said it believed that the attack could only be accomplished if a PC user had the "Outlook Today" folder as the default home page in Outlook 2002. Now, after being alerted by Jouko Pynonnen, the Finnish security researcher who found the flaw, it says the potential for attack is greater.

The flaw in Microsoft Office XP and Office 2002 seems to be much more serious than initially thought. For those users who use these products, it is important that you read this...