iDefense said in a note that the vulnerability is "really nasty" and that computer security professionals could be in for a rough ride. Microsoft issued its biggest group of patches in five years on Tuesday, and is not due for a regular patch release until Jan. 13, although it could opt to do an emergency release.
"Chances are this will be unpatched for around about a month, and that leaves plenty of time for attackers to take advantage," said Toralv Dirro, a security strategist based in Germany for McAfee's Avert Labs. "This should be taken pretty seriously."
iDefense said there aren't many options for users to defend themselves, but there is an easy one. The SANS Institute, which runs computer security training courses, recommended that people use a browser other than Internet Explorer.
In an advisory, Microsoft said users should put IE7 in "protected" mode, which causes warning prompts to appear if something tries to change system files or settings. But that protected mode is only available to users running Windows Vista.
Another mitigating factor is the default security level setting for IE7 running on Windows Server 2003 and Windows Server 2008. It is set to "high," which blocks file downloads, Microsoft said. Generally, administrators should not browse the Web from the server.
Toralv said it could be tough to get the Internet Service Providers hosting the dodgy Web sites to take them offline, since the process is time intensive and service providers can be slow to respond.
The IE vulnerability compounds what looks to be a tough month for Microsoft, with the publication of another 0-day vulnerability in Microsoft's WordPad application earlier this week.
That problem is somewhat less severe since a user would have be tricked into opening a maliciously-crafted document attached to an e-mail. It also does not affect computers running Windows XP Service Pack 3 and Vista.
It does, however, affect Windows 2000 Service Pack 4, Windows XP Service Pack 2, Windows Server 2003 Service Pack 1 and Windows Server 2003 Service Pack 2, according to Microsoft.