Both SMBv1 and its successor, SMBv2, contain the bug. "Windows Vista, Windows Server 2008, Windows XP, Windows Server 2003 and Windows 2000 are not affected," assured Forstrom.

Attacks could be aimed at any browser, not just Internet Explorer (IE), Microsoft warned. After tricking users into visiting a malicious site or a previously-compromised domain, hackers could feed them specially-crafted URIs (uniform resource identifier), and then crash their PCs with malformed SMB packets.

Microsoft said it may patch the problem, but didn't spell out a timetable or commit to an out-of-cycle update before the next regularly-scheduled Patch Tuesday of Dec. 8. Instead, the company suggested users block TCP ports 139 and 445 at the firewall. Doing so, however, would disable browsers as well as a host of critical services, including network file-sharing and IT group policies.

Gaffie's vulnerability was the first zero-day reported and confirmed by Microsoft in Windows 7 since the new operating system went on sale Oct. 22.