"Here's what we know: In order for this attack to be carried out, a user must first open a malicious Excel document that is sent as an email attachment or otherwise provided to them by an attacker," Reavey said.
"Remember remember to be very careful opening unsolicited attachments from both known and unknown sources," he added.
Within 24 hours, Microsoft is supposed to have an official response to give to customers, providing temporary work-arounds or patches. All versions of Windows (from 95 to XP to 2003 SP2) are affected, and after a compromised file is opened, a worm called Downloader.Booli.A is installed. The big AV suite producers have already provided detection and removal in their suites.