Attackers are exploiting the just-patched vulnerability in Internet Explorer (IE) by hiding malicious ActiveX controls in Microsoft Word documents.
"Inside the document is an ActiveX control, and in that control is a line that makes it call out to the site that's hosting the malware," said David Marcus, the director of security research and communications for McAfee Inc.'s Avert Labs.
"This is a pretty insidious way to attack people, because it's invisible to the eye, the communication with the site."
Embedding malicious ActiveX controls in Word documents isn't new -- Marcus said he had seen it "a time or two" -- but using an ActiveX control to ping a hacker's server for attack code is "definitely an innovation," he added. "They're stepping it up."
The rogue documents can be delivered as attachments to spam e-mail or offered up by hacked sites.
Attackers have been exploiting the IE bug since at least 9 December, when reports first surfaced about malicious code found in the wild and on several Chinese hacker servers. McAfee was one of the first security companies to report the emerging exploit.
Since then, Microsoft acknowledged the bug, then offered up a series of advisories urging users to take protective steps until a fix was available.
Although other researchers continue to claim that thousands of legitimate Web sites have been compromised, then used to serve "drive-by" attacks against unpatched browsers, Marcus wasn't certain about the numbers he's seen bandied about.
"But absolutely, there's been a lot of activity around this," he said. "A lot of the bad guys have embedded IFRAMES in their sites to attack IE."
According to other reports, the IE exploit has been added to one or more multi-strike hacker toolkits that try several different exploits when users visit a compromised or malicious site. "If it's not in one of those yet, it probably will be," said Marcus. "Some of the exploits in those kits are years old, so a good one like this, unpatched until yesterday, will make its way into them."
Marcus recommended that users be cautious about opening Word documents, keep their security software up-to-date, and apply the IE patch as soon as possible.