Images: Sites infected with WMF Trojan

A vulnerable Windows system could become infected if someone just views what appears to be a malicious image on this compromised Web site, according to Websense's posting.

Another example. Mikko Hypponen, chief research officer at F-Secure, estimates that 99 percent of computers worldwide are vulnerable to an attack using the WMF flaw.

Websense, which has been tracking malicious activity around the flaw, says it has discovered malicious sites in the United States, Russia, Netherlands, the United Kingdom, China and Japan.

Microsoft has said it plans to release a fix for the WMF vulnerability as part of its monthly security update cycle on Jan. 10.

This is how a machine running Knoppix, a version of Unix, behaves when a malicious Web site is visited using the open-source Firefox browser. It alerts the Web surfer that a file is being opened.