Finnish security firm F-Secure calculates more than 6 million newly infected systems with the Downadup worm in the past few days. This makes Downadup one of the most virulent infections we have witnessed in quite some time.It was only Tuesday when F-Secure estimated that the Downadup worm had infected about 3.5 million PCs by exploiting a vulnerability Microsoft had patched last October. That's right: the patch for the hole this worm exploits has been available since before the presidential election, Thanksgiving, and New Year's -- and not enough people have applied the patch. This doesn't say much for the current state of security awareness, or the auto-update process. But the fault for this doesn't rest with Microsoft: people are neglecting to update their systems, and not just for a few days or weeks, but for months.

As of yesterday, F-Secure estimated the number of infections to be 8,976,038. That's a staggering haul for the creators of this malware. If the controllers of this loosely-coupled network decided to, 9 million infected hosts could deliver a crippling blow to just about any Internet-connected server or Web site they chose.

Here's a link to the Microsoft MS08-067 security bulletin that plugs the hole Downadup infects.

Here's Microsoft's free malware cleansing tool.