Downadup, the superworm that attacks a patched vulnerability in Microsoft Windows, is making exponential gains if estimates from researchers at F-Secure are accurate. They show 6.5 million new infections in the past four days, bringing the total number of machines it has compromised to almost 9 million.

The astronomical growth stunned some researchers, although others cautioned the numbers could be inflated since the counting of infected computers is by no means an exact science. Most agreed F-Secure's estimate was certainly plausible and if it proved to be correct, represented a major development in the world of cyberthreats.

"This thing has gotten way out of hand," said Paul Ferguson, a security researcher for anti-virus provider Trend Micro who has spent the past several weeks tracking the worm's progress. "It seems pretty spectacular to me that there could be that much growth."

A confluence of factors are responsible for the growth of Downadup, which also goes by the name Conficker.

For one, the underlying vulnerability allows for self-replicating attacks in the 2000, XP, and Server 2003 versions of Windows. And for another, the malware authors have cleverly designed exploits that spread via flash and network drives, online trojans, and social engineering features that allow it to spread like wildfire within a local network once a single machine is compromised.

Another important contribution to the outbreak seems to be the legions of administrators and users of Windows machines who have failed to heed repeated admonitions to update. Despite Microsoft releasing an emergency patch for the vulnerability almost three months ago, nearly one in three Windows machines have yet to apply it, according to research from security provider Qualys.

Some Skepticism

Not all security watchers are convinced there really are 9 million machines infected by Downadup. Paul Royal, chief scientist with anti-botnet company Damballa, said his researchers have counted only about 500,000 unique IP addresses connecting to Downadup's master control server. That would imply an average of 18 infected machines behind each address, a number he says is unlikely.

The skepticism prompted F-Secure researchers to explain its methodology for the mind-boggling number. By infiltrating Downadup's control channel and analyzing logs of machines that connected, researchers discovered a counter believed to show the number of other PCs the compromised machine has infected.

After creating a script that totaled all those numbers together, F-Secure deduced 8.97 million machines have been compromised, up from 2.4 million on Tuesday.