The Downadup or "Conficker" worm has increased to over nine million infections over the weekend increasing from 2.4 million in a four-day period, according to F-Secure.
IT PRO has already followed the life of the worm closely, starting with isolated attacks forcing an out-of-band patch by Microsoft, and then experiencing such rapid growth that it could potentially claim the title of the world's biggest botnet.
Last Friday F-Secure outlined how it was calculating its estimates, while providing a to-the-point warning: "The situation with Downadup is not getting better. It's getting worse."
The worm has password cracking capabilities, which is often successful because company passwords sometimes match a predefined password list that the worm carries.
Corporate networks around the world have already been infected by the network worm, which is particularly hard to eradicate as it is able to evolve - making use of a long list of websites - by downloading another version of itself.
Rik Ferguson, solution architect at Trend Micro, told IT PRO that the worm was very difficult to block for security companies as they had to make sure that they blocked every single one of the hundreds of domains that it could download from.
He said: "It is very difficult to stop it connecting. It only needs to get it once right, so the odds are on their side."
Ferguson said that the worm was creating a staggering amount of infections, even if just the most conservative infection estimates are taken into account. He said: "What's particularly interesting about this worm is that it is the first hybrid with old school worm infection capabilities and command and control infrastructure."
Trend Micro threats analyst Robert McArdle said on the company's blog that the worm has become so successful because of poor security policies, such as administrators not properly rolling out the Microsoft patch when it was available.
He said that even having one unpatched machine was enough to have the worm spread throughout the entire network.
"Patch management is a critical component of any IT department's job today, and it is vitally important that it is applied in a timely fashion across all of the company's machines, including laptops and other mobile networks," McArdle added.
"Companies also need to have very clear policies on patch levels of external parties who access their network like partner companies and contractors. Like so many aspects of security, it only takes one hole to bring down an entire network."
