The tool -- which is updated and shipped once a month on Patch Tuesday -- removed malware associated with Storm Worm from 274,372 machines in the first week after September 11. In all the tool scanned more about 2.6 million Windows machines.
These numbers, released by Microsoft anti-virus guru Jimmy Kuo, puts the size of the botnet on the low end of speculation that Storm Worm has commandeered between 1 million and 10 million Windows machines around the world.
The MSRT numbers, though helpful, shouldn't be relied on as gospel. For starters, the tool targets a very specific known malware (it only finds exactly what it's looking for) and attackers constantly tweak malware files to get around detection. In addition, it is only delivered to Windows machines that have automatic updates turned on, which means there are liely tons and tons of hijacked machines that never gets a copy of the MSRT.
Still, Kuo claims that the September version of MSRT made a dent in the botnet.
Another antimalware researcher who has been tracking these recent attacks has presented us with data that shows we knocked out approximately one-fifth of Storm's Denial of Service (DoS) capability on September 11th. Unfortunately, that data does not show a continued decrease since the first day. We know that immediately following the release of MSRT, the criminals behind the deployment of the Storm botnet immediately released a newer version to update their software. To compare, one day from the release of MSRT, we cleaned approximately 91,000 machines that had been infected with any of the number of Nuwar components. Thus, the 180,000+ additional machines that have been cleaned by MSRT since the first day are likely to be home user machines that were not notably incorporated into the daily operation of the Storm botnet. Machines that will be cleaned by MSRT in the subsequent days will be of similar nature.
The September release of the MSRT probably cleaned up approximately one hundred thousand machines from the active Storm botnet. Such numbers might project that the strength of that botnet possibly stood at almost half a million machines with an additional few hundred thousand infected machines that the Storm botnet perhaps were not actively incorporating.
Kuo also confirmed fears that the botnet will slowly regain its strength once those cleaned machines become reinfected because those machines are likely unpatched and not equipped with any security software.