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Guessing at Compromised host Numbers

The fine art of pulling numbers from your nether regions.

After much waiting by end users, and a lot of hoping from interested Information Security watchers, Microsoft has finally added detection for the Storm Trojan (Nuwar, Zhelatin, etc) to their Malicious Software Removal Tool. The most recent update, released on September 11th, included detection for this malicious software, and the early results are not quite what most people were expecting.

With various claims being made that there are millions upon millions of compromised hosts that make up the available network that the botnet controllers have access to, it would be assumed that the Microsoft Malicious Software Removal Tool should be removing a large number of affected systems from the Internet.

That isn't quite what Microsoft found. In a post to their TechNet blog, Microsoft's Anti-Malware team discovered that the total number of Storm Trojan removals placed it third in the overall list of malware variants removed from the systems of end users. With more than 660,000 removals each, Renos and Zlob malware types led the Nuwar / Storm family of malware. Nuwar variants, and components of Nuwar were successfully removed from just under a quarter of a million systems.

A third party researcher notified Microsoft of a drop of almost 20% of the total DoS capability for the malware bot network in the days following the update to the MSRT, implying that it was directly related to the update. While correlation does not imply causation, it is an interesting data point that could point to an infection rate significantly lower than most have predicted. Microsoft's own assessment is that the quarter of a million machines cleaned up represented almost 100,000 machines in the active botnet. Combined with the 20% reduction, it suggests that the overall size of the botnet is only 500,000 machines, with another couple of hundred thousand that are not active, but are infected.

There is no doubt that the Nuwar menace is something that is causing a lot of users significant trouble. What is being called into doubt is the total spread of infected systems. It is possible that Microsoft have not successfully identified infected variants of Nuwar, and it is possible that most infected systems are not running any form of malware removal tool (either through ignorance, hubris, or just not being aware of them). It may also be that many infected systems have already been cleaned by third party antimalware software, though this should have been evident in an overall reduction of botnet operation.

23 September 2007

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