Jul 6, 2011 13:24 GMT  ·  By

Microsoft has released a new report about its progress in dismantling the Rustock botnet which shows that during the past three months its size was reduced by half.

The Rustock botnet, once the world's primary source of spam, was taken down in March in a coordinated effort that saw the participation of Microsoft's Digital Crimes Unit (DCU), the U.S. Marshals Service and other security firms.

According to Microsoft, a bit over 1.6 million IP addresses were generating Rustock traffic at the time.

Counting IP addresses is not a very accurate way to determine botnet size because it can lead to either underestimation or overestimation.

For example, multiple computers on the same local network can be infected with the malware, but researchers might see traffic from a single IP assigned to the network's gateway.

Similarly, infected computers might be assigned IP addresses dynamically by their ISP. This means that every time they reconnect, they can have a different IP, which skews the count.

But even under these circumstances, a significant reduction in the number of IPs suggests a significant reduction in the number of actual infected machines.

According to figures disclosed by Microsoft in its new "Battling the Rustock Threat" report, the average number of IPs showing signs of Rustock activity in mid-June was a bit over 700,000. That represents a 56% global reduction in under three months.

"Since the time of the initial takedown, we estimate the Rustock botnet is now less than half the size it was when we took it down in March," Microsoft says.

"That’s great news and the infection reduction has happened much more quickly than it did for Waledac over a similar period of time last year, but we still have a long way to go," it adds.

The highest reduction values were registered in Ukraine (71%), Russia (70%), Malaysia (69%) and India (69%). The United States registered only a 35% drop. The countries still harboring most Rustock infected machines are India with 99,032 IPs, USA with 55,731 and Turkey with 50,465.