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Only 50 Internet service providers (ISPs) host the majority of the world's spam, according to a new study, and that finding could reshape private and public approaches to combating the botnets that infect computers and then use them as spam mailers.

The study was conducted for the Organization for Economic Cooperation and Development (OECD) by researchers at Delft University of Technology in the Netherlands and Michigan State University, who examined 109 billion spam messages from 170 million unique IP addresses, gathered via a "spam trap" from 2005 to 2009.

One major finding is that where there's spam, you'll find an infected -- aka zombie -- machine. That's because according to the study data, on average 80% to 90% of the world's spam comes from infected machines.

Researchers also found that the 33 member countries that comprise the OECD, as well as Estonia, the Russian Federation, Brazil, China, India, Indonesia, and South Africa, "harbor over 60% of all infected machines worldwide registered by the spam trap." In other words, the majority of infected machines aren't laying low in countries nearly off the grid.

But perhaps the biggest surprise, said the researchers, was that "we discovered that infected machines display a highly concentrated pattern." In particular, "the networks of just 50 ISPs account for around half of all infected machines worldwide." In other words, "the bulk of the infected machines are not located in the networks of obscure or rogue ISPs, but in those of established, well-known ISPs."

The results suggest a formidable new way to block botnets. With a caution that historical data is no guarantee of future botnet behavior, the researchers said that "current efforts to bring about collective action -- through industry self-regulation, co-regulation, or government intervention -- might initially achieve progress by focusing on the set of ISPs that together have the lion's share of the market."

In other words, if policymakers want to maximize their bang for buck, start by improving the security practices of the 50 ISPs that host half the world's spam.

As perimeters melt away, security goes beyond encryption, authentication, and monitoring. We also need to ensure privileged users aren't betraying trust. In this report, we'll cover ways to track who did what to which system, and when. Download the report here (registration required).