Criminals operating online are doubling down--or to be more accurate, tripling down--on the infrastructure they maintain to infect PCs, control botnets, and launch follow-on attacks and spam campaigns.

That finding comes via hardware proxy appliance manufacturer Blue Coat Systems, which Tuesday released a report titled "The Vicious Cycle of Malnets," which reviews how attackers have employed malicious infrastructure during the first half of 2012.

"Last year we were tracking about 500 malnets, and currently we're up to about 1,500," said Tim Van Der Horst, a senior malware researcher at Blue Coat, via phone. Malnet refers to the server-side infrastructure used to infect PCs and sometimes also to control botnets--groups of infected PCs--via command-and-control (C&C) servers.

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According to Blue Coat, the largest known malnet is Shnakule, which has used up to 5,005 malicious hosts--or servers--at any given time, depending on the capabilities needed at any given moment by its operators. Blue Coat believes that Shnakule is controlled by a single gang, and it's been used to serve up just about every type of known attack, including "fake AV attack, fake code, fake Firefox updates, C&C servers, gambling, work at home stuff, porn," said Van Der Horst. "They've got their fingers in every evil pie out there."

Beyond Shnakule, the other four largest malnets seen in the first half of 2012 were all newcomers, and included Tricki (with 547 maximum hosts used at once), Rubol (476), Raskat (163), and Rongdac (105). All of those malnets are used primarily to poison search engine results or relay spam. Overall, the most prevalent type of attack launched via malnets, comprising 35% of all attacks seen, was to poison search engine results, followed by attacks launched via email (11%), pornography (4%), or Web-delivered exploits (also 4%).

Blue Coat's report also charts recent botnet changes. Interestingly, use of the Alureon botnet increased 517% over the first six months of 2012, which now makes it the world's most active botnet. According to Blue Coat, the increased use of Alureon is a consequence of information security firms recently having targeted Zeus botnets, leading criminal operators to shift their resources elsewhere.

Blue Coat, for its part, tries to fingerprint and track malnets, and then sells that information to businesses--or provides it free to consumers--via its cloud-based threat intelligence service, which Van Der Horst said includes safety ratings for about 95% of known websites on the Internet, with any unknown site encountered by a user being slated for review.

Why track servers that are known to be part of malnets? "Because it's there day in and day out, the only thing that's changing is the attack that they're doing," said Van Der Horst. "So if you're tracking the infrastructure, you care less and less about what it does."

Indeed, a malnet that's used to poison search engine results one week may be used the next week to launch targeted attacks that exploit a zero-day vulnerability. Last year, for example, a single malnet was being used to target MySQL.com with an exploit that could give attackers root-level access to vulnerable servers. But that same day, the same set of malicious servers was also being used to poison search engine results and sneak fake search engine results onto legitimate sites, leaving links that directed users back to exploits served via the very same malicious servers.

As that suggests, another impetus for tracking malnets is that attackers tend to keep reusing the same attack infrastructure, rather than constantly rebuilding it from scratch. "They many change their IP addresses, or the website content a little bit, but it takes a lot of time and effort to get a server up and running smoothly," Van Der Horst said. "So you want to reuse as much of this work as you can, over time."