An emerging category of malicious networks is responsible for a large portion of the malware that is delivered across the Web, researchers said this week.

In a report published Wednesday, researchers at Blue Coat Systems offered a deeper look into the growing class of malware delivery networks, including a list of the 10 largest.

"These malware delivery networks are typically hosted across multiple sites and are responsible for launching dynamic attacks on unsuspecting users," the report says.

For the first half of 2011, Shnakule was the leading malware delivery network, both by size and effectiveness. On average during that period, this network had 2,000 unique host names per day with a peak of more than 4,300 per day, according to the report. It also proved the most adept at luring users, with an average of more than 21,000 requests and as many as 51,000 requests in a single day.

"Shnakule is a broad-based malware delivery network whose malicious activities include drive-by downloads, fake anti-virus and codecs, fake flash and Firefox updates, fake warez, and botnet/command and controls," the Blue Coat researchers state. "Interrelated activities include pornography, gambling, pharmaceuticals, link farming, and work-at-home scams."

Shnakule also contains many large component malware delivery networks, including Ishabor, Kulerib, Rabricote, and Albircpana, which all appear on the top 10 list of largest malware delivery networks.

The report also outlines how victims are acquired. In the first half of 2011, search engine poisoning was the most popular malware vector. In nearly 40 percent of all malware incidents, search engines and portals were the entry point into malware delivery networks, the report said.

A growing portion of malware is delivered via categories of sites or applications that companies typically allow in acceptable use policies, the Blue Coat researchers say. Searching for images and pirated media often leads to malware delivery, and users engaging in these activities are especially vulnerable, Blue Coat states. Have a comment on this story? Please click "Comment" below. If you'd like to contact Dark Reading's editors directly, send us a message.