Malvertising networks were responsible for a 240 per cent increase in the number of malicious sites detected in 2011.
According to a Blue Coat Systems 2011 report, 74 per cent of infections were from legitimate websites and many infections were from social networking sites.
The report said malnets -- different from botnets -- were built, managed and maintained by cyber criminals for the purpose of launching long-term attacks against victims to steal personal information or build botnets
Malnets tricked users into downloading software to create a kind of botnet, the company said.
The company also claimed malnets were so effective at launching attacks through search engines and portals that one in 142 searches led to malicious links.
Last year, the London Stock Exchange was hit by a malvertising scare when a third party advertising provider named Unanimis was hit and the London Stock Exchange's website was one of many sites to be affected.
The report claimed that there are five main malnets, the largest being ‘Shnakule' which specialised in distributing drive-by download attacks, rogue anti-virus and fake Firefox and Adobe updates.
The Glomyn malnet mainly distributed spam while Naargo and Cinbric were focused on sending adult content.