Malvertising networks were responsible for a 240 per cent increase in the number of malicious sites detected in 2011.

According to the 2012 security report from Blue Coat Systems, ‘malnets' are different from botnets as they are within the internet and end-uers are going to trusted websites and end up infected.

Blue Coat said that malnets are built, managed and maintained by cyber criminals for the purpose of launching a variety of attacks against unsuspecting users over extended periods of time, with the ultimate result to steal personal information or transforming end-user systems into botnets.

Speaking to SC Magazine, David Albohair, senior product marketing manager at Blue Coat said that 74 per cent of infections were from legitimate websites and many infections were from social networking sites.

He also claimed that malnets have become so effective at launching attacks through search engines/portals that one in 142 searches leads 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 specialises in distributing drive-by download attacks, rogue anti-virus and fake Firefox and Adobe updates. The Glomyn malnet mainly distributes spam while Naargo and Cinbric are focused on sending adult content.

Albohair said: “The aim of these is to download software to the end-user to create a kind of botnet, so now everyone is targeted by a cyber criminal.”