Almost 30,000 websites have been hit by a mass injection attack, most of which are based on the WordPress content management system, Websense has found.
Over 200,000 webpages have been compromised by a small slice of code, which has been placed at the bottom of pages.
Visitors to infected pages have been redirected to fake antivirus sites, as cyber criminals look to dupe people into downloading Trojans onto their system by claiming their systems are already infected with lots of nasty malware.
After this access is maintained to the compromised website, the injected code keeps getting updated periodically.
The majority of the infected sites (85 per cent) are based in the US. Of those checked by Websense, nearly five per cent of visitors to infected sites were from the UK.
"The injected websites in this campaign have many different versions of WordPress CMS installed. Usually in this type of mass injections; vulnerabilities or security holes in certain versions and their accompanied infrastructure are abused to get initial access to those websites," a Websense spokesperson told IT Pro.
"Therefore after this access is maintained to the compromised website, the injected code keeps getting updated periodically i.e. in every new cycle of the mass injection.
"What is for sure is that because WordPress is so widely used all over the world, every version of it is studied and exploited by hackers. These rogue AV scams keep re-emerging and adapting to avoid detection."
