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Millions of visitors to porn sites could be affected by the malvertising campaign Getty Images

Hundreds of porn sites have been targeted by criminals using malicious advertising – commonly known as malvertising – to infect visitors' devices with malware. The latest campaign comes after similar attacks in recent months on a number of popular adult websites, including xHamster, RedTube and PornHub.

Malicious adverts were discovered by the US-based security firm Malwarebytes on what it said were moderately popular porn sites attracting millions of visitors each day. The sites included DrTuber, Nuvid, Eroprofile, IcePorn and Xbabe. The adverts are hosted and served by the adult ad network AdXpansion and do not need to be clicked on to potentially affect a visitor to a site.

"It isn't so much that pornography sites are targeted, so much as the adult advertising networks," Chris Boyd, an analyst at Malwarebytes, told IBTimes UK. "The end result of infected PCs is still the same, but as with most forms of malvertising, the websites themselves tend to have little control over which adverts are served to their visitors.

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A malicious advert served through the AdXpansion ad networkScreengrab/Malwarebytes

"Everyone is reliant on a chain of trust which is easily broken by unscrupulous individuals willing to pollute the stream with rogue adverts. The challenge is to find ways of reducing the threat to those at risk from the bad ads."

Various types of malware were used during the campaign, including Ransomware. Malwarebytes contacted the AdXpansion network to inform them of the malvertising campaign but no response had been given at the time of publication.