Visitors to porn sites could be exploited by cyber criminals, who may have booby-trapped the site with adware, spyware and viruses, a study has suggested.

The study found that many porn sites harboured malware or used "shady" practices to squeeze money out of their visitors.

By creating their own porn sites researchers found that many consumers were vulnerable to known bugs and loopholes.

Competition among porn sites makes the online adult industry ripe for abuse by hi-tech criminals, BBC quoted the study as saying.

"They have almost inadvertently created a whole ecosystem that's easy to abuse for cyber crime on a large scale," said Dr Gilbert Wondracek, a computer security expert from the International Secure System Lab, which led the study.

Wondracek said the team embarked on the study to find out the truth of the widely held view that porn sites are dangerous to visit.

As a first step the researchers trawled pornographic sites to classify what they found and how the industry was structured.

The big distinction was between free sites and those that charge for access. Typically pay sites produce content they give to free sites to drum up traffic.

More than 90 per cent of the 35,000 pornographic domains analysed in the study were free sites.

The researchers analysed the 269,000 websites hosted on the 35,000 domains to see which hosted malicious software.

About 3.23 per cent of these sites were booby-trapped with adware, spyware and viruses.

Many others used "shady" practices to keep visitors onsite. These included javascript catchers that made it hard for people to leave a page.

Others use scripts that re-direct visitors so when they click on a link they do not see the video or image they were expecting but are passed to an affiliate site.

"It's cut-throat competition," said Wondracek.

"Everybody tries to get as much traffic as possible."

It could also be a great way for hi-tech criminals to get a ready source of victims, said Wondracek.

The researchers also created two adult sites of their own, populated them with free content from porn producers and spent USD 160 to get traffic piped to these sites.

Analysis of the 49,000 visitors sent to their sample sites showed that 20,000 were using a computer and browser combination that was vulnerable to at least one known exploit.

With many porn sites appearing in the top 100 most popular sites on the web this could mean that huge numbers of people are caught out when they browse for adult content.

While relatively few porn sites were infecting visitors, it is difficult to spot good from bad, he said.

The researchers presented their results at the Workshop on the Economics of Information Security held at Harvard this week.