An analysis of sex-based websites has revealed that up to 80 percent of them are now being used to upload spyware, worms and Trojans to unsuspecting visitors.

The statistic emerged from a study by security company Aladdin Knowledge Systems, which trawled pornographic websites, analysing anything it was able to upload from them.

The company randomly sampled between 2,000 and 3,000 sites from a total database of 30,000. What it discovered was that a large number are now being used as fronts for criminal groups looking to dispense one or more of a variety of types of malware.

Similarly, an analysis of virus code revealed that 70 percent of viruses and worms are now being written to help in the spread of secondary programs such as Trojans, keyloggers and redirects, rather than for their own sake.

"We had a gut feeling that viruses have become more professional," said Shimon Gruper, Aladdin’s vice president of technologies. Until around two years ago, the company would have expect to see between 400 and 500 new viruses per month, a figure that was now greatly reduced.

"What we see now are only 20-35 new viruses per month but they are now much more sophisticated. The payloads are now more dangerous," he said.

There had been a dramatic shift away from viruses and Worms being written as nuisance software. They were now far more likely to be written by professionals for financial gain, a relatively recent trend.

"Our analysis over the last several months confirms that the majority of virus writers are shifting away from creating complicated viruses purely aimed at causing havoc, and instead, are creating spyware that is linked to organized crime and illegal money-making opportunities."

Gruper suggested that malware had moved to being distributed from websites because that this was one area that most corporate and consumer PCs were not well protected against. By contrast, e-mail systems would now routinely block attachments and most spam, making it harder for malware to reach users.