Hackers, possibly based in Russia, have hijacked the computers of unsuspecting internet users for pornographic Web sites.
Researchers believe more than 1,000 computers have been taken over.
Internet users are accustomed to junk e-mails which are now sent out from hijacked internet addresses.
But this new form of hacking uses the computer as the actual Web sites.
"Here people are sort of involved in the porno business and don't even know it," computer researcher Richard Smith told reporters.
The method allows porn site operators to hide their identity.
Pornographers increasingly face being shut down by service providers following complaints about the explicit material and even risk prosecution, making the use of unwitting participants attractive.
"There's no individual computer to shut down," said Smith, who has been tracking the scheme.
"We're dealing with someone here who is very clever," Smith said.
People who use high-speed internet connections are the most vulnerable, he said.
By monitoring the traffic to porn adverts, he had already found 1,000 computers affected.
The system is complex. The hacker employed by the porn site will plant a program that advertises explicit material for people to buy over the net.
The hijacked computers may advertise this pornographic material for only a few moments, making detection more difficult.
Potential customers, having found the site during a trawl for pornography, will sign up as members and give their credit card details for payment.
The hacker gets paid a percentage.
Credit card details thus obtained could be used for fraud purposes.
One advantage of getting a porn user's card details is that they are less likely to complain to the authorities if defrauded, particularly if the sites contain illegal activities.
In the US, using someone else's computer could breach the Computer Fraud and Abuse Act.
The enormous rise in pornography-related junk email has already led to new plans for legislation.
California is leading the way in creating this legislation and plans to allow recipients of such spam to sue the sender.
No one has yet been identified as the brains behind the scheme, although organized crime in Russia has developed similar techniques in the past.