"There is a whole underground world -- an online cottage industry -- that has grown up that allows anyone to commit mortgage fraud," said Constance Wilson of Interthinx, a financial fraud detection firm.

The sites offer better credit scores by linking customers to a stranger's credit card, or giving them pay stubs from a bogus company. One even offered a well-heeled bank account to rent for a couple of months, The New York Times reported Saturday.

Industry experts say the sites fueled the mortgage fraud that caused a spike in loan defaults in recent months when people bought buildings they could not afford.

Policing the shady Web sites is difficult because they can easily shut down and then quickly reopen for business under a different name on the Internet, the Times said.