Fraudsters have been using a similar technique for years, but until recently they tended to make minor purchases on online retail sites. Now, as these sites have become better at identifying and blocking these transactions, the criminals have begun looking elsewhere, said Zulfikar Ramzan, senior principal researcher with Symantec. "Using a charitable organization as a way to verify a credit card number is a relatively new technique, and it's probably being used by a minority of the more innovative guys," he said.

Credit card numbers are bought and sold in underground "carder" forums, which bring together the people who have stolen the credit card numbers with those who want to use them. These charitable donations are typically made by the person buying the card numbers as a final check to ensure that the numbers will work, Ramzan said.

Last month the Red Cross was forced to return nearly $7,000 that was donated in the course of 700 fraudulent transactions, said Carrie Martin, a spokeswoman for the American Red Cross. "We routinely see this kind of activity," she said. "We have someone in place who deals with this on a regular basis."

This fraud accounted for a tiny sliver of the Red Cross's $6 billion in revenue last year, but the organization also has to pay staff to stay on top of the fraud, Martin said.

This is not the only time that fraudsters have found ways to misuse charities. In another common scam, the criminal will give the charity a fake check and ask that a portion of it be returned in cash. Though the check will initially clear in the charity's bank account it will eventually be returned. By then, however, the charity will have already paid out to the thieves.

"These kinds of things have hit charities before," said Ramzan "I feel bad because all these charities are trying to do good and you have these fraudsters that try to take advantage of them because of the way they work."