Criminals can work out the card number, expiry date and security code for a Visa debit or credit card in as little as six seconds using guesswork, researchers have found.
Experts from Newcastle University said it was “frighteningly easy” to do with a laptop and an internet connection.
Fraudsters use a so-called Distributed Guessing Attack to get around security features put in place to stop online fraud, and this may have been the method used in the recent Tesco Bank hack.
Researchers found the system did not detect cyber criminals making multiple invalid attempts on websites in order to get payment card data.
According to a study published in the academic journal IEEE Security & Privacy, that meant fraudsters could use computers to systematically fire different variations of security data at hundreds of websites simultaneously.
Within seconds, by a process of elimination, the criminals could verify the correct card number, expiry date and the three-digit security number on the back of the card.
Mohammed Ali, a PhD student at the university’s School of Computing Science, said: “This sort of attack exploits two weaknesses that on their own are not too severe but when used together, present a serious risk to the whole payment system. Firstly, the current online payment system does not detect multiple invalid payment requests from different websites. This allows unlimited guesses on each card data field, using up to the allowed number of attempts – typically 10 or 20 guesses – on each website.
“Secondly, different websites ask for different variations in the card data fields to validate an online purchase. This means it’s quite easy to build up the information and piece it together like a jigsaw.
“The unlimited guesses, when combined with the variations in the payment data fields make it frighteningly easy for attackers to generate all the card details one field at a time.
“Each generated card field can be used in succession to generate the next field and so on.
“If the hits are spread across enough websites then a positive response to each question can be received within two seconds – just like any online payment.
“So even starting with no details at all other than the first six digits – which tell you the bank and card type and so are the same for every card from a single provider – a hacker can obtain the three essential pieces of information to make an online purchase within as little as six seconds.”
Visa said: “Visa is committed to keeping fraud at low levels and works closely with card issuers and acquirers to make it very difficult to obtain and use cardholder data illegally.”