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 that 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: "The research does not take into account the multiple layers of fraud prevention that exist within the payments system, each of which must be met in order to make a transaction possible in the real world.
"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. "We provide issuers with the necessary data to make informed decisions on the risk of transactions. There are also steps that merchants and issuers can take to thwart brute force attempts.
"For consumers, the most important thing to remember is that if their card number is used fraudulently, the cardholder is protected from liability." It said it also has the Verified by Visa system, which offers improved security for online transactions. Tesco Bank said the fraud last month affected 9,000 customers and cost £2.5m. A spokesman said: "We identified the fraud quickly and communicated immediately with our customers, the Financial Conduct Authority and National Crime Agency. This remains a criminal investigation."