Thieves take just six seconds to guess Visa card number

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.