It may take as little as six seconds for hackers to guess your credit or debit card number, expiry date and security code, say scientists who were able to circumvent all security features meant to protect online payments from fraud.
Exposing the flaws in the VISA payment system, researchers from Newcastle University in the UK, found neither the network nor the banks were able to detect attackers making multiple, invalid attempts to get payment card data.
By automatically and systematically generating different variations of the cards security data and firing it at multiple websites, within seconds hackers are able to get a 'hit' and verify all the necessary security data.
Investigators believe this guessing attack method is likely to have been used in the recent Tesco cyberattack which the Newcastle team describe as "frighteningly easy if you have a laptop and an internet connection."
"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," said Mohammed Ali, a PhD student at Newcastle University.
"Firstly, the current online payment system does not detect multiple invalid payment requests from different websites," said Ali.
"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," he said.
"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," Ali said.
"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," he said.
"Each generated card field can be used in succession to generate the next field and so on," Ali said.
"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," he said.
"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," he said.
To obtain card details, the attack uses online payment websites to guess the data and the reply to the transaction will confirm whether or not the guess was right.
Since the current online system does not detect multiple invalid payment requests on the same card from different websites, unlimited guesses can be made by distributing the guesses over many websites.
However, the team found it was only the VISA network that was vulnerable.
The research was published in the academic journal IEEE Security and Privacy.
