Tesco Bank left itself vulnerable to fraud by using sequential card numbers - FT

Tesco Bank left itself vulnerable to fraud by using sequential card numbers - FT

Tesco Bank may have left itself open to fraud by issuing debit cards with sequential numbers, according to a report by the FT.

Criminals last month drained £2.5 million from 9000 current accounts at the supermarket chain's banking operations in a hack that was described by Tesco Bank CEO Benny Higgins as "a systematic, sophisticated attack", and billed as "unprecedented in the UK" by the country's banking watchdog.

According to the FT, in the month since the Tesco Bank breach, the Financial Conduct Authority has contacted several British lenders to check if they too are using a sequential numbering scheme for their cards.

Researchers at Newcastle University earlier this month published a study which demonstrated how criminals could have speared the bank's defences by automatically and systematically generating different variations of the card security data and firing it at multiple websites.

Because the Visa card system does not detect multiple invalid payment requests on the same card from different websites, unlimited guesses can be made to find the correct expiry date and CVV code.

In a reply to an FT query, Tesco Bank refused to confirm or deny the report. “As this remains an ongoing investigation, we will not comment on specific questions regarding the incident," says the bank. "However, we will confirm that our first priority was, and remains, to ensure that our customers’ accounts are safe and secure, and that we communicate with our customers immediately and transparently.”

Comments: (3)

Ketharaman Swaminathan
Ketharaman Swaminathan - GTM360 Marketing Solutions - Pune 12 December, 2016, 17:46Be the first to give this comment the thumbs up 0 likes

How's this possible? I thought V/MC rules require card numbers to be compliant with MOD 10 / Luhn algorithm, which ensures that they won't be sequential?

A Finextra member
A Finextra member 13 December, 2016, 13:44Be the first to give this comment the thumbs up 0 likes

That is the point. All cards follow the Luhn algorithm, if you know that then the sequence of the cards can remain sequential. It doesn't mean literally increment by 1, rather what is the next valid number following the Luhn algorithm.

Even not being sequential doesnt solve the issue at all. If you are able to keep "pinging cards" with bots, then all you are doing is making the process longer, but you can still just guess card details.

Serious fault with the issuer to be fair in responding to x number of invalid requests. 

Ketharaman Swaminathan
Ketharaman Swaminathan - GTM360 Marketing Solutions - Pune 13 December, 2016, 17:40Be the first to give this comment the thumbs up 0 likes

@AFinextraMember:

TY for your clarification. I assumed sequential meant incremented by 1.

Any idea why FCA would need to contact British lenders to see "if they too are using a sequential numbering scheme for their cards"? Since they must all be generating their numbers using some algorithm, isn't it obvious that they're indeed using a sequential numbering scheme?

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