Unexpected items in the banking area
Analysis Security analysts have narrowed down the range of possible explanations for the Tesco Bank breach.
Earlier this month Tesco Bank was obliged to admit that an estimated £2.5m had been looted from 9,000 accounts.
Initially it was feared that money had been taken from 20,000 accounts, but this figure was revised downwards days after the breach was disclosed.
Tesco Bank temporarily froze online account operations and contactless payments for its current account holders in the immediate aftermath of the breach, one of the worst to ever affect a British retail bank.
The bank reimbursed funds to defrauded customers.
The newly established UK National Cyber Security Centre (NCSC) issued a statement on 7 November confirming that an investigation into the Tesco Bank breach was underway.
It said (by way of reassurance) it was “unaware” of any threat to the wider UK banking sector.
The ongoing investigation is being led by the National Crime Agency (NCA) with NCSC providing support and technical assistance.
Security intelligence firm Digital Shadows claims it has identified “multiple instances” of Tesco Bank customers claiming that fraudulent online transactions had been made from their accounts.
Some of these reported small fraudulent transactions of around £20 before larger transactions of £500 or more were attempted.
Another report talked about cash had been fraudulently withdrawn from a customer’s account from an ATM in Rio de Janeiro, Brazil.
Tesco Bank login pages were included as a target in the config files of three major banking trojans: Vawtrak, Dridex and Retefe.
In addition, Digital Shadows has identified a user on the forum linked to cybercrime bazaar AlphaBay claiming back in September that they were able to cash out Tesco Bank accounts with the assistance of an insider at the bank.
The claim remains unverified but nonetheless deserves to be taken seriously in the light of subsequent events.
Several competing theories about what might have happened have sprung up in the wake of the incident.
Security pundits have variously blamed credential stuffing, an inside job, and exploitation of a third-party supplier retail partner for the breach.
Digital Shadows has applied the technique of the Analysis of Competing Hypothesis (ACH) on the available data in an attempt to narrow down the possibilities between four competition theories.
This work suggests that either banking trojan or the cash-out using aggregated card information sit badly with available evidence.
Two other potential explanations – payment system compromise) and cash-out of cloned cards – both offer more promising theories.
NCSC’s statement that the Tesco Bank incident did not represent a threat to the wider UK banking sector, the short timeframe of the attack, and the focus of the breach on current accounts rather than credit accounts were key reference points that Digital Shadows applied to the competing theories it evaluated.
The digital intelligence firm is careful to say it was not possible to definitively rule out any of the four hypotheses examined.
Digital Shadows nonetheless reckons that cash-out of cloned cards is more likely than other possibilities it examined, essentially because it offered a simpler attack method.
“Cash-out of cloned cards would likely have been a simpler to execute than payment system compromise and, in operational terms, would have involved fewer moving parts… [and] may be the more plausible scenario,” Digital Shadows concludes.
The security intelligence firm further concludes that the heist was run by an organised criminal group that likely represents an ongoing threat.
“It is a realistic possibility that the actors responsible for these thefts will attempt to further monetise any Tesco Bank account information in their possession by attempting to sell it within the criminal ecosystem,” Digital Shadows warns.
“In the immediate future, it’s likely Tesco Bank customers will be targeted with phishing emails imitating law enforcement or Tesco Bank customer support.
Tesco Bank customers are advised to exercise caution when receiving calls or opening emails or SMS messages purporting to relate to this incident and to report any suspected phishing attempts to Tesco Bank via email@example.com,” it adds. ®
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