Two Cambridge University computer scientists have warned that corrupt bank employees could exploit computer hardware flaws to gain access to cash card Pins.
According to the New Scientist, Mike Bond and Piotr Zielinski claim that the vulnerability centres on employees who could exploit weaknesses in internal banking systems.
Under normal circumstances it would take around 5,000 attempts to guess a Pin, but cash machines lock out after three attempts.
But banks' internal systems are not all protected in the same way, so an unscrupulous employee could make many more attempts to guess the code.
By using simple brute force attacks on unprotected systems inside a bank's infrastructure it could be possible to crack a Pin in just 15 attempts.
According to the scientists' research, thieves with access to banking systems using a more complex technique could harvest 14,000 Pins in one hour.
The scientists warned that the fraud could generate millions from the sale of stolen codes, or from the manufacture of cloned cards used to access the compromised bank accounts.
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