Security researchers have highlighted how corrupt bank insiders might be able to obtain bank card PINs using as little as one or two guesses.

The flaw, which involves the way ATM PINs are encrypted and transmitted across international financial networks (by switches), is far more severe than previous attacks which created a means for insiders to crack PINs using around 15 guesses. By design, it shouldn't be possible to guess a four-digit pin in less than an average of 5,000 attempts.

Israeli academics Omer Berkman and Odelia Moshe Ostrovsky have published a paper, titled The Unbearable Lightness of PIN Cracking (PDF), which explains how the processing system used by banks is open to abuse. One of the attacks targets the translate function in switches. Another abuses functions that are used to allow customers to select their PINs online.

In either case, the flaws create a means for an attacker to discover PIN codes, for example, those entered by customers while withdrawing cash from an ATM providing they have access to the online PIN verification facility or switching processes.