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Millions of PIN Mailers are used by banks and other financial institutions to send new security codes linked to credit and debit cards to customers. Typically, the number will be printed on some kind of background pattern and hidden beneath a tear off strip to make it - in theory - impossible to tamper with and avoid detection.

However, Mike Bond and a team from Cambridge found that poor printing quality meant that many numbers could be read through the tear off strip simply by shining a bright light at a particular angle. In other cases, the PIN Mailer could be read by scanning it into a computer and using simple image enhancement features found in popular graphics packages such as Photoshop.

The team told the banks of their findings last year and the institutions have slowly been changing the style of the mailers to defeat Bond's techniques. However, the researchers say they are disturbed that the old style mailers are still be used in large quantities.

You can read the full details of the PIN Mailer vulnerability report in PDF format. No bright light required.