Consumers are being urged to be vigilant after a simple hack to redeem a gift card was discovered and exposed by a victim.

Sydney man Simon Dean purchased a $500 gift card from his local supermarket, but when he went to cash it in he found it had already been redeemed.

"The card had been redeemed about an hour after I bought it and that was really strange and annoying," he told 9News.

He insisted the card hadn't been physically tampered with, and that he checked to ensure the PIN was covered before purchasing it.

Dean, who has a background in computer science, began investigating the issuer, attempting to figure out exactly what had happened.

He bought a fresh card and devised to program to run through the 10,000 possible combinations for the four-digit pin.

His experiment worked, re

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