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A security researcher said he found a way to game Starbucks gift cards to generate unlimited amounts of money on them. Both he and the coffee chain are grumbling after he used a fraudulent card to make a purchase, then repaid the amount and reported the vulnerability.

Egor Homakov of the Sakurity security consultancy found a weakness known as a race condition in the section of the Starbucks website responsible for checking balances and transferring money to gift cards. To test if an exploit would work in the real world, the researcher bought three $5 cards. After a fair amount of experimentation, he managed to transfer the $5 balance from card A to card B, not just once as one would expect, but twice. As a result, Homakov now had a total balance of $20, a net—and fraudulent—gain of $5.

The researcher went on to visit a downtown San Francisco Starbucks location to make sure his attack would actually work. He used the two cards to make a $16.70 cent purchase. He went on to deposit an additional $10 from his credit card "to make sure the US justice system will not put us in jail over $1.70," he explained in a blog post. Here's where hurt feelings—and arguably an overreaction on the part of both parties—entered into the story. Homakov wrote:

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