Sakurity hacker Egor Homakov has found a way to dupe Starbucks into loading free cash onto the "coffee" chain's payment cards.
Homakov says a race condition within Starbuck's card purchase system means money can be transferred between cards without it being deducted.
The bug hunter exploited the bug and tested it by purchasing food and drink at Starbucks.
He says he pulled off the hack which he first quietly reported to the coffee house by opening two browser windows and simultaneously moving US$5 from one account to another in both sessions.
"So the transfer of money from card one to card two is stateful: the first POST request saves these values in the session and the second POST actually transfers the money and clears the session," Homakov says.
"This makes the exploitation harder because the session gets destroyed right after first confirmation request and second one fails."
"But this 'protection' is still easy to bypass: just use same account from two different browsers (with different session cookies)."
The result was that $10 was loaded onto the second account but only $5 was deducted.
Homakov tested that the cash transfer had actually occurred in a Starbucks cafe in San Francisco where he bought water, gum, and a chicken sandwich.
He added $10 onto the account to reimburse the coffee giant for the $1.70 stolen.
However Starbucks did not appreciate the exploitation nor the hacker's quiet disclosure despite closing the hole in about 10 days earlier this month.
"The unpleasant part is a guy from Starbucks calling me … mentioning 'fraud' and 'malicious actions' instead," Homakov says.
The push back is not unexpected since vulnerability tests conducted without explicit authorisation often irk organisations.
Security bods commenting on Reddit have debated Starbuck's response with most feeling that Homakov pushed the boundaries of acceptable white hat research through his cafe purchase.
The exploitation of discovered bugs is widely-considered bad practise and will usually result in the forfeiture of paid bounties, and occasionally in legal action.
Security professionals have long urged technology teams of all sizes to maintain a bug reporting security policy on their company websites, and assign an internal staff member to handle inquiries.
The New Zealand Internet Task Force offers a nice template for how to do this stuff right in its guidelines (PDF) on the handling of bug reports by researchers and organisations. ®