Polymarket's US platform suffered a massive fraud attack involving stolen debit cards, with losses reaching at least $10 million during the prediction market's rapid American expansion. The payment processor handling transactions for Polymarket discovered in February that fraudsters were systematically linking stolen cards to accounts to place bets and withdraw funds to controlled destinations. At the peak of the fraudulent activity, over 80% of deposits on the platform were determined to be illegitimate, vastly exceeding the industry standard fraud rate of approximately 1%. The incident highlights significant compliance and security challenges facing crypto-adjacent platforms as they scale operations within regulated US markets.