Mobile wallets promise a future where users do not need to carry physical payment cards. However, the slow adoption of contactless point of sales (POS) terminals by merchants limits the potential of Near-Field Communication (NFC) based payment devices. In this paper, we present Wild Card, a secure and backward compatible solution for making mobile payments at conventional magnetic stripe based POS terminals. Our solution resembles a traditional credit card in its physical dimensions and stays in the phone case. It can be programmatically set by an NFC-enabled mobile phone to any card number that the user owns. The key technologies that enable Wild Card are a fully programmable magnetic stripe, an energy harvesting system that allows the card to be charged and programmed by the phone through NFC, and a security mechanism that makes card information resilient to attacks on mobile devices. With a prototype, we evaluate the feasibility of Wild Card in terms of functionality and energy budget.