It is critical for you to download and install 2 previously-released security patches (SUPEE-5344 and SUPEE-1533) from the Magento Community Edition download page (https://www.magentocommerce.com/products/downloads/magento/).  Please do this immediately, as Check Point Software Technologies has published a technical description of how they discovered the issue, which we feel might serve as a tutorial for implementing an attack against your website. 