One of the main characteristics of Smart TVs are apps. Apps extend the Smart TVs menu with various functionalities, ranging from usage of social networks or payed streaming services, to buying articles on Ebay. These actions demand usage of critical data like authentication tokens and passwords, and thus raise the question of new attack scenarios and the general security of Smart TV apps. We investigate attack models for Smart TVs and their apps, and systematically analyze the security of Smart TV devices. We point out that some popular apps, including Facebook, Ebay or Watchever, send login data over unencrypted channels. Even worse, we show that an arbitrary app installed on devices of the market share leader Samsung can gain access to the credentials of a Samsung Single Sign-On account. Therefore, such an app can hijack a complete user account including all his devices, like smartphones and tablets, connected with it. Based on our findings, we provide recommendations of general importance and applicable to areas beyond Smart TVs.