According to explosion of smart-devices, demands on N-screen services based on cooperation of multiple screens are rapidly increasing. These N-screen services can provide new user-experience (UX) to users. That is, it can provide technical advances to users. On the other hand, it causes new security problems. In this paper, we analyze conventional web-security attacks, and we propose and analyze new security requirements for migration, combination, and separation of web-contents based on N-screen service scenarios. Also, we develop N-screen cooperation service framework in order to ensure user security.