A user device may strengthen the protection level of a digital content by dividing the security and normal modes and performing an operation. In order to further strengthen the protection level of the digital content, the user device may determine whether the main operating system is hacked or not, and blocks the operation in the secure mode. Otherwise, the device authorization information indicating the device security level of the user device is authorized by the content service server, and the user device blocks the operation in the secure mode according to the result.