The transformation of energy systems into smart grids will provide a lot of new possibilities to all participants. By upgrading energy distribution nets with information and communication technologies (ICT) customers and suppliers will have additional information. It integrates a bi-directional information transport between both, power supply company and customer. This real-time data is used to save costs by reducing energy consumption, to provide a good alignment with fluctuating renewable energy production, or to actively participate in the market by offering surplus production. However, ICT systems are always a subject to potential cyber-attacks and manipulations. Therefore, securing such systems is one of the most important objectives. In this paper we conduct the security analysis of our smart grid prosumer demonstrator, which is implemented using on a service oriented architecture. We analyze the attackers' motivations and derive an attacker model. We identify weak points of our system with respect to the attacker model and the system architecture. Finally, we propose counter measurements to improve the case study smart grid system.