Electric power grids are undergoing a modernization process. By relying on the ICT infrastructure and on Internet connectivity, these so-called Smart Grids are now able to provide new functionalities and to become more efficient. However, despite the existence of a few standards that aim to specify the secure operation of Smart Grids, utility companies do not have a comprehensive set of metrics and evaluation tools for assessing security properties in these infrastructures. Thus, it is necessary to develop new toolsets to provide support for vulnerability analysis in Smart Grids. This paper proposes ASTORIA, a framework developed to allow the simulation of attacks and the evaluation of their impact on Smart Grid infrastructures, using closely-related real devices and real topologies comprising both power grid elements as well as ICT and networking equipment. We anticipate that ASTORIA can be used by Smart Grid operators not only to analyze the impact of malicious attacks and other security threats in different components, but also to permit the development and evaluation of anomaly detection techniques in a simulation environment. Further, we present evaluation scenarios illustrating customizable Smart Grid topologies, comprising sensors, master and remote stations, and using an extensible set of attack profiles.