Presentation of the Cybernetic Proving Ground (CPG in short) project - a cloud-based security testbed for simulation of cyber attacks and evaluation of their impacts on real infrastructure. The artificial environment aims to resemble as much as possible the real world, yet it provides sufficient isolation and control over all activities performed within it. Every simulation is provided with a sandbox where attacks can be safely executed. Experiments are described by end users via scenarios. Data captured during the simulation are stored for later reuse, enabling the user to replay the experiment, analyze its course, etc. Visualization tool enables the end users to monitor the experiment by means of interactive views.