Demand response and active participation of residential houses are expected to play a critical role in the future smart grids. Market based transactive exchanges between prosumers are triggered by the increased deployments of renewable generations and microgrid architectures. Transactive Energy Systems (TES) employ economic and control mechanisms to dynamically balance the demand and supply across the full electrical grid. Effective transactive mechanisms leverage on a large number of distributed edge-computing and a communication architecture. Given the prolific usage of digital devices, the assets within a transactive environment are vulnerable to cyber threats. In this paper, various components within a transactive environment are represented and modelled. The effect of cyber attacks on the operation of a transactive energy systems is studied through proxy attacks simulated on the test system.