Microgrids are adopted to provide distributed generation of renewable energy resources and scalable integration of loads. To ensure the reliability of their power system operations, distributed and cooperative control schemes are proposed by integrating communication networks at their control layers. However, the information exchanged at the communication channels is vulnerable to malicious attacks aiming to introduce voltage instability and blackouts. In this paper, we design and evaluate a novel type of attacks on the cooperative control and communication layers in microgrids, where the attacker targets the communication links between distributed generators (DGs) and manipulates the reference voltage data exchanged by their controllers. We analyze the control-theoretic and detectability properties of this attack to assess its impact on reference voltage synchronization at the different control layers of a microgrid. Results from numerical simulation are presented to demonstrate this attack, and the maximum voltage deviation and inaccurate reference voltage synchronization it causes in the microgrid.