State estimation allows continuous monitoring of a power system by estimating the power system state variables from measurement data. Unfortunately, the measurement data provided by the devices can serve as attack vectors for false data injection attacks. As more components are connected to the internet, power system is exposed to various known and unknown cyber threats. Previous investigations have shown that false data can be injected on data from traditional meters that bypasses bad data detection systems. This paper extends this investigation by giving an overview of cyber security threats to phasor measurement units, assessing the impact of false data injection on hybrid state estimators and suggesting security recommendations. Simulations are performed on IEEE-30 and 118 bus test systems.