A class of malicious attacks against remote state estimation in cyber-physical systems is considered. A sensor adopts an acknowledgement (ACK)-based online power schedule to improve the remote state estimation performance under limited resources. To launch malicious attacks, the attacker can modify the ACKs from the remote estimator and convey fake information to the sensor, thereby misleading the sensor with subsequent performance degradation. One feasible attack pattern is proposed and the corresponding effect on the estimation performance is derived analytically. Due to the ACKs being unreliable, the sensor needs to decide at each instant, whether to trust the ACK information or not and adapt the transmission schedule accordingly. In the meanwhile, there is also a tradeoff for the attacker between attacking and not attacking when the modification of ACKs is costly. To investigate the optimal strategies for both the sensor and the attacker, a gametheoretic framework is built and the equilibrium for both sides is studied.