A spoofing attack for a wireless communication system is the most common attack method for unauthorized access and control. IEEE 802.15.4 is a standard that defines only physical and medium access control layers for low rate, low power, and low cost wireless systems. This standard is widely used as lower layers for not only several wireless communication standards but also customized protocols by manufacturers. However, security has not been considered seriously in these customized protocols, due to other important features including efficiency and cost. In this paper, in order to empirically analyze the real world threat in these systems, we chose to study three IEEE 802.15.4 based wireless communication systems as targets. We manually analyzed the customized protocols above IEEE 802.15.4 if there exist vulnerabilities to be exploited. For all three systems, we discover significant vulnerabilities. We implemented a spoofing attack for two targets, and we successfully controlled the targets by our spoofing attack. For the last target, we chose not to run the experiment due to significant safety reasons.