Virtualization is a technique to make the cloud computations secure. This technique can reduce the costs and increase the reliability of systems by means of sharing the physical resources between several VMs (Virtual Machines). However, virtualized environments are vulnerable to some security issues such as lack of performance isolation. These days, using the virtualization as a technique is growing slowly in large organizations. Despite these advantages, virtualizations have created several security challenges. So, the main concern of virtualization service providers and their customers is how to detect and encounter these security challenges. In this research, the resource freeing attack is selected among all available virtualization attacks because of its simplicity and its drastic effects. Resource freeing attack causes an unfair resource distribution among VMs. We use Cloudsim as a powerful cloud simulation toolkit to implement resource freeing attack. After simulating the attack, the CPU's efficiency, response time diagrams, and the obtained bandwidth are measured. The simulation results show the significant changes in VM behavior after occurring the attack in the virtualized environment. These changes help cloud providers to detect resource freeing attack in the cloud.