Devices, systems, and methods of detecting user identity, differentiating between users of a computerized service, and detecting a cyber-attacker. An end-user device (a desktop computer, a laptop computer, a smartphone, a tablet, or the like) interacts and communicates with a server of a computerized server (a banking website, an electronic commerce website, or the like). The interactions are monitored, tracked and logged. Communication interferences are intentionally introduced to the communication session; and the server tracks the response or the reaction of the end-user device to such communication interferences. The system determines whether the user is a legitimate human user; or a cyber-attacker posing as a legitimate human user but actually utilizing a Virtual Machine.