We study cyber-physical systems subject to dynamic sensor attacks, relating them to the system's strong observability. First, we find necessary and sufficient conditions for an attacker to create a dynamically undetectable sensor attack and relate these conditions to properties of the system dynamics eigenvectors. Next, we provide an index that gives the minimum number of sensors that must be attacked in order for an attack to be undetectable. Finally, we illustrate our results with a numerical example on the Quadruple Tank Process.