In this paper, based on our proposed mal-packet self-propagation models in wireless sensor networks, we use TOSSIM to study their propagation dynamics. We also present a preliminary study of the feasibility of mal-packet defense in sensor networks. Specifically, based on random graph theory and percolation theory, we propose the immunization of the highly-connected nodes in order to partition the network into as many separate pieces as possible, thus preventing or slowing down the mal-packet propagation. We study the percolation threshold of different network densities and the effectiveness of immunization in terms of connection ratio, remaining link ratio, and distribution of component sizes. We also present an analysis of the distribution of component sizes.