Authors: Luca Viganò, Maria-Camilla Fiazza, Michele Peroli, Matteo Zavatteri Dipartimento di Informatica, Università di Verona, Italy Title: How to defend vulnerable security protocols (a visionary approach) Abstract: Security protocols are often found to be vulnerable after their deployment. We propose an approach that aims at the neutralization or mitigation of the attacks to vulnerable protocols: it avoids the complete dismissal of the interested protocol and allows honest agents to continue to use it until a corrected version is released. Our approach is based on the knowledge of the network topology, which we model as a graph, and on the consequent possibility of creating an interference to an ongoing attack of a Dolev-Yao attacker, by means of non-collaboration actuated by an ad-hoc benign attacker that plays the role of network guardian. Such guardians, positioned in strategical points of the network, have the task of monitoring the messages in transit and discovering at runtime, through particular types of inference, whether an attack is ongoing, interrupting the run of the protocol in the positive case. We have been investigating the different network topologies that make security protocol defense feasible and illustrate our approach by means of concrete examples.