This work introduces probabilistic model checking as a viable tool-assisted approach for systematically quantifying DoS security threats. The proposed analysis is based on a probabilistic attacker model implementing simultaneous N zombie participants, which subvert secure authentication features in communication protocols and electronic commerce systems. DoS threats are expressed as probabilistic reachability properties that are automatically verified through an appropriate Discrete Time Markov Chain representing the protocol participants and attacker models. The overall analysis takes place in a mature probabilistic model checking toolset called PRISM. We believe that the applied quantitative verification approach is a valuable means for comparing protocol implementations with alternative parameter choices, for optimal resistance to the analyzed threats.