Previous analyses of IKEv2 concluded that the protocol was suffering from two authentication vulnerabilities: the penultimate authentication flaw and a vulnerability that leads to a reflection attack. In this paper we analyze the IKEv2 protocol specification using the Spin model checker. To do so we extend and improve an existing modeling method that allows analyzing security protocols using Spin. For completeness we indicate each abstraction we make when writing the model. As a result we show that the reflection attack is actually not applicable. We further discuss two modifications of the protocol that we prove to be effective against the penultimate authentication flaw.