The IEEE 802.11 standard de nes a 4-way handshake between a supplicant and authenticator for secure communication. Many attacks such as KRACK, cipher downgrades, and key recovery attacks have been recently discovered against it. These attacks raise the question as to whether the implementation violates one of the required security properties or whether the security properties are insucient. To the best of our knowledge, this is the rst work that shows how to answer this question using formal methods. We model and analyse a variety of these attacks using the Tamarin prover against the security properties mandated by the standard for the 4-way handshake. This lets us see which security properties are violated. We nd that our Tamarin models vulnerable to the KRACK attacks do not violate any of the standard's security properties, indicating that the properties, as speci ed by the standard, are insucient. We propose an additional security property and show that it is violated by systems vulnerable to KRACK attacks, and that enforcing this property is successful in stopping them. We demonstrate how to use Tamarin to automatically test the adequacy of a set of security properties against attacks, and that the suggested mitigations make 802.11 secure against these attacks.