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The degree to which a cryptographic protocol is susceptible to successful attacks or penetrations cannot be accurately calculated because of the more fine and fast algorithms to break ciphers, being introduced. To deal with the issue, analysis of a cryptographic protocol becomes more vital; and more exhaustive search for vulnerabilities is required. Kerberos is a widely used security protocol for authentication of the communicating parties. It is usually thought to be the most secure protocol. In this paper, it is shown that a man in the middle attack is possible against Kerberos protocol. This is done using the Strand Spaces, where a series of attacks are carried to verify the authenticity of participants in the Kerberos protocol using formal methods. To make the paper self-contained we also provide a solution on how to solve the problem.