Jeffrey Altman, Viktor Dukhovni, and Nicolas Williams reported that Heimdal, an implementation of Kerberos 5 that aims to be compatible with MIT Kerberos, trusts metadata taken from the unauthenticated plaintext , rather than the authenticated and encrypted KDC response. A man-in-the-middle attacker can use this flaw to impersonate services to the client. See https://orpheus-lyre.info/ for details.