Breaking and Fixing Public-Key Kerberos

Iliano Cervesato, Carnegie Mellon University
Aaron D. Jaggard, Tulane University of Louisiana
Andre Scedrov, University of Pennsylvania
Joe-Kay Tsay, University of Pennsylvania
Christopher Walstad, University of Pennsylvania

Article comments

A revised version appears in Information and Computation Volume 206, Issues 2-4, February-April 2008, Pages 402-424

Abstract

We report on a man-in-the-middle attack on PKINIT, the public key extension of the widely deployed Kerberos 5 authentication protocol. This flaw allows an attacker to impersonate Kerberos administrative principals (KDC) and end-servers to a client, hence breaching the authentication guarantees of Kerberos. It also gives the attacker the keys that the KDC would normally generate to encrypt the service requests of this client, hence defeating confidentiality as well. The discovery of this attack caused the IETF to change the specification of PKINIT and Microsoft to release a security update for some Windows operating systems. We discovered this attack as part of an ongoing formal analysis of the Kerberos protocol suite, and we have formally verified several possible fixes to PKINIT—including the one adopted by the IETF—that prevent our attack as well as other authentication and secrecy properties of Kerberos with PKINIT.