Man-in-the-Middle in Tunnelled Authentication Protocols

Deploying a new security protocol is expensive. This encourages system designers to look for ways of re-using existing infrastructure. When security protocols and components are re-used, it is critical to re-examine the security of the resulting system as a whole. For example, it has become a common paradigm to run a legacy client authentication protocol within a server-authenticated tunnel. The commonest example of such composition is the use of HTTP authentication inside a TLS tunnel. In this paper, we describe a man-in-the-middle attack on such protocol composition. The vulnerability arises if the legacy client authentication protocol is used both in tunnelled and untunnelled forms. We propose to solve the discovered problem by using a cryptographic binding between the client authentication protocol and the protection protocol.