On the Effective Prevention of TLS Man-in-the-Middle Attacks in Web Applications

In this paper we consider TLS Man-In-The-Middle (MITM) attacks in the context of web applications, where the attacker is able to successfully impersonate the legitimate server to the user, with the goal of impersonating the user to the server and thus compromising the user's online account and data. We describe in detail why the recently proposed client authentication protocols based on TLS Channel IDs, as well as client web authentication in general, cannot fully prevent such attacks. Nevertheless, we show that strong client authentication, such as Channel ID-based authentication, can be combined with the concept of server invariance, a weaker and easier to achieve property than server authentication, in order to protect against the considered attacks. We specifically leverage Channel ID-based authentication in combination with server invariance to create a novel mechanism that we call SISCA: Server Invariance with Strong Client Authentication. SISCA resists user impersonation via TLS MITM attacks, regardless of how the attacker is able to successfully achieve server impersonation. We analyze our proposal and show how it can be integrated in today's web infrastructure.