Designed to Be Broken: A Reverse Engineering Study of the 3D Secure 2.0 Payment Protocol

3 Domain Secure 2.0 (3DS 2.0) is the most prominent user authentication protocol for credit card based online payment. 3DS 2.0 relies on risk assessment to decide whether to challenge the payment initiator for second factor authentication information (e.g., through a passcode). The 3DS 2.0 standard itself does not specify how to implement transaction risk assessment. The research questions addressed in this paper therefore are: how is transaction risk assessment implemented for current credit cards and are there practical exploits against the 3DS 2.0 risk assessment approach? We conduct a detailed reverse engineering study of 3DS 2.0 for payment using a browser, the first study of this kind. Through experiments with different cards, from different countries and for varying amounts, we deduct the data and decision making process that card issuers use in transaction risk assessment. We will see that card issuers differ considerable in terms of their risk appetite. We also demonstrate a practical impersonation attack against 3DS 2.0 that avoids being challenged for second factor authentication information, requiring no more data than obtained with the reverse engineering approach presented in this paper.