Maginot Line: Assessing a New Cross-app Threat to PII-as-Factor Authentication in Chinese Mobile Apps

—Authentication is one of the established practices to ensure user security. Personally identifiable information (PII), such as national identity card number (ID number) and bank card number, is used widely in China’s mobile apps as an additional secret to authenticate users, i.e., PII-as-Factor Au-thentication ( PaFA ). In this paper, we found a new threat that calls on the cautiousness of PaFA : the simultaneous usages and business-related interactions of apps make the authentication strength of a target app weaker than designed. An adversary, who knows fewer authentication factors (only SMS OTP) than a PaFA system required, can break the authentication by gathering information or abusing cross-app authorization from other apps. To systematically study the potential risks, we proposed a semi-automatic system, MAGGIE , to evaluate the security of PaFA in target apps. By measuring 234 real-world apps in Chinese app markets with the help of MAGGIE , we found 75.4% of apps that deployed PaFA can be bypassed, including the popular and sensitive ones (e.g., AliPay, WeChat, UnionPay), leading to severe consequences like hijack user accounts and making unauthorized purchases. Additionally, we conducted a survey to demonstrate the practical implications of the new risk on users. Finally, we reported our findings to the vendors and provided several mitigation measures.