Phone-based mobile money is becoming the dominant paradigm for financial services in the developing world. For example, mPesa has a cash flow of over thirty billion USD, equivalent to nearly half of Kenya's GDP. Inside of these markets, competitors have appeared who leverage ThinSIMS, small SIM-card add-ons, to provide alternative mobile money implementations. However, the security implications of ThinSIMs are not well understood. To resolve this, we explore the security of phone-based mobile money systems against attacks via the SIM interface, the 3GPPdefined interface between a SIM card and a phone. Using a ThinSIM to intercept and initiate communication over the SIM interface, we demonstrate that a malicious ThinSIM can steal a user's mPesa credentials and initiate transactions without the user's consent or knowledge. We also demonstrate a similar ThinSIM-based attack against USSD-based mobile money systems that allows for similar transactions without the user's knowledge or participation. Lastly, we propose and implement modifications to both STK and USSDbased mobile money systems to limit the impact of our discovered ThinSIM-based attacks.
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