# Two-Factor Authentication Scheme for Mobile Money: A Review of Threat Models and Countermeasures
## 1 Introduction  
The exponential demand for mobile money services is radically transforming the lives of the large unbanked population in sub-Saharan Africa. The mobile money system (MMS) makes it possible to render diversified services at affordable costs to remote areas and low-income people. Therefore, [PERSON] [1] defined a mobile money system as a payment account that sits on the user's mobile phone and is used to engage in financial transactions by going through a menu. In sub-Saharan Africa, MMS has spread at a remarkable speed and extends many benefits such as convenience, reliability, speed,flexibility, and affordability [2]. Furthermore, it settles domestic financial matters, avoids security hazards of carrying hard cash, and eliminates standing in long queues at banks [3].  
In Uganda, mobile money payment systems such as MTN Uganda, Airtel, UTL, Africell, M-Cash, Ezeey Money, and Micropay have a combined network of approximately 200,857 agents who act as intermediaries [4; 5; 6]. These MMSs provide services ranging from sending and receiving the money to checking account balances [7; 8]. Mobile money agents and users interact with the MMS using the unstructured supplementary service data (USSD) protocol interface that has a main menu and short message service (SMS) sent by the telecommunication company for notification purposes.  
Although MMS plays an essential part in bridging the financial inclusion gap, unfortunately, the security of MMS has been a rising concern with the evolution of mobile technology. Despite the considerable effort invested in providing a more robust and secure system, most of the existing MMSs still rely on a weak two-factor authentication (2 FA) scheme. Various attacks to mobile money's 2 FA scheme include man-in-the-middle (MITM) attack, authentication attack, replay attack, identity theft, USSD technology vulnerabilities, brute force attack, social engineering attacks, and denial of service (DoS) attack [8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. [PERSON] et al. [23] also observed that the current MMS uses nonstandard cryptography, which is easily compromised, thus limiting the integrity and privacy guarantees of the software, giving rise to the threat of forged transactions and loss of transaction privacy.  
In Uganda, many customers and businesses have lost billions of shillings to adversaries because of the sophisticated technologies used to attack MMS's 2 FA, which is the weakest authentication method [24]. The purpose of this study is to analyse critically the previously published literature on the threat models and countermeasures for the mobile money's 2 FA scheme. The MMS primarily deals with high-level private financial and confidential information, where the security has to be top-notch as threats cannot be accepted [7]. Therefore, deploying highly secure MMS will ensure secure authentication, access control, integrity, confidentiality, availability, nonrepudiation, and privacy of the confidential information [25; 26].  
There are related survey papers that focused on mobile 2 FA schemes. For example, [PERSON] et al. [27] presented a comprehensive review of smart mobile device authentication schemes. [PERSON] et al. [20] explored