## 2 Materials and Methods
### Background and Related Work
#### 2.1.2 Threat Model
In general, this division into three parts is intended to be more didactic and understandable, bringing with it the ability to be reusable in future works that may extend or implement MFA_R. As for the scope of the attack surface, it is important to note that the networking technology used in the LPWAN security domain was not specified. Several technologies bring embedded problem solutions as challenges to be overcome for the proper and secure authentication implementation. As some problems shown in the services were mitigated during the mechanism design, others may arise in this detail of the surface link. This fact will vary according to the technology used as some already have security measures implemented in their protocols while others do not. The next item in the methodology is threat modeling (2), which is a tool that brings possible threats to the artifact under development at design time. Thus, although there are several threat-modeling methodologies, we chose to use three different methodologies, organized into three steps: **STRIDE (2.1)** classification, severity assessment with **CVSS (2.2)**, \begin{table}
\begin{tabular}{c c c c} \hline \hline \# & **Group** & **Surface** & **Vulnerabilities** \\ \hline s01 & actors & Device & impersonation, sleep privation, and jamming \\ \hline s02 & & RegServ & information disclosure, leakage e DoS \\ s03 & services & AuthServ & device impersonation, brute force attack \\ s04 & & DataServ & device impersonation, and tampering data \\ \hline s05 & links & HTTPS & information leakage, MITM \\ s06 & & LPWAN & message tampering, message fabrication, and unauthorized access \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Description of attack surface for MFA_R—surfaces are grouped into three categories (external actors, services, and data links). The data are organized in the surface column and the vulnerability column, with the latter being a set of all vulnerabilities listed for the related surface. Figure 1: MFA_R Threat-modeling steps—in green, the macro steps are detailed; in purple, the three threat analysis (TM) steps are detailed. The workflow was divided into three macro-steps, with the second step being detailed in three other sub-steps. and risk analysis with **DREAD (2.3)**, and the result is detailed in Tables 3 and 4, and is revealed in Table 5.  
The first methodology is **STRIDE (2.1)**, which was used to categorize vulnerabilities contained in MFA_R and contains six different categories of threats [60]: spoofing, tampering, repudiation, information disclosure, denial of service, and elevation of privilege. Data flow diagrams (DFD) are used in its methodology to describe the system, its components, and the security domains through which the data travels. Comparatively, as it is a more consolidated approach, it has great acceptance by the community, many tools that support it, and many published articles about this methodology or that use it in their modeling. The second methodology used was the common vulnerability score system, **CVSS (2.2)**, adopted for prioritizing the severity of the potential impact of vulnerabilities. This tool provides a systematic approach to assessing the severity of vulnerabilities using weights [61] ranging from zero to three (no severity and high severity, respectively) and is commonly used with other methodologies such as common vulnerabilities exposure (CVE) [62; 63; 64]. Therefore, due to not being a tool that classifies or proposes attack vectors, it is very commonly used in conjunction with other threat modeling methodologies (TMMs) during the threat assessment process in a project. Third, the risk assessment **DREAD (2.3)** was used. This is an acronym for damage, reproducibility, exploitability, affect users, and discoverability [65; 66], which are the categorizations used to quantify risk in this methodology. Furthermore, such a methodology can be viewed as a way of measuring the vulnerability risk associated with each associated threat [67] through the use of numerical values.