## 8 Security and Privacy Risk Assessment
### Security and Privacy Risk Assessment at the Requirements Analysis Phase
#### 8.3.2.2 Determine Likelihood  
The likelihood represents the probability that a threat event will occur by exploiting one or more vulnerabilities. To estimate the likelihood, the assessor team needs to consider factors such as:  
* Adversary intent and skill level.
* The affected asset.
* Historical evidence about the threat.  
The same threat can have a different likelihood score based on the source of the threat and assets affected. For example, a DoS attack can compromise the availability of the web server and sensor devices. Initiating a DoS attack on a web server will be easier than the sensor device, as an attack on a sensor device will require advanced level skills and tools. In this scenario, the likelihood level will be different on both assets. So, during the assessment the assessor team needs to assign the likelihood level based on the available evidence, experience and expert judgement. Table 7 outlines the assessment scale for calculating likelihood level.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Impact Factor**} & \multirow{2}{*}{**Impact Description**} & \multicolumn{4}{c}{**Impact Level**} \\ \cline{3-5}  & & & **Semi-Quantitative** \\ \cline{3-5}  & & **Qualitative** & **Scale** & **Bins** \\ \hline \multirow{2}{*}{\begin{tabular}{c} Harm to user \\ health \\ \end{tabular} } & Only the person who is using the device will be in risk & Very High & 100 & 10 \\ \hline \multirow{2}{*}{\begin{tabular}{c} Operational \\ impacts \\ \end{tabular} } & Only that device will be out & & & \\  & of operation, it will not & & & \\  & severely affect the overall & & & \\  & application operation & & & \\ \hline \multirow{2}{*}{\
\begin{tabular}{c} Financial loss \\ \end{tabular} } & Loss of a single device will & & & \\  & have limited financial impact & & & \\ \hline \multirow{2}{*}{
\begin{tabular}{c} Reputational \\ harm \\ \end{tabular} } & Loss of a single sensor device & & & & \\  & will not create severe & & & \\  & reputational harm & & & \\ \hline Loss of assets & Only one sensor device & & & \\ \hline \multicolumn{5}{c}{Average} & Medium & 70 & 5.2 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Impact analysis for physical attack on a sensor node.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{
\begin{tabular}{c} **Qualitative** \\ **Values** \\ \end{tabular} } & \multicolumn{2}{c}{**Semi-Quantitative**} & \multirow{2}{*}{**Impact Definition**} \\ \cline{2-2}  & **Scale** & **Bins** & \\ \hline Very Low (1) & 0–4 & 0 & Threat event will have negligible adverse effects \\ \hline Low (2) & 5–20 & 2 & Threat event will have limited adverse effects \\ \hline Medium (3) & 21–79 & 5 & Threat event will have serious adverse effects \\ \hline High (4) & 80–95 & 8 & Threat event will have catastrophic adverse effects \\ \hline Very High (5) & 96–100 & 10 & Threat event will have multiple catastrophic effects \\ \hline \hline \end{tabular}
\end{table}
Table 5: Assessment scale for impact.  
Table 8 illustrates an example for identifying the likelihood level for a DoS attack on a web server. During the calculation, the likelihood level value is assigned to each likelihood factor and then the average of all the factors is calculated.