## 4 Case Study of an IoT Enabled Bridge
### Assessment of Vulnerability, Threat Source and Physical Impact  
The assessment of the individual characteristics of vulnerability (i.e., _Attack vector_ (X\({}_{\text{AV}}\), W\({}_{\text{AV}}\)), _Attack complexity_ (X\({}_{\text{AC}}\), W\({}_{\text{AC}}\)), _User interaction_ (X\({}_{\text{UL}}\), W\({}_{\text{UL}}\)). _Priiileges required_ (X\({}_{\text{PR}}\), W\({}_{\text{PR}}\)), _Interoperability_ (X\({}_{\text{IO}}\), W\({}_{\text{IO}}\))), threat source (i.e., _Knowledge_ (X\({}_{\text{KN}}\), W\({}_{\text{KN}}\)). _Resources_ (X\({}_{\text{RE}}\), W\({}_{\text{RE}}\)), _Psychology_ (X\({}_{\text{PS}}\), W\({}_{\text{PS}}\)), _and Terrorism experience_ (X\({}_{\text{Te}}\), W\({}_{\text{Te}}\))) and type of physical impact (X\({}_{\text{PI}}\)) are based on the description of the experimental study as presented in [56]. Concerning the assessment of the MAC layer vulnerability, the exploitation of this vulnerability based on the identified cyber-physical attack scenario, does not require the threat source to interact with any other user (_User interaction_), to have any privileges (_Privileges required_), or to gain access to other interacting IoT devices (_Interoperability_). Thus, assessors can assign W\({}_{\text{UL}}\), W\({}_{\text{PR}}\), W\({}_{\text{IO}}\) a score in [0-0.20] corresponding to a Very low level. The threat source exploits the MAC layer vulnerability without the need to interact with other users, without requiring additional privileges, and without the need to gain entry point through another IoT device, since the IoT based WSN operates under one IoT application. Thus, assessors can assign X\({}_{\text{UI}}\) and X\({}_{\text{PR}}\) a score corresponding to a High or Very High level (i.e., 8-10). They can also assign X\({}_{\text{IO}}\) a score corresponding to a Very low level (0-1). To perpetrate the energy depletion attack successfully, the threat source needs to have a sufficient level of physical proximity with the IoT devices and act without being identified as a malicious sender, in order to send bogus messages targeting the Zigbee protocol. Thus, assessors should assign both W\({}_{\text{AV}}\), W\({}_{\text{AC}}\) values corresponding to a Very High level in [0.81, 1]. In Scenario 1 (i.e., _No control barriers_), the threat source can be in close proximity to the ZigBee device without performing any additional step to communicate with them. Thus, assessors can assign X\({}_{\text{AV}}\), X\({}_{\text{AC}}\) scores corresponding to High or Very high levels. In Scenario 2, the existence of control barriers in physical space only and the lack of control barriers in cyber space reduce X\({}_{\text{AV}}\) that should be assigned a score ranging from Very low to Low level. In Scenario 3, the existence of control barriers in cyber space only, can eliminate the MAC layer vulnerability and thus reduces \(\mathrm{X_{AC}}\) that now should be assigned a score corresponding to a Very low level. As Scenario 4 combines control barriers in both cyber and physical spaces, \(\mathrm{X_{AV}}\) should be assigned a score ranging from Very Low to Low level and \(\mathrm{X_{AC}}\) should be assigned a score corresponding to a Low level. Regarding assessment of the threat source, the exploitation of the MAC layer vulnerability does require a moderate level of knowledge in the three steps, although it does not necessitate resources (e.g., advanced computational modules) or manpower. Thus, assessors should assign \(\mathrm{W_{KN}}\) a value in [0.61, 1.0] corresponding to a High to Very high level. They should also assign \(\mathrm{W_{RE}}\) a value in [0, 0.4] corresponding to a Very low or Low level. Additionally, the exploitation of the MAC layer vulnerability requires strong motives and terrorism experience. Thus, assessors should assign \(\mathrm{W_{PS}}\) and \(\mathrm{W_{Te}}\) a value in [0.81, 1.0] corresponding to a Very high level.