  Cybersecurity risks are often managed by reducing the system's attack
surface, which includes minimizing the number of interconnections, privileges,
and impacts of an attack. While attack surface reduction techniques have been
frequently deployed in more traditional information technology (IT) domains,
metrics tailored to cyber-physical systems (CPS) have not yet been identified.
This paper introduces attack surface analysis metrics and algorithms to
evaluate the attack surface of a CPS. The proposed approach includes both
physical system impact metrics, along with a variety of cyber system properties
from the software (network connections, methods) and operating system
(privileges, exploit mitigations). The proposed algorithm is defined to
incorporate with the Architecture Analysis \& Design Language (AADL), which is
commonly used to many CPS industries to model their control system
architecture, and tools have been developed to automate this analysis on an
AADL model. Furthermore, the proposed approach is evaluated on a distribution
power grid case study, which includes a 7 feeder distribution system, AADL
model of the SCADA control centers, and analysis of the OpenDNP3 protocol
library used in many real-world SCADA systems.
