  Small Unmanned Aerial Systems (sUAS) must meet rigorous safety standards when
deployed in high-stress emergency response scenarios; however many reported
accidents have involved humans in the loop. In this paper, we, therefore,
present the HiFuzz testing framework, which uses fuzz testing to identify
system vulnerabilities associated with human interactions. HiFuzz includes
three distinct levels that progress from a low-cost, limited-fidelity,
large-scale, no-hazard environment, using fully simulated Proxy Human Agents,
via an intermediate level, where proxy humans are replaced with real humans, to
a high-stakes, high-cost, real-world environment. Through applying HiFuzz to an
autonomous multi-sUAS system-under-test, we show that each test level serves a
unique purpose in revealing vulnerabilities and making the system more robust
with respect to human mistakes. While HiFuzz is designed for testing sUAS
systems, we further discuss its potential for use in other Cyber-Physical
Systems.
