Noncompliance as Deviant Behavior: An Automated Black-box Noncompliance Checker for 4G LTE Cellular Devices

ABSTRACTThe paper focuses on developing an automated black-box testing approach called DIKEUE that checks 4G Long Term Evolution (LTE) control-plane protocol implementations in commercial-off-the-shelf (COTS) cellular devices (also, User Equipments or UEs) for noncompliance with the standard. Unlike prior noncompliance checking approaches which rely on property-guided testing, DIKEUE adopts a property-agnostic, differential testing approach, which leverages the existence of many different control-plane protocol implementations in COTS UEs. DIKEUE uses deviant behavior observed during differential analysis of pairwise COTS UEs as a proxy for identifying noncompliance instances. For deviant behavior identification, DIKEUE first uses black-box automata learning, specialized for 4G LTE control-plane protocols, to extract input-output finite state machine (FSM) for a given UE. It then reduces the identification of deviant behavior in two extracted FSMs as a model checking problem. We applied DIKEUE in checking noncompliance in 14 COTS UEs from 5 vendors and identified 15 new deviant behavior as well as 2 previous implementation issues. Among them, 11 are exploitable whereas 3 can cause potential interoperability issues.