Survivable Software for Cyber-Physical Systems presented at OWASP AppSecDC 2012

by Karen Mercedes Goertzel,

Tags: Security

Summary : Industrial control systems (ICS), embedded systems such as weapons systems, medical devices, avionics, etc., can be characterized as logical/physical systems in which software, firmware, or hardware logic directly interfaces with and, often, controls the physical processes of the electrical or mechanical components of the systems. The physical elements of such systems often implement processes on which human physical well-being relies. Because of the potential consequences (loss of life or limb, damage to health or environment, etc.) should such systems fail, the main imperative for both the logical and physical components of these systems has traditionally been safety, which in system engineering terms is an extreme manifestation of quality and dependability. Also, because many logical/physical systems, or components thereof, have "real time" requirements, another imperative for such systems is performance. Because logical/physical systems have traditionally operated either stand-alone or have been connected only to self-contained, wholly isolated (non-Internet connected), dedicated networks, little if any attention has been paid to their security for most of their history. In the late 20th and 21st centuries, however, this isolation has gradually eroded, to the extent that a new term has been adopted for logical/physical systems that now use the Internet or another public network (e.g., the cellular telephone network) to enable communications among system elements: cyber-physical systems. Coincidental with their increasing "cyberization" has been the same trend in the engineering of such systems that has long been the norm for conventional information and communications technology (ICT) systems: the minimization of wholly proprietary, custom-built software and hardware elements and the maximization of commodity technologies and components. This shift means that many of the elements from which cyber-physical systems are built are readily available for inspection and study, so that those of their flaws and defects, weaknesses and vulnerabilities, that have not simply been published by their vendors or users have been discovered through reverse engineering and other analyses. Knowledge and understanding of the vulnerabilities in commodity technologies, combined with accessibility from the public network, means that cyber-physical systems are exposed to security threats that have long targeted more conventional information and communications technology (ICT) systems, but were unknown to previous generations of logical/physical systems. Unlike conventional ICT systems, the components of cyber-physical systems often retain the small size (both in terms of physical size and size of embedded logic) and low resource footprints, real time performance imperatives, and operational scenarios of stand-alone or isolated predecessors. These constraints often make it impossible for the operators of the systems to employ the full range of security measures and countermeasures commonly used to protect more conventional ICT systems against threats. Moreover, the safety imperative often means that blocking of inputs and shutting down of processes are simply not an option when responding to perceived hazards (threats can be characterized intentional hazards). With emergence of Advanced Persistent Threats (APTs), including those crafted specifically to target cyber-physical systems (e.g., Stuxnet), as well as other less complex but invidious methods of intrusion and attack, even the operators of conventional ICT systems are finding that their traditional protect-detect-respond-recover security strategies are increasingly ineffective. Many of these organizations are recognizing that a "paradigm shift" is needed, that does not rely on trying to detect, understand, or second-guess the attacker, but instead which requires a new approach to software and system engineering. This new approach will enable the systems to survive most, if not all, attacks, including those that the attackers have yet to think up. Whether the main imperative for system is security and or safety, or an equal combination of the two, the logic and mechanical components of a cyber-physical system must remain dependable under all anomalous conditions that arise, whether those conditions are accidental or result from intentional threats. Unlike most conventional ICT systems, cyber-physical systems must continue operating at the necessary level of performance under virtually any condition, at least up to the time at which a graceful shut down can be safely accomplished (e.g., after the plane has been safely landed, or the nuclear reactor taken offline). By contrast, the operators of most conventional ICT systems have a much greater tolerance for port blocking, performance degradation, and even temporary failure. One can see this when one compares the software in a safety-critical logical/physical system, in which the proportion of explicit exception handling logic to actual operational logic is often 3:1 or 4:1, as compared with conventional ICT software, in which programmers frequently rely on throwing a general exception and hoping for the best. In this context, a few things distinguish safety-critical software from security-critical software: (1) what constitutes an "anomalous condition" and what is likely to cause such a condition, (2) what is at stake if the software fails as the result of such a condition, and how many (if any) such failures can be tolerated; (3) the measures that can be taken to avoid such failures, or to recover from them, and the acceptable latency between failure and recovery. What both safety- and security-critical software share is a need to minimize to the absolute lowest possible degree the possibility of a software failure. In short, both safety-critical and security-critical software needs to be able to survive the anomalous conditions it is faced with. Engineers of safety-critical software are very good at understanding and writing software that can deal with unintentional anomalies, but are not usually familiar with the more complex, byzantine (non-contiguous) nature of intentional anomalies. Developers of security-critical software are skilled at dealing with intentional anomalies, but not with doing so under the extreme constraints placed on safety-critical software engineers. For survivability engineered into cyber-physical systems, and into the software that makes up the majority of the logic in such systems, engineers would do well to familiarize themselves with both software safety and software security strategies, practices, and tools, and more importantly, to analyze the aspects of each that render them beneficial or impractical for adoption in cyber-physical system software engineering. The place for engineers to begin is a study of the other community\'s development practices and techniques that have direct counterparts in their own, e.g., Safety Engineering | Security Engineering hazard analyses | threat models safety properties| security objectives safe design principles (e.g., failure avoidance and redundancy)| secure design principles (e.g., least privilege, separation of roles) safe coding practices, standards, languages, and tools (shared by both) stochastic (accidental), simple, contiguous, directly propagating faults | non-stochastic (intentional), byzantine (non-contiguous) faults static and dynamic analysis, fault injection | same, plus fuzz testing, vulnerability scans, and penetration tests assurance cases | someday...maybe