  Concerns for the resilience of Cyber-Physical Systems (CPS)s in critical
infrastructure are growing. CPS integrate sensing, computation, control, and
networking into physical objects and mission-critical services, connecting
traditional infrastructure to internet technologies. While this integration
increases service efficiency, it has to face the possibility of new threats
posed by the new functionalities. This leads to cyber-threats, such as
denial-of-service, modification of data, information leakage, spreading of
malware, and many others. Cyber-resilience refers to the ability of a CPS to
prepare, absorb, recover, and adapt to the adverse effects associated with
cyber-threats, e.g., physical degradation of the CPS performance resulting from
a cyber-attack. Cyber-resilience aims at ensuring CPS survival by keeping the
core functionalities of the CPS in case of extreme events. The literature on
cyber-resilience is rapidly increasing, leading to a broad variety of research
works addressing this new topic. In this article, we create a systematization
of knowledge about existing scientific efforts of making CPSs cyber-resilient.
We systematically survey recent literature addressing cyber-resilience with a
focus on techniques that may be used on CPSs. We first provide preliminaries
and background on CPSs and threats, and subsequently survey state-of-the-art
approaches that have been proposed by recent research work applicable to CPSs.
In particular, we aim at differentiating research work from traditional risk
management approaches based on the general acceptance that it is unfeasible to
prevent and mitigate all possible risks threatening a CPS. We also discuss
questions and research challenges, with a focus on the practical aspects of
cyber-resilience, such as the use of metrics and evaluation methods as well as
testing and validation environments.
