  Digital Manufacturing (DM) refers to the on-going adoption of smarter, more
agile manufacturing processes and cyber-physical systems. This includes modern
techniques and technologies such as Additive Manufacturing (AM)/3D printing, as
well as the Industrial Internet of Things (IIoT) and the broader trend toward
Industry 4.0. However, this adoption is not without risks: with a growing
complexity and connectivity, so too grows the cyber-physical attack surface.
Here, malicious actors might seek to steal sensitive information or sabotage
products or production lines, causing financial and reputational loss. Of
particular concern are where such malicious attacks may enter the complex
supply chains of DM systems as Trojans -- malicious modifications that may
trigger their payloads at later times or stages of the product lifecycle.
  In this work, we thus present a comprehensive overview of the threats posed
by Trojans in Digital Manufacturing. We cover both hardware and software
Trojans which may exist in products or their production and supply lines. From
this, we produce a novel taxonomy for classifying and analyzing these threats,
and elaborate on how different side channels (e.g. visual, thermal, acoustic,
power, and magnetic) may be used to either enhance the impact of a given Trojan
or utilized as part of a defensive strategy. Other defenses are also presented
-- including hardware, web-, and software-related. To conclude, we discuss
seven different case studies and elaborate how they fit into our taxonomy.
Overall, this paper presents a detailed survey of the Trojan landscape for
Digital Manufacturing: threats, defenses, and the importance of implementing
secure practices.
