Software Security: Software security has been/is an active area of research for ages in computer systems. But, with medical devices becoming more complex with more software functionality the risk factor for software attacks has exponentially increased with medical devices in the loop and IoT in general. Software security is an expansive topic by itself and it is hard to cover. The following reference gives the high level requirement of assessing the integrity of medical device software. Though they introduce tools to check software, the bigger picture is that software in medical devices is something to worry about (it's written by people at the FDA!).
Link: Static Analysis of Medical Device Software using CodeSonar

Hardware Security: Hardware security as stated in the Malware section has been growing aggressively, mainly attributed to the face that computer systems nowadays have been manufactured by several companies and especially outsourced. With this into account it is not possible to detect hardware threats that may have been implanted into the computer system. The following links give a brief picture of hardware risks and hardware layers.
Links:

• Generic Infusion Pump Hazard Analysis and Safety Requirements Version 1.0
• SoK: Security and Privacy in Implantable Medical Devices and Body Area Networks

 Miscellaneous: This topic considers the aspects of the following:

1. Resources: This constraint has been discussed as a double edged sword as security algorithms are resource hungry and medical devices are severely short of that. This was the case in smartphone in the early 2000's but with the advent of technology it is no longer a problem, however that can be the testbench for the current IoT/medical devices.  
   Links:
• Low-Energy Encryption for Medical Devices: Security Adds an Extra Design Dimension
• Security in resource-constrained computer systems
2. Algorithms: Analysis of various algorithms for security which can be encryption or decyption or unique algorithms for security in such environments like medical devices or sensor nodes. Again, sensor nodes can be generalized to medical devices because they have roughly the same characteristics.
   Links:
• Analysis of Hardware Encryption Versus Software Encryption on Wireless Sensor Network Motes
• Absence Makes the Heart Grow Fonder: New Directions for Implantable Medical Device Security (You can see the pun :D, this article is worth a read!)