In John's example of friends and enemies, what is the name used to refer to the intruder?

The name that can be used to refer to the intruder in John's example of friends and enemies is:

a cyber-criminal.

Explanation:

A cyber-criminal is an individual who uses technology to commit malicious activities on digital systems or networks with the intention of stealing sensitive data for profit, either financial or otherwise.  In addition to committing data theft, a cyber-criminal may be involved in other malicious activities, including cyberbullying or planting worms or viruses.  Cyber-crimes are the latest wave of criminal activities that are erupting from the internet of things.

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