What is an example of non-propagating malicious code?

A back door is an example of non-propagating malicious code because it functions as a way for unauthorized users to access a system or network without the need for the usual authentication process. Essentially, once a back door is installed on a system, it does not replicate itself to spread to other systems or networks. Its primary purpose is to allow continued access to an attacker, often allowing them to control the compromised system or exfiltrate data without raising alarms.

In contrast, other options like worms are designed specifically to replicate themselves across networks, actively seeking out vulnerabilities to exploit in order to spread, which makes them propagating threats. Similarly, a Trojan horse disguises itself as legitimate software or files and can lead to harmful effects, but it doesn't self-replicate either. However, it is generally categorized as malicious software that facilitates other harmful actions rather than being a non-propagating threat. A hoax, on the other hand, may deceive users but doesn't consist of malicious code in the sense of executing harmful actions on a system. It is more about misinformation than direct system compromise. Therefore, a back door stands out as it does not propagate beyond its initial installation.