Exactly, if it isn't networked then it is basically just an appliance. Just as you don't need to update the software in your microwave oven, you probably don't need to update your CNC machine. The exception to this is probably new features, such as supporting new file formats perhaps.
There might be a slight concern that the CNC machine could be infected by USB-borne malware. The worse case there is probably just infecting other computers through additional USB sticks. That, or industrial sabotage, but that sort of threat was relatively unheard of until recently.
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Exactly, if it isn't networked then it is basically just an appliance. Just as you don't need to update the software in your microwave oven, you probably don't need to update your CNC machine. The exception to this is probably new features, such as supporting new file formats perhaps.
There might be a slight concern that the CNC machine could be infected by USB-borne malware. The worse case there is probably just infecting other computers through additional USB sticks. That, or industrial sabotage, but that sort of threat was relatively unheard of until recently.