I sadly suspect if a large corporation with a very small ethics department or a state actor wanted to subvert these networks, they would have very little trouble doing so. Another comment mentioned Stuxnet which proves that point rather well.
Fortunately, these plants I am talking about are food plants (yoghurt, pudding, and such), so the risk of some foreign government wanting to shut down that plant is rather low. ;-)
And the risk of becoming infected by drive-by malware is contained by not letting these machines talk to the Internet.
(There is one connection to the regular corporate network, which does have Internet access, to tell the ERP system how much of each ingredient is left so the Purchasing department will order new ingredients on time. But in my benevolent imagination that connection is one teeeny-tiny hole through a humongous firewall.)
Fortunately, these plants I am talking about are food plants (yoghurt, pudding, and such), so the risk of some foreign government wanting to shut down that plant is rather low. ;-)
And the risk of becoming infected by drive-by malware is contained by not letting these machines talk to the Internet. (There is one connection to the regular corporate network, which does have Internet access, to tell the ERP system how much of each ingredient is left so the Purchasing department will order new ingredients on time. But in my benevolent imagination that connection is one teeeny-tiny hole through a humongous firewall.)