Potential threats to SCADA systems come from a variety of sources including commercial rivals, ex-employees, hackers and script kiddies, terrorists and malware such as viruses, trojans and worms.
A number of high profile incidents involving control systems, such as the Slammer worm's intrusion into Davis-Besse Ohio nuclear power plant network have unfortunately given SCADA systems bad publicity. Not only has this publicity painted the current security of these systems in a negative light, but it also publicises their very existence.
Whereas in the past a SCADA system could be said to operate with a sense of "security by obscurity", it is no longer the case that they can be considered invisible to the outside world.