Specialized AI is unlikely to be a threat. The issue will be with general AI, and simple but not fully thought out utility functions. This short video makes the concern much more real: https://www.youtube.com/watch?v=kopoLzvh5jY

Consider what would happen if you took that same AI, make its utility function similar to this: https://en.wikipedia.org/wiki/Core_War, and released it on the internet.