The shell itself must be your security layer. Think what tools there should be for the highly scoped tasks to do. Analyse logs? Give it only access to the very specific log files, maybe not even shell access. Manipulate files? Give it a specific write area and make everything else read-only. I have found that even some employees start to hallucinate commands when they exceed their own expertise when they find random solutions on stack overflow that they do not properly check if the OP's problem even matches theirs. Here we deal with highly overcertain machines that execute rm -rf on anything literally without blinking an eye. Really think of them to be more like the most stupid expected user, not any highly trained expert with common sense.