Common problem: you might consider it trivial to define a rule that forbids access to any file path outside the working directory. Allowing "safe" shell access is already a heuristic, there is no way around it:
- shell scripting is Turing-complete
- there are platform-specific paths that need to be at least readable to perform trivial tasks: Everything is a File!
- security models to make such low-level file access transparent exist, and generally work, but: they are never perfect (container environments, chroot etc). Because...