Well, on Linux, there are various hardening mechanisms that lock down ptrace, as well as things like /proc/PID/mem/, but I don't think the same restrictions apply to grabbing fds from other processes via /proc/PID/fd/. So in theory a process running as the same user (but which can't just debug your process due to hardening) could steal your pipe and exploit your program... probably not a terribly realistic threat, but a contrived objection to a contrived scenario :)