When running a user space program, instead of executing the instruction in the assembly, the CPU will at best raise an exception, and at worse attempt to execut

When running a user space program, instead of executing the instruction in the assembly, the CPU will at best raise an exception, and at worse attempt to execute some other instruction. If the process doesn't have privileges to execute that other instruction, then the CPU will just raise an exception (unless if it has a big hardware bug). I don't see how this could be a security flaw.