Still, most of these commands will also have bugs. Even if they didn't have convenient built-in ways to escape a restricted shell, I would not expect that these command-line programs, which weren't built to be run in an adversarial environment, weren't vulnerable to buffer overflows or logic errors.

The lesson appears to be that if you want to put users in a restricted environment, don't rely on unaudited unix command-line programs to enforce it. Instead do something at a level below that, such as using a kernel-enforced mechanism.

This is also the problem with sudo of course.