Single-user mode is intended as a system recovery method, providing a single user root access to the system by providing a boot option at startup. By default, no authentication is performed if single-user mode is selected. This provides a trivial mechanism of bypassing security on the machine and gaining root access.

To require entry of the root password even if the system is started in single-user mode, add the following line to the /etc/inittab file:

~:S:wait:/sbin/sulogin