Change the password on someone else's Mac OS

Every modern OS has a mode that grants you superuser permissions, without the need for a password. This mode is supposed to be used to run tasks that require exclusive access to shared resources or if you lost your password, for example.

The only reason this mode exists is because these OS's are seen as multiuser and the environment in which the computer is set should be a friendly one. Nevertheless this mode, the single user mode, can be used for some very nasty things if you now your BASH (or the existing shell) basics. Here we're going to change the root password.

First, you have to turn the computer off, because this mode is entered at boot time. While the system is booting keep cmd+S pressed, until a console appears. When it does, you're good to go. Just mount the file system with mount -wu /.

Now for the final step, type passwd and return. You should be asked to enter the new root password twice. Once that's done, you have successfully gained root rights to the OS. Reboot it and try it out!

If you want to be "protected" from this, you can install SecureIt, that requests you to enter a password before showing the console, when booting.

Notes:

1. You cannot enter single user or verbose mode if the computer owner or administrator has enabled Open Firmware Password Protection.
2. When in single-user mode, the keyboard layout is US English.
3. In Linux systems you have to change the configuration of the booter, can be easily done
4. In Windows there's the Safe Mode