Recently, I had a Mac which fried its video logic board. Luckily, Apple had concluded that this was a design flaw and was fixing the affected models for free (see more here). However, I did not find this page for a while, and during that time had to think about recovering my data. So, I looked around the interwebs and found single-user mode.

When the computer is off, press the Power On button while holding the down the command and s keys. Keep holding these down, and instead of booting to the Apple loading screen, it boots to the underlying Unix terminal. Once there, you can enter the following commands:

mount -uw /
cd /Users/
ls


And all of the users' home folders are displayed. Continuing to cd into these folders and ls to view contents, you can browse all of the users' files, without needing a password.

I then found that you are also able to plug in a USB stick and copy files to it (or from it), or perform actions on the files such as move and delete.



While this was helpful for me recovering data from my fried Mac, how is this a good idea? If I ever got hold of the MacBook of a friend and it was locked, I could just shut it down, boot into single-user mode and mess with their files - or even make a copy of them to a USB stick for later use. Macs are used by many people, a lot of whom have very important files that they need to protect.

This obviously isn't a bug, as Apple has a support article on how to enter single-user mode. I also know that one of the original purposes of single-user mode is to reset your password if you lost it, but giving access to the entire computer through the command line does not seem like a good way to go about it.

So, is this a problem? Is single-user mode bad? As far as I see it it is a security hole, but I could be missing something.

Physical access is total access, right?  How is this any worse than a boot CD or yanking the hard drive and popping it into another system?

Not that I'm a fan of OSX or this particular feature, but if someone has physical access to a computer with an unencrypted disk, they have access to everything on that disk anyway, so single user mode doesn't make that any worse, either.
