I’ve heard someone can change the root password by entering single user mode on my computer. How can I prevent that from happening?
I agree with Fallen Heart. You can secure your machine by disabling remote root logins and even disabling local root logins, but with physical access to the machine, there are alot of cd and usb tools out there to accomplish getting into any machine. A step you could take to make it a little more difficult might be to make sure that your boot order in your bios points directly at your local hard drive first (for example, boot order 1: hard drive, 2: cdrom, 3: usb, 4: whatever) and then password protecting your bios to keep it from being changed by anyone else. This is just one more layer of security. There IS a way around that, though. The point of it is, though, with physical access to the machine, there is no such thing as safe to a person who wants what you have and knows what they are doing.
to hack through single user mode one needs physical access to your computer. one thing you can do to save it just lock your cpu in some locker.
change some BIOS settings
is there any way to hack the system even after applying the measures you have mentioned...