Ever accidentally lose the ability to become root on your own system?
Perhaps you accidentally changed root's shell to /bin/bish because of a
typo when adding that new user. Perhaps you simply forgot what you
changed the root password to since you always use sudo. (Sudo == good --
logging in as root == bad.) Or maybe when trying to remove that old
/etc.bak directory you hit a space instead of dot. Whoops.
When these mistakes happen, it's nice to know that you have options to
get on your machine as root to clean up your mess and I'll show you some
of them. When you see how easy they are, you may make the logical (and
disturbing realization) that even unauthorized folks can use these same
techniques to take over your machine.
All the methods I'm going to talk about here are possible if an attacker
has physical access to your machine. With physical access, the baddie
has some pretty impressive power. I usually just call this the 'Boot
access is Root access' effect.
When your Linux box boots, it generally goes through several stages.
First the BIOS  gives you a chance to hit F8, F5, or whatever other
fun key combo it likes to let you mess with things. It'll probably
initialize some hardware, and you'll get a nice text splash screen or
two. Eventually, you get to the Lilo prompt. 
Most likely you've never spent time at the Lilo (Linux loader) prompt,
since it will boot Linux automatically if you don't press a key right
away. However, this is your opportunity to tell Linux how it should
boot. Say your default kernel is called 'linux' in lilo.conf, but you
have an older kernel, say 2.2.15 named linux.old. All a malicious hacker
needs do at the console is type:
at the lilo prompt and the older kernel will boot.
Since 2.2.15 has several instant ways for a normal user to become root,
the attacker can simply log in (this assumes they have an account, or
method of obtaining one) and they'll have root access in seconds.
To prevent someone at the console from booting an alternate kernel you
have several options:
Comment out the Alternate Kernel Definitions from /etc/lilo.conf
The definition starts with 'image=' and goes until the next 'image='
line (or end of file). For example to make our linux.old kernel
unavailable, you'd find the kernel definition, which would look like
Comment these out by putting a '#' at the beginning of the line. When
you're done editing the file, run lilo to make the changes:
Added linux *
Make sure that you don't see 'linux.old' as one of the entries.
Whichever is marked with a '*' is the default kernel, the one that will
launch if you don't explicitly specify one.
Password Protect the Other Kernel Definitions
You simply add a 'password' line to the configuration, such as this:
When attempting to boot the linux.old kernel, lilo will require the
password you specify in lilo.conf. I only suggest you do this with old
kernels that could be useful at some time and which are hopefully not
known to have instant root bugs. All completely unneeded kernels should
be removed from lilo.conf, and from your hard drive for that matter.
Again, make sure to re-run lilo when finished.
Next Week: Single User Mode
 Yes, I'm being PC centric. Forgive me, fellow non-x86 users.
 Now I must apologize to all non-lilo users -- I can't win.
Other boot loaders have similar security precautions: RTFM.