(newbie) Question about core linux security

From: Sam Joharay (public_viewer_at_indiatimes.com)
Date: 02/11/04 

Date: 10 Feb 2004 20:54:19 -0800

Hi,

A friend of mine pointed this out to me
(which I believe must be common knowledge
though I'm kinda surprised by it):

Claim: If you have physical access to a linux
machine, you can gain root access. Furthermore
you can create a user with any name which will
have root access (thus leave a backdoor entry
for remote root access).

How to: Reboot the PC. Hit Ctrl-X when the login
screen appears. You will get a command prompt
that says "boot:"
Type "linux 1"

Once thus logged in, you have permissions to edit
all files. Edit /etc/passwd and add line:

myuser::0:0:root:/root:/bin/bash

Save file and exit (logout/reboot).
Now you have created a user with user name myuser
which has root privileges (try it).

==============================================

I want to know whether:

a. this is a well known security flaw
b. if there is someway to prevent this.
c. is there anything that distinguishes a user thus
created from the root user (in terms of privileges)

=============================================

If the claim is true (I have tried it on the redhat
linux installation that I have) then IMHO, it means
that a linux machine is only as secure as the lock
that guards it.

Please comment,
[Sam Jo]

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