i got this linux redhat 9.0 used and don't know the password

please help

Boot into rescue mode (use cd1 of the install discs)
Mount the disk.
Mount your Linux disk to mountpoint, for example /mnt

mount /mnt /dev/hdXx

Now you can go to the mounted disk

cd /mnt

Use vi, vim, or nano to edit the files

What your need to edit
Try now to edit your file /etc/passwd by vi or sed. But, attention!
Your file /etc/password is in fact /mnt/etc/passwd. You need to edit
the line contains the word "root". For example, if your /etc/passwd
contains the line

root:x3FrTg6uYT7.:0:0:root:/:/bin/sh

edit it and blank out the password field to the next:

root::0:0:root:/:/bin/sh

You should be able to login as root and you needn't any password for root.
After that, don't forget to change your root password to new one.

passwd <newpassword>

i don't have the boot cd

Use a live cd. knoppix or something. If you don't have one, download one, they are nice to have around

Does this mean that I could break into the root of any Linux system with just a boot cd? Or can a system be protected against this?

Yes. If you want, you can disable the "boot from CD" option in the BIOS, then password-protect the BIOS settings, but even that can be overcome with enough effort. Such heroic measures are really unnecessary in most cases.

It's like securing your house; a good deadbolt or rimlock on the door combined with other standard security measures will usually suffice to keep the undesirables out, or you can go nuts and install multiple locks on the doors and bars on the windows in an effort to keep out a few more.

Either way, a really determined person can still get in by driving a car through your wall.

Try this

At the grub menu, type 'e' (edit)

which has kernel in it, and type 'e'
goto end of kernel line & type 'init 1'
and press enter

then type 'b'(boot)

The above is for no password boot,
u can add users & password later

Then type 'init 5' & login as new user

go to single user mode and there u type your new root passwd and then enter