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Re: Discovery: Gain access to root on Linux via NIS
From: cellwood () gauss elee calpoly edu (Chris Ellwood)
Date: Thu, 7 Sep 1995 03:58:39 -0700


Ken Weaverling said...
-----BEGIN PGP SIGNED MESSAGE-----
A user here stumbled upon a nice gaping hole in Linux using NIS. I sent
mail to CERT about it TUESDAY LAST WEEK, and got a form letter back to
send.
[...]
Anyway, the Linux used here is Slackware 2.2.0. Not sure if the hole
exists on others, and I've never seen it discussed elsewhere. I've tested
my DG/UX systems and they are fine.

This hole is incredibly simple.  If you are running NIS on Linux, I
can get root on your machine as easily as the famous -froot bug. No
exploit scripts, poking at ports, or peeking at packets. Darn simple.
[...]
I know this is a full disclosure list, and I worry that others already know,
especially since numerous people here apparently already know,
so I am seriously considering posting details unless CERT stops ignorning
me. I emailed them again today about it as well.

I am in a real tizzy about this. I can't even tell you how to protect
yourself without giving it away. Just disabling NIS will not be enough,
believe it or not. :-(  If you have *EVER* run NIS on your Linux box,
you may be vulnerable :-(

Since I believe in full disclosure, I'll go ahead and take a stab at it.

I would guess that the problem is if you have "+::0:0:::" in your
/etc/passwd file, anyone can do 'su +' and get root access.  This
hole seems to meet your criteria of being very simple and existing
even with NIS disabled.  However, the Linux yp-client v1.6 docs clearly
state that you should add an entry like "+:*:0:0:::" to your passwd
file, which would not allow you to 'su +' and get root access.

The real problem seems to be that Linux will recognize '+' as being a
valid user.  Most other OS's (such as SunOS and Ultrix) do not.

Best of luck,

- Chris  <cellwood () gauss calpoly edu>



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