Re: Discovery: Gain access to root on Linux via NIS

[cellwood () gauss elee calpoly edu (Chris Ellwood)]

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>