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Re: Privilege escalation (lpadmin -> root) in cups
From: Yves-Alexis Perez <corsac () debian org>
Date: Sun, 11 Nov 2012 10:01:35 +0100

On dim., 2012-11-11 at 00:18 -0700, Kurt Seifried wrote:
On 11/10/2012 05:49 AM, Yves-Alexis Perez wrote:
Hi,

a Debian user reported a bug in our BTS concerning cupsd. The bug
is available at
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=692791 and 
upstream bug at http://www.cups.org/str.php?L4223 (restricted
because it's tagged security).

I'm unsure right now if it's an upstream issue or specific to
Debian.

On Red Hat Enterprise 6 and Fedora 16 the file is owned by root:sys,
and the cupsd.conf defaults to:

<Location /admin/conf>
  AuthType Default
  Require user @SYSTEM
  Order allow,deny
</Location>

As far as I can tell, @SYSTEM is defined using SystemGroup and defaults
to lpadmin.

so that should be like "root", "bin" and "adm" so yeah it would appear
to be vendor specific.

Well, in Debian (and upstream) case it's lpadmin -> root but in your
case it'd be bin -> root and adm -> root. Maybe adm is intended to be
root anyway but I guess that's not the case for bin?

The whole point is that people with access to the admin web interface
can force cupsd to read or write files with the user running cupsd
(root).

Basically, members of the lpadmin group (which is the group having
admin rights to cups, meaning they're supposed to be able to
add/remove printeers etc.) have admin access to the web interface,
where they can edit the config file and set some “dangerous”
directives (like the log filenames), which enable them to read or
write files as the user running the cupsd webserver.

In Debian case at least, it's run as root, meaning we have a
privilege escalation issue from lpadmin group to root.

I think as a rule cupsd runs as root, to touch the various files/dirs/etc.

A fix would be to not run cupsd web server as root, and maybe to 
restrict it to some kind of chroot so it doesn't have access to 
sensitive files

Tricky, /dev/*, log dirs, etc. Probably better to just use a print
specific user/group and make all the standard locations owned by it,
and require the admin to setup anything like say
/non-standard/log/printers/ and so on.

Can a CVE be allocated for this?

Please use CVE-2012-5519 for this issue. Also if other vendors could
check the permissions/configs/etc. and reply if they are vulnerable
that would be good.

Thanks.

Regards,
-- 
Yves-Alexis

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