Winbind's rfc2307 & SFU nss_info plugin in Samba 3.0.25[a-c] assigns users a primary gid of 0 by default

Advisory Content :

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==========================================================
==
== Subject: Incorrect primary group assignment for
== domain users using the rfc2307 or sfu
== winbind nss info plugin.
==
== CVE ID#: CVE-2007-4138
==
== Versions: Samba 3.0.25 - 3.0.25c (inclusive)
==
== Summary: When the "winbind nss info" parameter in
== smb.conf is set to either "sfu" or "rfc2307",
== Windows users are incorrectly assigned
== a primary gid of 0 in the absence of the
== RFC2307 or Services or Unix (SFU) primary
== group attributes.
==
==========================================================

===========
Description
===========

The idmap_ad.so library provides an nss_info extension to Winbind
for retrieving a user's home directory path, login shell and
primary group id from an Active Directory domain controller. This
functionality is enabled by defining the "winbind nss info"
smb.conf option to either "sfu" or "rfc2307".

Both the Windows "Identity Management for Unix" and "Services for
Unix" MMC plug-ins allow a user to be assigned a primary group
for Unix clients that differs from the user's Windows primary group.
When the rfc2307 or sfu nss_info plugin has been enabled, in
the absence of either the RFC2307 or SFU primary group attribute,
Winbind will assign a primary group ID of 0 to the domain user
queried using the getpwnam() C library call.

==================
Patch Availability
==================

A patch addressing this defect has been posted to

http://www.samba.org/samba/security/

Additionally, Samba 3.0.26 has been issued as a security
release to correct the defect.

==========
Workaround
==========

Samba and Active Directory administrators may avoid this security
issue by two methods:

(a) Ensure that all user's stored in AD are properly assigned a
Unix primary group, or
(b) Discontinue use of the sfu or rfc2307 "winbind nss info" plugin
until a patched version of the idmap_ad.so library can be
installed.

Note that the problem is only evident on servers using the sfu
or rfc2307 "winbind nss info" plugin and not those only making
use of Winbind's idmap_ad IDMap backend interface.

=======
Credits
=======

This vulnerability was reported to Samba developers by Rick King
as Samba Bug #4927.

The time line is as follows:

* Aug 29, 2007: Initial report from Rick King.
* Aug 29, 2007: First response from Samba developers confirming
the bug along with a proposed patch.
* Sep 4, 2007: Announcement to vendor-sec mailing list.
* Sep 11, 2007: Public security advisory made available.

==========================================================
== Our Code, Our Bugs, Our Responsibility.
== The Samba Team
==========================================================
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Winbind's rfc2307 & SFU nss_info plugin in Samba 3.0.25[a-c] assigns users a primary gid of 0 by default