===========================================================
== Subject:     A user in an AD Domain could become root on
==              domain members
==
== CVE ID#:     CVE-2020-25717
==
== Versions:    All Samba versions since Samba 3.0
==
== Summary:     Samba may map domain users to local users
==              in an undesired way.
===========================================================

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

Windows Active Directory (AD) domains have by default a feature to
allow users to create computer accounts, controlled by
ms-DS-MachineAccountQuota.

In addition some (presumably trusted) users have the right to create
new users or computers in both Samba and Windows Active Directory
Domains.

These features can be quite dangerous in the wrong hands, as the user
who creates such accounts has broad privileges to not just create them
and set their passwords, but to rename them at a later time with the
only contraint being they may not match an existing samAccountName in
AD.

When Samba as an AD Domain member accepts a Kerberos ticket, it must
map the information found therein to a local UNIX user-id (uid). This
is currently done via the account name in the Active Directory
generated Kerberos Privileged Attribute Certificate (PAC), or the
account name in the ticket (if there is no PAC).

For example, Samba will attempt to find a user "DOMAIN\user" before
falling back to trying to find the user "user".

If the DOMAIN\user lookup can be made to fail, then a privilege
escalation is possible.

The easiest exaple to illustrate this is if an attacker creates an
account named root (by renaming a MachineAccountQuota based machine
account), and asks for a login without a Kerberos PAC. Between
obtaining the ticket and presenting it to a server, the attacker
renames the user account to a different name. Samba attempts to look
up "DOMAIN\root", which fails (as this no longer exists) and then
falls back to looking up user "root", which will map to the privileged
UNIX uid of 0.

This patch changes Samba to use the SID in the PAC instead of the
account name, as with NTLM based logins. The SID is unique and
non-repeating and so can't be confused with another user.

Additionally, a new parameter has been added "min domain uid" (default
1000), and no matter how we obtain the UNIX uid to use in the process
token (we may eventually read /etc/passwd or similar), by default no
UNIX uid below this value will be accepted.

============
Beyond Samba
============

Samba is almost unique in the Linux/Unix landscape in that it can read
the PAC and make authorisation choices based on the SIDs in the PAC.

Administrators and those responsible for the deployment of other
applications that use Kerberos, and which can only rely on the "cname"
(Client name) in the Kerberos ticket should take care not to map these
into the same namespace as system users.

NFS-Ganesha has a mode to map AD domain users via Samba's winbindd.
Use of this mode is encouraged, and (for all NFS servers in AD
domains) the default idmapd.conf "Method = nsswitch" is discouraged.

Likewise special care should be taken in the general configuration of
"auth_to_local" in a krb5.conf as might be used by other services.

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

Patches addressing both these issues have been posted to:

    https://www.samba.org/samba/security/

Additionally, Samba 4.15.2, 4.14.10 and 4.13.14 have been issued
as security releases to correct the defect.  Samba administrators are
advised to upgrade to these releases or apply the patch as soon
as possible.

==================
CVSSv3 calculation
==================

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N (8.1)

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

Setting "gensec:require_pac=true" in the smb.conf makes the
DOMAIN\user lookup succeed, due to a cache prime in winbind, provided
nss_winbind is in use and no error paths are hit.

It would be prudent to pre-create disabled users in Active Directory
matching on all privileged names not held in Active Directory, eg

 samba-tool user add root -H ldap://$SERVER -U$USERNAME%$PASSWORD --random-password
 samba-tool user add ubuntu -H ldap://$SERVER -U$USERNAME%$PASSWORD --random-password
 ...
 (repeat for eg all system users under 1000 in /etc/passwd or special
 to any other AD-connected services, eg perhaps "admin" for a web-app)

Setting ms-DS-MachineAccountQuota to 0, in the Active Directory domain
is also advised, if possible.

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

Originally reported by Andrew Bartlett of Catalyst and the Samba Team.

Patches provided by:
 * Stefan Metzmacher of SerNet and the Samba Team
 * Samuel Cabrero of SuSE and the Samba Team
 * Joseph Sutton of Catalyst and the Samba Team
 * Alexander Bokovoy of Red Hat and the Samba Team
 * Ralph Boehme of SerNet and the Samba Team

Andrew wishes to give much thanks to NetSPI for the blog
"MachineAccountQuota is USEFUL Sometimes: Exploiting One of Active
Directory's Oddest Settings" by Kevin Robertson[1], on which the full
horror of MachineAccountQuota became clear.

[1] https://www.netspi.com/blog/technical/network-penetration-testing/machineaccountquota-is-useful-sometimes/

==========================================================
== Our Code, Our Bugs, Our Responsibility.
== The Samba Team
==========================================================