====================================================================
== Subject:     Subsequent DCE/RPC fragment injection vulnerability
==
== CVE ID#:     CVE-2021-23192
==
== Versions:    Samba 4.10.0 and later.
==
== Summary:     If a client to a Samba server sent a very large
                DCE/RPC request, and chose to fragment it, an
		attacker could replace later fragments with
		their own data, bypassign the signature requirements.
=====================================================================

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

Samba implements DCE/RPC, and in most cases it is provided over and
protected by the underlying SMB transport, with protections like 'SMB
signing'.

However there are other cases where large DCE/RPC request payloads are exchanged
and fragmented into several pieces. If this happens over untrusted transports
(e.g. directly over TCP/IP or anonymous SMB) clients will typically
protect by an explicit authentication at the DCE/RPC layer, e.g. with
GSSAPI/Kerberos/NTLMSSP or Netlogn Secure Channel.

Because the checks on the fragment protection were not done between
the policy controls on the header and the subsequent fragments, an attacker
could replace subsequent fragments in requests with their own data, which
might be able to alter the server behaviour.

As active directory domain controller this issue affects Samba versions greather
or equal to 4.10.0.

As NT4 classic domain controller, domain member or standalone server
this issue affects Samba versions greather or equal to 4.13.0.

==================
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:H/PR:N/UI:N/S:U/C:L/I:L/A:N (4.8)

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

Setting "dcesrv:max auth states=0" in the smb.conf will provide
some mitigation against this issue.

There are no known problems with this change as
NT4 classic domain controller, domain member or standalone server.

But it disables "Security Context Multiplexing" and may reopens
https://bugzilla.samba.org/show_bug.cgi?id=11892.
which means domain members running things like Cisco ISE or
VMWare View may no longer work. This applies only to
active directory domain controllers.

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

Originally reported by Stefan Metzmacher of SerNet

Patches provided by Stefan Metzmacher of SerNet and the Samba Team.
Advisory by Andrew Bartlett of Catalyst and the Samba Team.

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