Man in the middle attacks possible with NTLMSSP

Description

There are several man in the middle attacks possible with
NTLMSSP authentication.

E.g. NTLMSSP_NEGOTIATE_SIGN and NTLMSSP_NEGOTIATE_SEAL
can be cleared by a man in the middle.

This was by protocol design in earlier Windows versions.

Windows Server 2003 RTM and Vista RTM introduced a way
to protect against the trivial downgrade.

See MsvAvFlags and flag 0x00000002 in
https://msdn.microsoft.com/en-us/library/cc236646.aspx

This new feature also implies support for a mechlistMIC
when used within SPNEGO, which may prevent downgrades
from other SPNEGO mechs, e.g. Kerberos, if sign or
seal is finally negotiated.

The Samba implementation doesn’t enforce the existence of
required flags, which were requested by the application layer,
e.g. LDAP or SMB1 encryption (via the unix extensions).
As a result a man in the middle can take over the connection.
It is also possible to misguide client and/or
server to send unencrypted traffic even if encryption
was explicitly requested.

LDAP (with NTLMSSP authentication) is used as a client
by various admin tools of the Samba project,
e.g. “net”, “samba-tool”, “ldbsearch”, “ldbedit”, …

As an active directory member server LDAP is also used
by the winbindd service when connecting to domain controllers.

Samba also offers an LDAP server when running as
active directory domain controller.

The NTLMSSP authentication used by the SMB1 encryption
is protected by smb signing, see CVE-2015-5296.

The following vulnerabilities are related:
CVE-2016-2112 and CVE-2016-2113

Patch Availability

A patch addressing this defect has been posted to

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

Additionally, Samba 4.4.2, 4.3.8 and 4.2.11 have been issued as
security releases to correct the defect. Samba vendors and administrators
running affected versions are advised to upgrade or apply the patch as
soon as possible.

Note that Samba 4.4.1, 4.3.7 and 4.2.10 were privately released to vendors,
but had a regression, which is fixed in 4.4.2, 4.3.8 and 4.2.11.

Workaround

None.

Credits

This vulnerability was discovered and researched by Stefan Metzmacher of
SerNet (https://samba.plus) and the Samba Team (https://www.samba.org).
He provides the fixes in collaboration with the Samba Team.