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             AUSCERT External Security Bulletin Redistribution

                               ESB-2017.0329
Microsoft Windows SMB Tree Connect Response memory corruption vulnerability
                              3 February 2017

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        AusCERT Security Bulletin Summary
        ---------------------------------

Product:           Microsoft Windows SMB
Publisher:         CERT/CC
Operating System:  Windows
Impact/Access:     Execute Arbitrary Code/Commands -- Remote/Unauthenticated
                   Denial of Service               -- Remote/Unauthenticated
Resolution:        Mitigation

Original Bulletin: 
   http://www.kb.cert.org/vuls/id/867968

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Vulnerability Note VU#867968 
Microsoft Windows SMB Tree Connect Response memory corruption vulnerability

Original Release date: 02 Feb 2017 | Last revised: 02 Feb 2017

Overview

Microsoft Windows contains a memory corruption bug in the handling of SMB 
traffic, which may allow a remote, unauthenticated attacker to cause a denial
of service or potentially execute arbitrary code on a vulnerable system.

Description

Microsoft Windows fails to properly handle traffic from a malicious server. In
particular, Windows fails to properly handle a server response that contains 
too many bytes following the structure defined in the SMB2 TREE_CONNECT 
Response structure. By connecting to a malicious SMB server, a vulnerable 
Windows client system may crash (BSOD) in mrxsmb20.sys. It is not clear at 
this point whether this vulnerability may be exploitable beyond a 
denial-of-service attack. We have confirmed the crash with fully-patched 
Windows 10 and Windows 8.1 client systems.

Note that there are a number of techniques that can be used to trigger a 
Windows system to connect to an SMB share. Some may require little to no user
interaction.

Exploit code for this vulnerability is publicly available.

Impact

By causing a Windows system to connect to a malicious SMB share, a remote 
attacker may be able to cause a denial of service or potentially execute 
arbitrary code with Windows kernel privileges.

Solution

The CERT/CC is currently unaware of a practical solution to this problem. 
Please consider the following workarounds:

Block outbound SMB

Consider blocking outbound SMB connections (TCP ports 139 and 445 along with 
UDP ports 137 and 138) from the local network to the WAN.

Vendor Information (Learn More)

Vendor Status Date Notified Date Updated

Microsoft Corporation Affected 01 Feb 2017 02 Feb 2017

If you are a vendor and your product is affected, let us know.

CVSS Metrics (Learn More)

Group Score Vector

Base 10.0 AV:N/AC:L/Au:N/C:C/I:C/A:C

Temporal 9.0 E:POC/RL:U/RC:C

Environmental 9.0 CDP:ND/TD:H/CR:ND/IR:ND/AR:ND

References

https://github.com/lgandx/PoC/tree/master/SMBv3%20Tree%20Connect 
https://msdn.microsoft.com/en-us/library/cc246499.aspx 
https://www.us-cert.gov/ncas/current-activity/2017/01/16/SMB-Security-Best-Practices

Credit

This vulnerability was publicly reported by PythonResponder.

This document was written by Will Dormann.

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