Vulnerability Note VU#867968
Microsoft Windows SMB Tree Connect Response denial of service vulnerability
Original Release date: 02 Feb 2017 | Last revised: 17 Mar 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 on a vulnerable system.

Description
Microsoft Windows fails to properly handle traffic from a malicious server.
In particular, Windows fails to properly handle a specially-crafted 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. We have confirmed the crash with fully-patched Windows 10 and Windows 8.1 client systems, as well as the server equivalents of these platforms, Windows Server 2016 and Windows Server 2012 R2.
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 by crashing Windows.

Solution
Apply an update

This issue is addressed in Microsoft Security Bulletin MS17-012. Please also consider the following best practices, which also function as 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)

VendorStatusDate NotifiedDate UpdatedMicrosoft CorporationAffected01 Feb 201717 Mar 2017If you are a vendor and your product is affected, let
us know.

CVSS Metrics (Learn More)

Group
Score
Vector

Base
7.8
AV:N/AC:L/Au:N/C:N/I:N/A:C

Temporal
7.0
E:POC/RL:U/RC:C

Environmental
7.0
CDP:ND/TD:H/CR:ND/IR:ND/AR:ND

References

https://technet.microsoft.com/library/security/ms17-012
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.

Other Information

CVE IDs:
CVE-2017-0016

Date Public:
01 Feb 2017

Date First Published:
02 Feb 2017

Date Last Updated:
17 Mar 2017

Document Revision:
26

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