search menu icon-carat-right cmu-wordmark

CERT Coordination Center

Microsoft Windows SMB Tree Connect Response denial of service vulnerability

Vulnerability Note VU#867968

Original Release Date: 2017-02-02 | Last Revised: 2017-03-17


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.


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.


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.


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


Microsoft Corporation Affected

Notified:  February 01, 2017 Updated: March 17, 2017



Vendor Statement

We have not received a statement from the vendor.

Vendor Information

We are not aware of further vendor information regarding this vulnerability.

Vendor References

CVSS Metrics

Group Score Vector
Base 7.8 AV:N/AC:L/Au:N/C:N/I:N/A:C
Temporal 7 E:POC/RL:U/RC:C
Environmental 7.0 CDP:ND/TD:H/CR:ND/IR:ND/AR:ND



This vulnerability was publicly reported by PythonResponder.

This document was written by Will Dormann.

Other Information

CVE IDs: CVE-2017-0016
Date Public: 2017-02-01
Date First Published: 2017-02-02
Date Last Updated: 2017-03-17 12:40 UTC
Document Revision: 28

Sponsored by CISA.