Software Engineering Institute

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.