Software Engineering Institute

Overview

Microsoft SMBv3 contains a vulnerability in the handling of compression, which may allow a remote, unauthenticated attacker to execute arbitrary code on a vulnerable system. This vulnerability is being referred to as "SMBGhost and CoronaBlue."

Description

Microsoft Server Message Block 3.1.1 (SMBv3) contains a vulnerability in the way that it handles connections that use compression. This vulnerability may allow a remote, unauthenticated attacker to execute arbitrary code on a vulnerable system. It has been reported that this vulnerability is "wormable."

Impact

By connecting to a vulnerable Windows machine using SMBv3, or by causing a vulnerable Windows system to initiate a client connection to a SMBv3 server, a remote, unauthenticated attacker may be able to execute arbitrary code with SYSTEM privileges on a vulnerable system.

Solution

Apply an update

This issue has been addressed in the Microsoft update for CVE-2020-0796. Please also consider the following workarounds:

Disable SMBv3 compression

According to Microsoft Security Advisory ADV200005 :

You can disable compression to block unauthenticated attackers from exploiting the vulnerability against an SMBv3 Server with the PowerShell command below.

Set-ItemProperty -Path "HKLM:\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters" DisableCompression -Type DWORD -Value 1 -Force

Notes:

1. No reboot is needed after making the change.
2. This workaround does not prevent exploitation of SMB clients.

You can disable the workaround with the PowerShell command below.

Set-ItemProperty -Path "HKLM:\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters" DisableCompression -Type DWORD -Value 0 -Force

Block inbound and outbound SMB

Consider blocking outbound SMB connections (TCP port 445 for SMBv3) from the local network to the WAN. Also ensure that SMB connections from the internet are not allowed to connect inbound to an enterprise LAN.

Acknowledgements

This document was written by Will Dormann.

Vendor Information

References

• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2020-0796
• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/adv200005