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                             CERT-Renater

                 Note d'Information No. 2020/VULN120
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DATE                : 11/03/2020

HARDWARE PLATFORM(S): /

OPERATING SYSTEM(S): Windows running Microsoft SMBv3.

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https://kb.cert.org/vuls/id/872016/
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Microsoft SMBv3 compression remote code execution vulnerability
Vulnerability Note VU#872016

Original Release Date: 2020-03-11 | Last Revised: 2020-03-11


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.


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 to execute arbitrary code
with SYSTEM privileges on a vulnerable system.


Solution

The CERT/CC is currently unaware of a practical solution to this
problem. Please 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.


Vendor Information

No information available at this time.


CVSS Metrics
Group           Score     Vector
Base            10        AV:N/AC:L/Au:N/C:C/I:C/A:C
Temporal        8.1       E:U/RL:W/RC:C
Environmental   8.1       CDP:ND/TD:ND/CR:ND/IR:ND/AR:ND


References

https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/adv200005


Acknowledgements

This document was written by Will Dormann.


Other Information

CVE IDs:                CVE-2020-0796
Date Public:            2020-03-10
Date First Published:   2020-03-11
Date Last Updated:      2020-03-11 00:43 UTC
Document Revision:      14


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+ 23/25 Rue Daviel    | fax : 01-53-94-20-41            +
+ 75013 Paris         | email:cert@support.renater.fr   +
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