Microsoft leaked info on a security update for a 'wormable' pre-auth remote code execution vulnerability found in the Server Message Block 3.0 (SMBv3) network communication protocol that reportedly should have been disclosed as part of this month's Patch Tuesday.

The vulnerability is due to an error when the SMBv3 handles maliciously crafted compressed data packets and it allows remote, unauthenticated attackers that exploit it to execute arbitrary code within the context of the application.

Even though the vulnerability advisory was not published by Microsoft (no explanation for this was released by Redmond so far), a number of security vendors part of Microsoft Active Protections Program who get early access to vulnerability information did release details on the security flaw tracked as CVE-2020-0796.

Desktop and server Windows 10 versions impacted

Devices running Windows 10 Version 1903, Windows Server Version 1903 (Server Core installation), Windows 10 Version 1909, and Windows Server Version 1909 (Server Core installation) are impacted by this vulnerability according to a Fortinet advisory, although more versions should be affected given that SMBv3 was introduced in Windows 8 and Windows Server 2012.

"An attacker could exploit this bug by sending a specially crafted packet to the target SMBv3 server, which the victim needs to be connected to," Cisco Talos explained in their Microsoft Patch Tuesday report — this was later removed by the Talos security experts.

"The exploitation of this vulnerability opens systems up to a 'wormable' attack, which means it would be easy to move from victim to victim," they also added.

Fortinet says that upon successful exploitation, CVE-2020-0796 could allow remote attackers to take full control of vulnerable systems.

Due to Microsoft's secrecy, people are coming up with their own theories regarding the malware and its severity, some comparing it to EternalBlue, NotPetya, WannaCry, or MS17-010 (1, 2).

Others have already started coming up with names for the vulnerability such as SMBGhost, DeepBlue 3: Redmond Drift, Bluesday, CoronaBlue, and NexternalBlue.

Available CVE-2020-0796 mitigations

Until Microsoft will release a security update designed to patch the CVE-2020-0796 RCE vulnerability, Cisco Talos shared that disabling SMBv3 compression and blocking the 445 TCP port on client computers and firewalls should block attacks attempting to exploit the flaw.

While no proof-of-concept exploits have been released yet for this wormable SMBv3 RCE, we recommend implementing the mitigation measures shared by Cisco Talos until Microsoft will release an out-of-cycle security update to fix it seeing that almost all the info is out anyway.

BleepingComputer has reached out to Microsoft for more details but had not heard back at the time of this publication.

If you're Microsoft you basically have little choice now but to release the patch for 2020-0796 out-of-cycle as soon as it meets quality standards, right? There's too much info out there to just hope somebody won't find it before April.



Fun times for sysadmins everywhere. — Brian in Pittsburgh (@arekfurt) March 10, 2020

Update: Microsoft published a security advisory with details on how to disable SMBv3 compression to protect servers against exploitation attempts.

You can disable compression on SMBv3 servers with this PowerShell command (no reboot required, does not prevent the exploitation of SMB clients):

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