Microsoft has released the KB4551762 update to address an exploit in the file sharing service SMBv3 which, if left unpatched, could allow for an attacker to inflict malicious code.
The fix, delivered outside of Microsoft’s Patch Tuesday update, is for Windows 10 versions 1903 and 1909 for 32-bit, ARM64-based and x64-based systems, as well as Windows Server versions 1903 and 1909.
Exploiting the vulnerability in an SMB server would require an attacker sending a crafted packet to the server, while exploiting the vulnerability in an SMB client would require a user being tricked into connecting to a malicious SMBv3 server.
Leaving the vulnerability unpatched could pave the way for a significant attack, as SMB security flaws have historically been exploited to execute ransomware attacks.
In addition to the update, Microsoft also suggested to disable SMB compression to block unauthenticated attackers from exploiting the vulnerability via the PowerShell command: Set-ItemProperty -Path "HKLM:\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters" DisableCompression -Type DWORD -Value 1 –Force
The disabling of SMB compression is not expected to have any negative impact on performance, as SMB compression is not currently used by Windows or Windows Server.
However, this does not prevent the exploitation of SMB clients. Instead, Microsoft also recommended to prevent SMB traffic from lateral connections entering or leaving networks by blocking unsolicited communication and outgoing traffic to TCP ports 139 and 445 and UDP ports 137 and 138.
Meanwhile, for Windows clients and servers that do not host SMB shares, all inbound SMB traffic can be blocked via Windows Defender Firewall.