SMBGhost CVE-2020-0796 (wormable SMBv3.1.1 compression bug in Windows 10/Server 1903 and 1909, CISA KEV): which systems are affected, what's the update, and is there a workaround?

Applies to: Windows 10 1903/1909 and Windows Server 1903/1909 without the March 2020 update (KB4551762); SMB server and client

How it was checked: Compiled on 2026-10-31 from CISA's June 2020 SMBGhost alert, NVD (affected versions) and the CISA KEV catalog; workaround and KB per Microsoft advisory ADV200005. Not reproduced by Exuvia.

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What it is: CVE-2020-0796, nicknamed SMBGhost, is a remote code execution flaw in how Windows' SMB 3.1.1 protocol handles compressed messages. It affects both the SMB server and client: an unauthenticated attacker who can reach port 445 could run code on a vulnerable server, and a malicious server could attack a client that connects to it. Microsoft fixed it in March 2020 (advisory ADV200005, update KB4551762). Working exploit code became public, CISA warned in June 2020 that unpatched systems were being attacked, and it's in the KEV catalog. Because it's wormable, any unpatched machine on a flat network is a risk to the rest. Am I affected: - Windows 10 version 1903 and 1909 and Windows Server version 1903 and 1909 (Server Core), per NVD and Microsoft, without the March 12, 2020 update or a later cumulative update. Those releases are long out of support, so any machine still running them is missing many other fixes too. - Check with winver (version and OS build) or Get-ComputerInfo | Select WindowsVersion, OsBuildNumber. Fix: 1. Install the March 2020 security update (KB4551762) or, better, upgrade to a currently supported Windows release with current updates. 2. Microsoft's server-side workaround until patched: disable SMBv3 compression with Set-ItemProperty -Path "HKLM:\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters" DisableCompression -Type DWORD -Value 1 -Force. It doesn't protect the SMB client. 3. Block TCP 445 at the internet edge (CISA's advice) and limit SMB between network segments; don't let clients connect to SMB servers on the internet. 4. If an unpatched machine was exposed, check it for compromise and rebuild if in doubt. Verify: the OS build includes the March 2020 or later update (or the machine runs a supported Windows version), and vulnerability scans no longer flag CVE-2020-0796. Sources: https://www.cisa.gov/news-events/alerts/2020/06/05/unpatched-microsoft-systems-vulnerable-cve-2020-0796 https://nvd.nist.gov/vuln/detail/CVE-2020-0796 https://www.cisa.gov/known-exploited-vulnerabilities-catalog

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