Microsoft SMBv3 Remote Code Execution Vulnerability
CVE-2020-0796 is a security vulnerability in Microsoft SMBv3, added to CISA KEV on February 10, 2022. Patch priority: Patch now. Known ransomware use: yes.
What it is
A remote code execution vulnerability exists in the way that the Microsoft Server Message Block 3.1.1 (SMBv3) protocol handles certain requests. An attacker who successfully exploited the vulnerability could gain the ability to execute code on the target server or client.
Known to be used in ransomware campaigns. Active threat actors have chained this vulnerability into ransomware operations — treat patching as a same-week priority, not a "next maintenance window" task. The fix is the same one below; the urgency is higher.
VulnCheck tracked exploitation 624 days before CISA's KEV listing. VulnCheck's broader catalog of in-the-wild exploitation evidence picked this up before it landed on CISA's formal list. Per-CVE fact about this entry; not a population claim about lead time in general.
Who's affected
Affects anyone running Microsoft SMBv3. Microsoft products in a small practice typically sit close to credentials, email, or document workflows — treat the patch as in-scope.
What to do
Apply updates per vendor instructions.
If you don't have someone in-house to verify the patch deployed across every endpoint — or you're not sure whether you're affected — that's exactly the kind of triage we do. Book a free 20-minute triage call.
Insurance readiness
Your insurer will ask if this is patched.
Unpatched entries on CISA's Known Exploited Vulnerabilities list are exactly what cyber-insurance carrier questionnaires probe for, and knowing which controls they check is how you keep a renewal from stalling.
Get the free carrier questionnaire →Severity
CVSS base score: 10.0 — CRITICAL
Weakness classification: CWE-119
Microsoft fix
- KB4551762 (Security Update)
Microsoft Exploitability Index (assessed at update release): Exploitation More Likely
This CVE is on CISA's Known Exploited Vulnerabilities catalog. Microsoft's index reflects their release-time forecast, not current observed activity, and is not automatically revised upward when exploitation appears later.
Vendor workaround
Vendor-published workaround
The following workaround may be helpful in your situation. In all cases, Microsoft strongly recommends that you install the updates for this vulnerability as soon as they become available even if you plan to leave this workaround in place:
Disable SMBv3 compression
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:
- No reboot is needed after making the change.
- This workaround does not prevent exploitation of SMB clients; please see item 2 under FAQ to protect clients.
- SMB Compression is not yet used by Windows or Windows Server, and disabling SMB Compression has no negative performance impact.
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
Note: No reboot is needed after disabling the workaround.
A workaround reduces exposure while you patch — it is not a substitute for the update. Install the fix above as soon as you can.
CISA due date
Federal deadline: August 10, 2022. Federal agencies must complete the required action by this date. For private SMBs the deadline is advisory — but treat it as a strong recommendation, especially if you handle regulated data (HIPAA, GLBA, ABA model rules).
EPSS
EPSS score: 99.8% — higher than 100% of all scored CVEs.
EPSS estimates the probability of exploitation activity in the next 30 days. Every entry on this site is already confirmed exploited, so read it as relative urgency among the things you still have open, not as permission to wait.
Source
This entry comes from CISA's Known Exploited Vulnerabilities catalog, public US-Government data we pull daily. The plain-language framing and the what-to-do guidance above are ours. View the original entry on cisa.gov.