Is CVE-2021-31166 being actively exploited?

Yes. CVE-2021-31166 is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, added on April 6, 2022.

What is the CVSS score for CVE-2021-31166?

CVE-2021-31166 has a CVSS 3.1 base score of 9.8 (CRITICAL).

CVE-2021-31166

CRITICAL · CVSS 9.8CISA KEV: actively exploitedEPSS 99.9%EU EUVD: exploited

HTTP Protocol Stack Remote Code Execution Vulnerability

Key facts

CVSS score: 9.8 (v3.1)
Severity: CRITICAL
CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Weakness (CWE): CWE-416
EPSS score: 99.9% (100th percentile)
In CISA KEV: Yes, added April 6, 2022
In EU EUVD: Yes (EUVD-2021-18079)
Published: May 11, 2021
NVD last modified: June 17, 2026
Status: Analyzed

CISA Known Exploited Vulnerability

Microsoft HTTP Protocol Stack Remote Code Execution Vulnerability

Added to the KEV catalog on April 6, 2022; federal remediation due April 27, 2022.

EU Vulnerability Database (EUVD)

This vulnerability is also tracked by ENISA as EUVD-2021-18079 in the EU Vulnerability Database. ENISA flags it as known exploited. Last updated in EUVD on October 21, 2025.

View on the EU Vulnerability Database

Affected products

microsoft / windows 10 2004
microsoft / windows 10 20h2
microsoft / windows server 2004
microsoft / windows server 20h2

Certified products that may be affected

1 FIPS 140 or Common Criteria certified product, from automated matching by sec-certs.org.

Microsoft Windows 10 version 2004 (May 2020 Update), Microsoft Windows Server version 2004 (May 2020 Update) and others - Microsoft Corporation - Common Criteria 2020-50-INF-3671 - active

References

http://packetstormsecurity.com/files/162722/Microsoft-HTTP-Protocol-Stack-Remote-Code-Execution.html(Third Party Advisory, VDB Entry)
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31166(Patch, Vendor Advisory)
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-31166(US Government Resource)

Last updated September 17, 2026 · Source: NVD, CISA KEV, FIRST.org EPSS

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