How severe is CVE-2021-31166?

CVE-2021-31166 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2021-31166 being actively exploited?

Yes. CVE-2021-31166 is on CISA's Known Exploited Vulnerabilities (KEV) catalog, added on 2022-04-06, which means active exploitation has been confirmed. It should be prioritised for remediation.

What products are affected by CVE-2021-31166?

CVE-2021-31166 primarily affects Microsoft Windows 10 2004. In total, 4 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2021-31166?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Because this CVE is known to be actively exploited, treat remediation as urgent — CISA KEV typically sets a short remediation deadline.

Does CVE-2021-31166 have an EU (EUVD) identifier?

Yes. CVE-2021-31166 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2021-18079. It is also flagged as exploited in the EUVD (since 2022-04-06).

When was CVE-2021-31166 published?

CVE-2021-31166 was published on 2021-05-11 and last updated on 2026-06-17.

CVE-2021-31166

CVE-2021-31166 is a critical-severity vulnerability in Microsoft Windows 10 2004 with a CVSS 3.x base score of 9.8. It is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, confirming it has been exploited in the wild (added 2022-04-06). The underlying weakness is classified as CWE-416.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v2: 7.5
EPSS exploit prediction: 100% (100th percentile)
Actively exploited: Yes — listed in CISA KEV (added 2022-04-06)
EU (EUVD) id: EUVD-2021-18079
EU exploitation: Flagged exploited in the ENISA EU Vulnerability Database (since 2022-04-06)
Weakness: CWE-416
Affected product: Microsoft Windows 10 2004
Published: 2021-05-11
Last modified: 2026-06-17

Description

HTTP Protocol Stack Remote Code Execution Vulnerability

Frequently asked questions

What is CVE-2021-31166?: HTTP Protocol Stack Remote Code Execution Vulnerability
How severe is CVE-2021-31166?: CVE-2021-31166 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2021-31166 being actively exploited?: Yes. CVE-2021-31166 is on CISA's Known Exploited Vulnerabilities (KEV) catalog, added on 2022-04-06, which means active exploitation has been confirmed. It should be prioritised for remediation.
What products are affected by CVE-2021-31166?: CVE-2021-31166 primarily affects Microsoft Windows 10 2004. In total, 4 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2021-31166?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Because this CVE is known to be actively exploited, treat remediation as urgent — CISA KEV typically sets a short remediation deadline.
Does CVE-2021-31166 have an EU (EUVD) identifier?: Yes. CVE-2021-31166 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2021-18079. It is also flagged as exploited in the EUVD (since 2022-04-06).
When was CVE-2021-31166 published?: CVE-2021-31166 was published on 2021-05-11 and last updated on 2026-06-17.

References

Affected products (4)

cpe:2.3:o:microsoft:windows_10_2004:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_10_20h2:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_server_2004:*:*:*:*:*:*:*:*
cpe:2.3:o:microsoft:windows_server_20h2:*:*:*:*:*:*:*:*

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