What is CVE-2021-31166?

CVE-2021-31166 is a critical-severity vulnerability in Microsoft Windows 10 2004, classified under Use After Free. CVSS score: 9.8/10. Published 2021-05-11.

How severe is CVE-2021-31166?

Critical severity. CVSS v3 base score is 9.8 out of 10.

Is CVE-2021-31166 known to be exploited?

Yes. CVE-2021-31166 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2022-04-06), indicating it is being actively exploited. 76 public proof-of-concept repositories are indexed.

Use After Free in Microsoft Windows 10 2004

CVE-2021-31166

HTTP Protocol Stack Remote Code Execution Vulnerability

Vulnerability class: Use-After-Free

Published 2021-05-11 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.999 (100.0th percentile), scored 2026-10-15.

Very high probability of exploitation in the next 30 days — among the top 0.1% most-likely-to-be-exploited CVEs. 100th percentile — 100.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.998 · newest: 0.999 · change: +0.001

Affected products

Microsoft Windows 10 2004
Microsoft Windows 10 20h2
Microsoft Windows 10 Version 2004 — versions 10.0.0
Microsoft Windows 10 Version 20h2 — versions 10.0.0
Microsoft Windows Server 2004
Microsoft Windows Server 20h2
Microsoft Windows Server Version 2004 — versions 10.0.0
Microsoft Windows Server Version 20h2 — versions 10.0.0

Weakness classification (CWE)

CWE-416 — Use After Free

CISA KEV (Known Exploited Vulnerabilities)

This CVE is on the CISA KEV catalog, added on 2022-04-06. CISA KEV inclusion means CISA has confirmed in-the-wild exploitation; US federal agencies are required to remediate within a published due date.

BOD 22-01 due date: 2022-04-27.

Required action: Apply updates per vendor instructions.

Public proof-of-concept exploits

References

portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31166 (Patch, Vendor Advisory)
packetstormsecurity.com/files/162722/Microsoft-HTTP-Protocol-Stack-Remote-Code-… (VDB Entry, Third Party Advisory)
134c704f-9b21-4f2e-91b3-4a467353bcc0 (US Government Resource)

Frequently asked questions

What is CVE-2021-31166?: CVE-2021-31166 is a critical-severity vulnerability in Microsoft Windows 10 2004, classified under Use After Free. CVSS score: 9.8/10. Published 2021-05-11.
How severe is CVE-2021-31166?: Critical severity. CVSS v3 base score is 9.8 out of 10.
Is CVE-2021-31166 known to be exploited?: Yes. CVE-2021-31166 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2022-04-06), indicating it is being actively exploited. 76 public proof-of-concept repositories are indexed.