# HTTP Protocol Stack Remote Code Execution Vulnerability

CVE record · `CVE-2021-31166`

- **Identifiers:** `CVE-2021-31166`
- **State:** Confirmed
- **Severity:** CVSS 9.8 Critical
- **Critical:** Yes: KEV, CVSS 9+, CVSS 7+ and EPSS
- **CNA:** microsoft
- **Weakness (CWE):** [CWE-416](https://cwe.mitre.org/data/definitions/416.html)
- **Tags:** [Memory corruption](https://agentprime.io/advisories/tag/memory-corruption.md)
- **Technologies:** [Microsoft Windows 10 Version 2004](https://agentprime.io/advisories/technology/microsoft-windows-10-version-2004.md) · [Microsoft Windows Server version 2004](https://agentprime.io/advisories/technology/microsoft-windows-server-version-2004.md) · [Microsoft Windows 10 Version 20H2](https://agentprime.io/advisories/technology/microsoft-windows-10-version-20h2.md) · [Microsoft Windows Server version 20H2](https://agentprime.io/advisories/technology/microsoft-windows-server-version-20h2.md)
- **Published:** 11 May 2021
- **Updated:** 21 Oct 2025
- **Reviewed by a person:** No

Published by the CVE Program (CNA: microsoft) at 11 May 2021, 19:11 UTC. agentprime has not analysed this yet, so whether it affects any app is not known. This CVE names a vendor and product rather than a package, so the lockfile check doesn’t match it.

## Critical

Critical, because:

- In CISA’s KEV catalogue since 6 Apr 2022: it is known to be exploited.
- CVSS score 9.8: 9.0 or more.
- CVSS score 9.8 (7.0 or more) and EPSS 1.00 (0.1 or more): a high score and a real chance of exploitation in the next 30 days.

How we decide what’s critical: <https://agentprime.io/advisories/critical.md>

## Affected products, as the CNA wrote them

- Vendor: Microsoft. Product: Windows 10 Version 2004.
  - affected: `10.0.0 to before 10.0.19041.982` (custom)
- Vendor: Microsoft. Product: Windows Server version 2004.
  - affected: `10.0.0 to before 10.0.19041.982` (custom)
- Vendor: Microsoft. Product: Windows 10 Version 20H2.
  - affected: `10.0.0 to before 10.0.19042.982` (custom)
- Vendor: Microsoft. Product: Windows Server version 20H2.
  - affected: `10.0.0 to before 10.0.19042.982` (custom)

## Not analysed yet

We haven’t published our own analysis of this CVE. If a person reviews it, this page will say what’s affected and what has to be true for it to matter.

## Severity and exploitation

- 9.8 Critical, CVSS 3.1: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C`, from CNA (microsoft)

- **CISA KEV:** Listed since 6 Apr 2022: known to be exploited.
- **EPSS:** 1.00, the 99th percentile, on 5 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

This record came from the full history of the CVE Program, published 11 May 2021. New records show when each source confirmed them.

## Sources

- [CVE List: CVE-2021-31166](https://www.cve.org/CVERecord?id=CVE-2021-31166)
- [NVD: CVE-2021-31166](https://nvd.nist.gov/vuln/detail/CVE-2021-31166)

### References from the CNA

- <https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31166>
- <http://packetstormsecurity.com/files/162722/Microsoft-HTTP-Protocol-Stack-Remote-Code-Execution.html>

Facts on this page come from the CVE List as the CNA published them, with CVSS from the sources named above, CISA KEV and FIRST EPSS. Vendors, products and versions are shown as the CNA wrote them. We don’t reproduce the CVE’s description.

## Check whether this affects you

This CVE names a vendor and product rather than a package, so the lockfile check doesn’t match it. Compare the affected versions above with what you run.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), `get_advisory` returns this record and `search_advisories` finds related ones. Setup: <https://agentprime.io/mcp.md>
- As JSON: <https://api.agentprime.io/v1/advisories/CVE-2021-31166>
- Everything that needs attention now: <https://agentprime.io/advisories/critical.md>

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The web page: <https://agentprime.io/advisories/CVE-2021-31166/>. Everything for agents: <https://agentprime.io/llms.txt>.