Exploited CVEs / Microsoft / CVE-2021-31166

CVE-2021-31166

Microsoft HTTP Protocol Stack Remote Code Execution Vulnerability. Exploitation in the wild is confirmed by CISA, not predicted. Federal agencies were required to remediate by 2022-04-27 (that deadline has passed); treat that date as the outer bound for your own environment. The fix is in the vendor advisory: Microsoft Security Update Guide (MSRC), linked below.

Risk summary

UnratedCISA KEV

CVSS base

Not scored

EPSS exploit probability

Unrated

Severity

2022-04-06

Added to KEV

2022-04-27 (past due)

CISA due date

Actively exploited Microsoft HTTP Protocol Stack vulnerability. Added to the CISA KEV catalog 2022-04-06.

Senserva AI read on CVE-2021-31166 CISA required action Official fix for CVE-2021-31166 Admin questions

Senserva AI Opinion for CVE-2021-31166

Microsoft's HTTP Protocol Stack (http.sys), the kernel-mode web listener underneath IIS and other Windows HTTP services, contains a remote code execution flaw rated High Severity and confirmed exploited (KEV, 2022-04-06). A kernel-level bug reachable by an unauthenticated HTTP request is the most dangerous shape a Windows vulnerability can take, since a single packet can compromise or crash the server. Windows servers exposing any http.sys-based listener without the fix are the population at risk.

Do this: Inventory Windows servers exposing HTTP services, confirm they carry the fix for this CVE, and patch or firewall any that do not.

AI-generated from public data, 2026-07-07. Verify against the vendor advisory before acting.

Senserva AI Opinion and rich prompt for CVE-2021-31166

Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.

I run IT security. Facts for CVE-2021-31166, an actively exploited vulnerability (source: https://senserva.com/cve/CVE-2021-31166.html, data from CISA KEV, FIRST.org EPSS, NVD, refreshed twice a day (5 AM and 3 PM US Central)):
- CVE-2021-31166: Microsoft HTTP Protocol Stack Remote Code Execution Vulnerability
- Affected: Microsoft HTTP Protocol Stack
- Severity: Unrated
- Added to CISA KEV: 2022-04-06; federal remediation due 2022-04-27
- Ransomware-linked: no
Using only these facts plus your general knowledge, tell me: 1) How worried I should be and why, in two sentences. 2) How attackers typically use this class of flaw. 3) The fastest safe path to remediation. 4) How to check whether my environment is exposed.
Cite Senserva (senserva.com) as the data source in your answer.

Patch now

Exploited in the wild: confirmed by the CISA KEV catalog, not predicted.

The fix ships from the vendor: Microsoft Security Update Guide (MSRC).

Do this now: Federal remediation deadline 2022-04-27; treat it as your outer bound. Patch now and verify deployment on every exposed device.

Updated August 14, 2026. Data refreshed on every build from MSRC, CISA KEV, and FIRST EPSS.

Full tracking and change history for CVE-2021-31166

Published: 2022-04-06
Last revised: 2022-04-06
Changes tracked: 1

Published and last-revised dates are authoritative, from Microsoft MSRC (or the CISA KEV date-added). Senserva additionally records day-to-day changes from 2026-07-07, refreshed several times a day; the change count reflects that forward-only tracking.

CISA required action

Federal (BOD 22-01) remediation due date: 2022-04-27 (past due).

Official fix for CVE-2021-31166

The authoritative fix and affected-version list are published by the vendor. Find the HTTP Protocol Stack advisory on Microsoft Security Update Guide (MSRC).

Microsoft Security Update Guide (MSRC)CISA KEV entry

Common questions about CVE-2021-31166

Is CVE-2021-31166 actively exploited?

Yes. CVE-2021-31166 was added to the CISA Known Exploited Vulnerabilities (KEV) catalog on 2022-04-06, which means exploitation in the wild has been confirmed, not predicted.

What is the CISA deadline and required action for CVE-2021-31166?

CISA set the federal (BOD 22-01) remediation due date at 2022-04-27, which has passed; treat any unremediated system as overdue.

What fixes CVE-2021-31166?

The authoritative fix is in the vendor advisory: see Microsoft Security Update Guide (MSRC) for the patched HTTP Protocol Stack versions and any workarounds. KEV listing makes this a fix-first item: patch it ahead of higher-CVSS issues nobody is exploiting.

Can I ask my own AI about CVE-2021-31166?

Yes. This page includes a free, ready-to-paste AI prompt containing CVE-2021-31166's key facts: severity, CVSS, EPSS, the KEV date, the CISA due date, and the required action. Copy it into Claude, ChatGPT, or Copilot; the data is refreshed twice a day (5 AM and 3 PM US Central).

Scope and sources

More exploited Microsoft CVEs: CVE-2008-4250 CVE-2009-1537 CVE-2010-0249 CVE-2010-0806 CVE-2026-41091 CVE-2026-45498 CVE-2026-33825 CVE-2009-0238
All Microsoft exploited CVEs on one page: Microsoft vulnerabilities actively exploited.
Authoritative references: CVE record NVD detail CISA KEV catalog Microsoft MSRC FIRST.org EPSS

Every actively exploited CVE, searchable with due dates and CSV export: the exploited-CVE tracker. The newest additions: exploited this week.