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
Actively exploited Microsoft HTTP Protocol Stack vulnerability. Added to the CISA KEV catalog 2022-04-06.
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.
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 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).
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 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 (morning and afternoon US Central).
Scope and sources
- More exploited Microsoft CVEs
- CVE-2019-1068CVE-2008-4250CVE-2009-1537CVE-2010-0249CVE-2010-0806CVE-2026-41091CVE-2026-45498CVE-2026-33825
All Microsoft exploited CVEs on one page: Microsoft vulnerabilities actively exploited.
- Authoritative references
Every actively exploited CVE, searchable with due dates and CSV export: the exploited-CVE tracker. The newest additions: exploited this week.