---
id: "CVE-2021-31166"
kind: "vulnerability"
aliases: ["CVE-2021-31166"]
title: "Microsoft HTTP Protocol Stack: Microsoft HTTP Protocol Stack Remote Code Execution Vulnerability"
severity: "critical"
cvss: 9.8
epss: 0.99867
exploited: true
status: "open"
category: "infrastructure"
products: [{"name": "Microsoft HTTP Protocol Stack", "fixed": [], "affected": "", "ecosystem": "vendor"}]
ecosystems: ["vendor"]
tags: ["exploited", "iis", "infrastructure", "microsoft-http-stack", "windows"]
not_affected_if: ["unknown"]
published_at: "2021-05-11T19:15:09+00:00"
first_seen_at: "2026-10-23T19:11:52+00:00"
updated_at: "2026-10-23T21:57:35+00:00"
sources: [{"source": "kev", "url": "https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-31166", "first_seen_at": "2026-10-23T19:11:52+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2021-31166", "first_seen_at": "2026-10-23T19:13:44+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2021-31166", "first_seen_at": "2026-10-23T21:57:35+00:00"}]
---

# Microsoft HTTP Protocol Stack: Microsoft HTTP Protocol Stack Remote Code Execution Vulnerability

Microsoft's HTTP Protocol Stack (http.sys) contains a flaw that allows remote code execution. An attacker who can send crafted HTTP requests to a vulnerable Windows server can run code on it; CISA lists it as actively exploited.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- vendor:Microsoft HTTP Protocol Stack affected ? fixed ?

## Fix

Apply the Microsoft updates for CVE-2021-31166 per vendor instructions (CISA required action). Affected build numbers: unknown from the sources.

## Timeline

- 2026-10-23T19:11:52+00:00 kev https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-31166
- 2026-10-23T19:13:44+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2021-31166
- 2026-10-23T21:57:35+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2021-31166

## From the sources

Microsoft HTTP Protocol Stack contains a vulnerability in http.sys that allows for remote code execution.

CISA required action: Apply updates per vendor instructions.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_