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Actively exploited

CVE-2021-31166 Microsoft HTTP Protocol Stack Remote Code Execution Vulnerability

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

From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.

Our Assessment

Should You Worry?

Patch nowCybergen assessment
  • CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 6 Apr 2022.
  • Rated Critical severity (CVSS 9.8, scored by Vendor).
  • It has been in the catalogue for 1,675 days. Systems still unpatched now have almost certainly been scanned for it.
The Vulnerability

What It Is

HTTP Protocol Stack Remote Code Execution Vulnerability

Affected Versions

ProductVersions
Microsoft Windows 10 Version 200410.0.0 to before 10.0.19041.982
Microsoft Windows Server version 200410.0.0 to before 10.0.19041.982
Microsoft Windows 10 Version 20H210.0.0 to before 10.0.19042.982
Microsoft Windows Server version 20H210.0.0 to before 10.0.19042.982

From the public CVE record, as supplied by the vendor. Always confirm against the vendor advisory.

From Watch to Action

What To Do Now

  1. 1

    Find it

    Search your asset inventory and external attack surface for Microsoft HTTP Protocol Stack. Include cloud images, appliances and anything a supplier runs for you.

    Attack Surface Management
  2. 2

    Fix it

    Apply updates per vendor instructions.

    Vulnerability Management
  3. 3

    Hunt for compromise

    If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.

    Incident Response
  4. 4

    Prove it is fixed

    Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.

    Penetration Testing

Vendor Advisories

Regulation

The Compliance Clock

Cyber EssentialsRated critical, so if it affects devices in your Cyber Essentials scope, the update must be installed within 14 days of the vendor releasing it.NIS2If exploitation causes a significant incident, essential and important entities owe their CSIRT an early warning within 24 hours and a notification within 72 hours.DORAFinancial entities must classify ICT incidents and send an initial notification for a major incident within strict deadlines, with exploited flaws a common cause.US federal (BOD 22-01)Federal civilian agencies had until 27 Apr 2022 to remediate. A sensible benchmark for everyone else.

General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.

Stay Ahead

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Evidence

Sources

  1. CISA Known Exploited Vulnerabilities catalogue
  2. NVD: CVE-2021-31166
  3. CVE record: CVE-2021-31166
  4. FIRST EPSS
  5. portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31166
  6. packetstormsecurity.com/files/162722/Microsoft-HTTP-Protocol-Stack-Remote-Code-Execution.html