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Vulnerabilities

Vulnerability · CVE-2021-31166

Microsoft HTTP Protocol Stack Remote Code Execution Vulnerability

Published May 11, 2021 · catalog updated Jun 16, 2026 · Microsoft HTTP Protocol Stack

Patch now

CVE-2021-31166 is in the CISA known-exploited catalog, due Apr 27, 2022. Confirm Microsoft HTTP Protocol Stack against the vendor advisory and apply that fix. The CVSS score does not change the priority.

Gaps on this record

  • No version range is stored. A customer cannot check an installed version against this record.
Severity
Critical9.8
Known exploitation
CISA KEV and VulnCheck KEV
EPSS
99.87%
Remediation
Remediation guidance
Affected products
cpe microsoft/windows 10 2004 · cpe microsoft/windows 10 20h2 · cpe microsoft/windows server 2004 · cpe microsoft/windows server 20h2 · vendor Microsoft HTTP Protocol Stack

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

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • cpe microsoft/windows 10 2004version unspecified
  • cpe microsoft/windows 10 20h2version unspecified
  • cpe microsoft/windows server 2004version unspecified
  • cpe microsoft/windows server 20h2version unspecified
  • vendor Microsoft HTTP Protocol Stackversion unspecified

How to fix CVE-2021-31166

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

CVE-2021-31166 is in the CISA known-exploited catalog, due Apr 27, 2022. Confirm Microsoft HTTP Protocol Stack against the vendor advisory and apply that fix. The CVSS score does not change the priority.

Which versions are affected?

Affected version ranges are not on this record. Apply updates per vendor instructions.

What should an analyst verify?

CWE-416: Use After Free. EPSS 99.87%. The CISA KEV listing is the exploitation evidence. A public exploit reference is linked in Sources. That is not the same as confirmed exploitation. Use the source links for the original advisory.

Technology hubs

Windows security

Code

The project this record applies to. Exploit listings stay in References.

No public source repository is identified for this product.

Timeline

Detection, updates, and resolution for this record.

  1. May 11, 2021 · Catalog

    Detected

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

  2. Apr 6, 2022 · CISA KEV

    Known exploited

    Apply updates per vendor instructions.

  3. Apr 27, 2022 · CISA KEV

    Remediation due

    Apply updates per vendor instructions.

  4. Jun 16, 2026 · Catalog

    Updated

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

Sources

  • CISA KEVConfirmed exploitationCVE-2021-31166

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

    cisa.gov
  • CVE.orgVulnerability recordCVE-2021-31166

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2021-31166

    30-day exploitation probability 99.9%

    first.org
  • Metasploit ModulesPublic exploit moduleauxiliary_dos/windows/http/http_sys_accept_encoding_dos_cve_2021_31166

    Public module reference (normal). Module source is not copied into this catalog.

    rapid7.com
  • NIST NVDVulnerability recordCVE-2021-31166

    HTTP Protocol Stack Remote Code Execution Vulnerability

    nvd.nist.gov
  • Packet Storm SecurityCited referencehttp://packetstormsecurity.com/files/162722/Microsoft-HTTP-Protocol-Stack-Remote-Code-Execution.html

    Packet Storm link already published on the finding.

    packetstormsecurity.com
  • VulnCheck KEVConfirmed exploitationCVE-2021-31166

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

    vulncheck.com

References

Freshness

Last verified in this catalog Oct 17, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.