CVE-2021-31166

Name: CVE-2021-31166
Creator: NVD / NIST
Published: 2021-05-11T19:15:09.3+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

CRITICALCVSS 9.8/10Actively ExploitedEPSS 99.66%

Last modified Jun 17, 2026

CVE-2021-31166 is a critical-severity vulnerability rated 9.8/10 on the CVSS scale. HTTP Protocol Stack Remote Code Execution Vulnerability. CISA has confirmed active exploitation in the wild. EPSS estimates a 99.66% chance of exploitation in the next 30 days.

Description

HTTP Protocol Stack Remote Code Execution Vulnerability

Metrics

CVSS 3.1

9.8/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

EPSS Probability

99.66%

99.9th percentile

Probability of exploitation in the next 30 days. Learn more

Exploitation Status

This vulnerability is listed in CISA’s Known Exploited Vulnerabilities catalog, confirming active exploitation in the wild. Federal agencies must remediate by Apr 27, 2022.

Weakness Enumeration

Affected Software

Vendor	Product	Versions
Microsoft	Windows 10 2004	< 10.0.19041.982
Microsoft	Windows 10 20h2	< 10.0.19042.982
Microsoft	Windows Server 2004	< 10.0.19041.982
Microsoft	Windows Server 20h2	< 10.0.19042.982

References

http://packetstormsecurity.com/files/162722/Microsoft-HTTP-Protocol-Stack-Remote-Code-Execution.htmlThird Party Advisory, VDB Entry
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31166Patch, Vendor Advisory
http://packetstormsecurity.com/files/162722/Microsoft-HTTP-Protocol-Stack-Remote-Code-Execution.htmlThird Party Advisory, VDB Entry
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31166Patch, Vendor Advisory
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-31166US Government Resource

Timeline

Published: May 11, 2021
Last Modified: Jun 17, 2026
Status: Analyzed

Frequently Asked Questions

What is CVE-2021-31166?

HTTP Protocol Stack Remote Code Execution Vulnerability

How severe is CVE-2021-31166?

CVE-2021-31166 has a CVSS score of 9.8/10 (CRITICAL severity). The EPSS model estimates a 99.66% probability of exploitation in the next 30 days. This vulnerability is listed in CISA's Known Exploited Vulnerabilities catalog.

How do I fix CVE-2021-31166?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-31166?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST