How severe is CVE-2021-31166?

CVE-2021-31166 has a CVSS score of 9.8/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-31166?

Affected package information for CVE-2021-31166 is not yet fully mapped. Check the official advisory links for details.

How do I fix CVE-2021-31166?

No fixed version has been published for CVE-2021-31166 yet. Monitor the advisory for updates, apply mitigations recommended by the vendor, and use O3 Security to track when a fix becomes available in your stack.

Is CVE-2021-31166 actively exploited in the wild?

Yes. There are 25 known exploit references for CVE-2021-31166, including 17 in-the-wild exploitations observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-31166?

CVE-2021-31166 has an EPSS (Exploit Prediction Scoring System) score of 93.1%, placing it in the 100th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. A score above 50% indicates significantly elevated exploitation risk — immediate patching is strongly recommended.

Is CVE-2021-31166 in the CISA Known Exploited Vulnerabilities catalog?

Yes. CVE-2021-31166 (Microsoft HTTP Protocol Stack Remote Code Execution Vulnerability) was added to CISA's Known Exploited Vulnerabilities (KEV) catalog on 2022-04-06. CISA required all US federal civilian agencies to remediate this vulnerability by 2022-04-27. The required action was: Apply updates per vendor instructions. Being listed in the KEV catalog is the strongest possible confirmation of active exploitation — treat this as a critical priority.

What type of vulnerability is CVE-2021-31166?

CVE-2021-31166 is classified as Use After Free (CWE-416). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

Which products and versions are affected by CVE-2021-31166?

CVE-2021-31166 affects microsoft windows 10 20h2; microsoft windows 10 2004; microsoft windows server 20h2; microsoft windows server 2004. Check the affected-products list on this page against your deployed version to confirm exposure.

When was CVE-2021-31166 published, and has it been updated?

CVE-2021-31166 was published on May 11, 2021 and was last updated on October 30, 2025. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CISA KEV·Added 2022-04-06 — agencies required to remediate by 2022-04-27

CVE-2021-31166

CRITICAL

HTTP Protocol Stack Remote Code Execution Vulnerability

Published

May 11, 2021

Updated

Oct 30, 2025

Affected

0 pkgs

Patched

None yet

Exploits

25 known

EPSS Exploitation Probability

via FIRST.org ↗

93.1%probability of exploitation in next 30 days

Very High Risk100th percentile0.00%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Description

HTTP Protocol Stack Remote Code Execution Vulnerability

Affected Products

4 products · 4 configurations

windows 10 20h2microsoft

< 10.0.19042.982

range

windows 10 2004microsoft

< 10.0.19041.982

range

windows server 20h2microsoft

< 10.0.19042.982

range

windows server 2004microsoft

< 10.0.19041.982

range

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

HTTP Protocol Stack Remote Code Execution Vulnerability

github.comGithubMay 2021

HTTP Protocol Stack Remote Code Execution Vulnerability

github.comGithubMay 2021

HTTP Protocol Stack Remote Code Execution Vulnerability

github.comGithubMay 2021

HTTP Protocol Stack Remote Code Execution Vulnerability

github.comGithubJul 2021

HTTP Protocol Stack Remote Code Execution Vulnerability

github.comGithubMay 2021

HTTP Protocol Stack Remote Code Execution Vulnerability

github.comGithubMay 2021

HTTP Protocol Stack Remote Code Execution Vulnerability

github.comGithubMay 2021

HTTP Protocol Stack Remote Code Execution Vulnerability

github.comGithubSep 2021

HTTP Protocol Stack Remote Code Execution Vulnerability

github.comGithubMay 2021

HTTP Protocol Stack Remote Code Execution Vulnerability

github.comGithubMay 2021

Frequently Asked Questions

HTTP Protocol Stack Remote Code Execution Vulnerability

O3 Security · Impact-Aware SCA

Is CVE-2021-31166 in your stack?

O3 detects CVE-2021-31166 across dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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