CVE-2021-31166

CRITICALNVD 9.89.8—Trending — 5 sources updated this weekExploited in the wild

9.8

HTTP Protocol Stack Remote Code Execution Vulnerability

CVSS v3: 9.8
EG Score: 9.8(high)
EPSS: 99.8%
KEV: ⚠ Exploited

Published

May 11, 2021

Last Modified

October 30, 2025

Advisory Details (3)

Auto-updated May 26, 2026

⚠️ Active exploitation confirmed. Patch available. Sources: microsoft, cisa.

cisa Patch Available🔴 Active Exploitation

Known Exploited Vulnerabilities Catalog | CISA

Known Exploited Vulnerabilities Catalog | CISA. Listed in CISA Known Exploited Vulnerabilities catalog.

https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-31166

microsoft Patch Available

Security Update Guide - Microsoft Security Response Center

Security Update Guide - Microsoft Security Response Center. Patch available via Microsoft Security Update

https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31166

generic

Packet Storm

http://packetstormsecurity.com/files/162722/Microsoft-HTTP-Protocol-Stack-Remote-Code-Execution.html

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

CWE-416Use After Free

All Vendor Advisories

(1)

Every vendor that published an advisory referencing this CVE — pulled from our cve_vendor_advisories aggregation. Click any row for the vendor's original advisory page.

Microsoft MSRCCVE-2021-311662021-05-11
HTTP Protocol Stack Remote Code Execution Vulnerability

Data Freshness Timeline

(refreshed 20× in last 7d / 20× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

2026-06-10 06:38 UTCEG score recompute
2026-06-10 06:38 UTCVendor advisory
2026-06-10 06:38 UTCGHSA enrichment
2026-06-10 02:53 UTCEG score recompute
2026-06-10 02:53 UTCVendor advisory
2026-06-10 02:53 UTCGHSA enrichment
2026-06-09 23:09 UTCEG score recompute
2026-06-09 23:09 UTCVendor advisory
2026-06-09 23:09 UTCGHSA enrichment
2026-06-09 19:25 UTCEG score recompute
2026-06-09 19:25 UTCVendor advisory
2026-06-09 19:25 UTCGHSA enrichment
2026-06-09 18:42 UTCCISA KEV update
2026-06-09 17:12 UTCCISA KEV update
2026-06-09 15:41 UTCEG score recompute
2026-06-09 15:41 UTCVendor advisory
2026-06-09 15:41 UTCGHSA enrichment
2026-06-09 11:57 UTCEG score recompute
2026-06-09 11:57 UTCVendor advisory
2026-06-09 11:57 UTCGHSA enrichment

Publicly available exploits

(10 references)

Working exploit code is in the public domain (1 Metasploit module) (9 GitHub PoCs). Defenders should treat patch urgency accordingly — public PoCs typically lead to mass-exploitation within 24-72 hours.

GitHub PoC0xmaximus/Home-Demolisher
First seen Nov 22, 2022
PoC for CVE-2021-31166 and CVE-2022-21907
Open source ↗
GitHub PoCiranzai/CVE-2021-31166-exploit
First seen Oct 20, 2021
Just a simple CVE-2021-31166 exploit tool
Open source ↗
GitHub PoCZZ-SOCMAP/CVE-2021-31166
First seen Sep 27, 2021
Windows HTTP协议栈远程代码执行漏洞 CVE-2021-31166
Open source ↗
GitHub PoCy0g3sh-99/CVE-2021-31166-Exploit
First seen Jul 3, 2021
Exploit for MS Http Protocol Stack RCE vulnerability (CVE-2021-31166)
Open source ↗
GitHub PoCzecopro/CVE-2021-31166
First seen May 19, 2021
simple bash script for exploit CVE-2021-31166
Open source ↗
GitHub PoCzha0gongz1/CVE-2021-31166
First seen May 17, 2021
PoC for CVE-2021-31166, a remote HTTP.sys use-after-free triggered remotely. Although it was defined as remote command execution, it can only cause the system to crash.
Open source ↗
GitHub PoCcorelight/CVE-2021-31166
First seen May 17, 2021
HTTP Protocol Stack CVE-2021-31166
Open source ↗
GitHub PoCmvlnetdev/CVE-2021-31166-detection-rules
First seen May 17, 2021
Different rules to detect if CVE-2021-31166 is being exploited
Open source ↗
GitHub PoC0vercl0k/CVE-2021-31166
First seen May 16, 2021
Proof of concept for CVE-2021-31166, a remote HTTP.sys use-after-free triggered remotely.
Open source ↗
Metasploitauxiliary/dos/windows/http/http_sys_accept_encoding_dos_cve_2021_31166✓ verified
First seen May 11, 2021
Windows IIS HTTP Protocol Stack DOS
Open source ↗

Frequently asked(6)

What is CVE-2021-31166?

CVE-2021-31166 is a critical vulnerability published on May 11, 2021. HTTP Protocol Stack Remote Code Execution Vulnerability

When was CVE-2021-31166 disclosed?

CVE-2021-31166 was first published in the National Vulnerability Database on May 11, 2021, with the most recent update on October 30, 2025. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.

Is CVE-2021-31166 actively exploited?

Yes. CISA added CVE-2021-31166 to the Known Exploited Vulnerabilities catalog on April 6, 2022, affecting Microsoft HTTP Protocol Stack. KEV listing indicates confirmed exploitation in the wild; this CVE warrants immediate patching attention.

What is the CVSS score of CVE-2021-31166?

CVE-2021-31166 has a CVSS v3 base score of 9.8 (NVD).

Which products are affected by CVE-2021-31166?

CVE-2021-31166 affects Microsoft HTTP Protocol Stack. The full affected-products list, including version ranges and fixed versions, is shown in the Affected Packages section of this page.

How do I remediate CVE-2021-31166?

Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-31166, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

See which npm, PyPI, Go, and Maven packages are affected by CVE-2021-31166

Explore →

Is Your Infrastructure Affected by CVE-2021-31166?

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CVE-2021-31166

Published

Last Modified

Advisory Details (3)

Known Exploited Vulnerabilities Catalog | CISA

Security Update Guide - Microsoft Security Response Center

Packet Storm

Weakness Classification(1)

All Vendor Advisories

Data Freshness Timeline

Publicly available exploits

Frequently asked(6)

Dependency Blast Radius

Is Your Infrastructure Affected by CVE-2021-31166?

Same vendor

Same CWE

CVE-2021-31166

📅 Published

🔄 Last Modified

📋 Advisory Details (3)

Known Exploited Vulnerabilities Catalog | CISA

Security Update Guide - Microsoft Security Response Center

Packet Storm

Weakness Classification(1)

All Vendor Advisories

Data Freshness Timeline

Publicly available exploits

❓ Frequently asked(6)

Dependency Blast Radius

Is Your Infrastructure Affected by CVE-2021-31166?

🔗 Related CVEs(same vendor + same CWE)

Same vendor

Same CWE

Published

Last Modified

Advisory Details (3)

Frequently asked(6)

Related CVEs(same vendor + same CWE)