CVE-2021-31166

CRITICALNVD 9.89.8—
EchelonGraph scoreHIGH confidence

Score elevated to 9.8 because this CVE is listed on the CISA Known Exploited Vulnerabilities catalog (added 2022-04-06), indicating real-world exploitation has been confirmed by US federal agencies. NVD baseline CVSS 9.8 retained for reference. Confidence: HIGH.

Triggered by: CISA KEV (actively exploited)
Sources: cisa_kev, epss, ghsa, nvd
Trending — 5 sources updated this weekExploited in the wild
9.8EG
EchelonGraph verdictPatch nowTreat as an emergency — this is being exploited.
  • Actively exploited in the wild (CISA-KEV)
CISA-KEV: ExploitedEPSS PROB: 100%CVSS: 9.8Exploit: cisa kev · ssvc active · Metasploit · epss top5pct · epss high · public exploitExposed services: Not assessed

A fix is available — apply it.

HTTP Protocol Stack Remote Code Execution Vulnerability

CVSS v3
9.8
EG Score
9.8CRITICALhigh confidence
EG Risk
99
EG Risk 99/100CISA SSVC

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity98% × 45%
Exploitation100% × 40%
Automatability100% × 15%
CISA SSVC: Attend at low mission impact; Act at medium or high.
Action: A fix is available. Apply it sooner than your standard update timelines at low mission impact and as soon as possible at medium or high.
EPSS PROB
100%
EPSS %ILE
99th
KEV
⚠ Exploited

CISA SSVCAttend at low mission impact; Act at medium or high.

A fix is available. Apply it sooner than your standard update timelines at low mission impact and as soon as possible at medium or high.

Exploitation active (CISA Vulnrichment) · Automatable yes (CISA Vulnrichment) · Technical impact total (CISA Vulnrichment). Mission impact is CISA's Mission & Well-being decision point, and only you can judge it: high means the affected system is essential to your organisation's mission, or its compromise could cause irreversible harm to people. CISA's decision table

Published

May 11, 2021

Last Modified

October 30, 2025

Advisory Details (3)

Auto-updated Oct 1, 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 - Human Validation

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

Vendor Advisories for CVE-2021-31166(1)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Weakness Classification(1)

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

Data Freshness Timeline

(refreshed 94× in last 7d / 399× 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.

Showing the most recent 100 of 1,621 total refreshes for this CVE.

  1. 2026-10-04 10:54 UTCVendor advisory
  2. 2026-10-04 10:54 UTCGHSA enrichment
  3. 2026-10-04 07:10 UTCVendor advisory
  4. 2026-10-04 07:10 UTCGHSA enrichment
  5. 2026-10-04 03:26 UTCVendor advisory
  6. 2026-10-04 03:26 UTCGHSA enrichment
  7. 2026-10-03 23:42 UTCVendor advisory
  8. 2026-10-03 23:42 UTCGHSA enrichment
  9. 2026-10-03 19:57 UTCVendor advisory
  10. 2026-10-03 19:57 UTCGHSA enrichment
  11. 2026-10-03 16:13 UTCVendor advisory
  12. 2026-10-03 16:13 UTCGHSA enrichment
  13. 2026-10-03 12:29 UTCVendor advisory
  14. 2026-10-03 12:29 UTCGHSA enrichment
  15. 2026-10-03 08:44 UTCVendor advisory
  16. 2026-10-03 08:44 UTCGHSA enrichment
  17. 2026-10-03 04:58 UTCVendor advisory
  18. 2026-10-03 04:58 UTCGHSA enrichment
  19. 2026-10-03 01:14 UTCVendor advisory
  20. 2026-10-03 01:14 UTCGHSA enrichment
  21. 2026-10-02 21:30 UTCVendor advisory
  22. 2026-10-02 21:30 UTCGHSA enrichment
  23. 2026-10-02 17:45 UTCVendor advisory
  24. 2026-10-02 17:45 UTCGHSA enrichment
  25. 2026-10-02 14:00 UTCVendor advisory
Show 75 more
  1. 2026-10-02 14:00 UTCGHSA enrichment
  2. 2026-10-02 10:16 UTCVendor advisory
  3. 2026-10-02 10:16 UTCGHSA enrichment
  4. 2026-10-02 06:32 UTCVendor advisory
  5. 2026-10-02 06:32 UTCGHSA enrichment
  6. 2026-10-02 02:47 UTCVendor advisory
  7. 2026-10-02 02:47 UTCGHSA enrichment
  8. 2026-10-01 23:00 UTCVendor advisory
  9. 2026-10-01 23:00 UTCGHSA enrichment
  10. 2026-10-01 19:16 UTCVendor advisory
  11. 2026-10-01 19:16 UTCGHSA enrichment
  12. 2026-10-01 15:32 UTCVendor advisory
  13. 2026-10-01 15:32 UTCGHSA enrichment
  14. 2026-10-01 11:48 UTCVendor advisory
  15. 2026-10-01 11:48 UTCGHSA enrichment
  16. 2026-10-01 08:01 UTCVendor advisory
  17. 2026-10-01 08:01 UTCGHSA enrichment
  18. 2026-10-01 04:17 UTCVendor advisory
  19. 2026-10-01 04:17 UTCGHSA enrichment
  20. 2026-10-01 00:33 UTCVendor advisory
  21. 2026-10-01 00:33 UTCGHSA enrichment
  22. 2026-09-30 20:48 UTCVendor advisory
  23. 2026-09-30 20:48 UTCGHSA enrichment
  24. 2026-09-30 17:04 UTCVendor advisory
  25. 2026-09-30 17:04 UTCGHSA enrichment
  26. 2026-09-30 13:20 UTCVendor advisory
  27. 2026-09-30 13:20 UTCGHSA enrichment
  28. 2026-09-30 09:35 UTCVendor advisory
  29. 2026-09-30 09:35 UTCGHSA enrichment
  30. 2026-09-30 05:52 UTCVendor advisory
  31. 2026-09-30 05:52 UTCGHSA enrichment
  32. 2026-09-30 02:07 UTCVendor advisory
  33. 2026-09-30 02:07 UTCGHSA enrichment
  34. 2026-09-29 22:23 UTCVendor advisory
  35. 2026-09-29 22:23 UTCGHSA enrichment
  36. 2026-09-29 18:36 UTCVendor advisory
  37. 2026-09-29 18:35 UTCGHSA enrichment
  38. 2026-09-29 14:49 UTCVendor advisory
  39. 2026-09-29 14:49 UTCGHSA enrichment
  40. 2026-09-29 11:04 UTCVendor advisory
  41. 2026-09-29 11:04 UTCGHSA enrichment
  42. 2026-09-29 07:20 UTCVendor advisory
  43. 2026-09-29 07:20 UTCGHSA enrichment
  44. 2026-09-29 03:36 UTCVendor advisory
  45. 2026-09-29 03:36 UTCGHSA enrichment
  46. 2026-09-28 23:51 UTCEG score recompute
  47. 2026-09-28 23:51 UTCVendor advisory
  48. 2026-09-28 23:51 UTCGHSA enrichment
  49. 2026-09-28 20:06 UTCVendor advisory
  50. 2026-09-28 20:05 UTCGHSA enrichment
  51. 2026-09-28 16:20 UTCVendor advisory
  52. 2026-09-28 16:20 UTCGHSA enrichment
  53. 2026-09-28 12:35 UTCVendor advisory
  54. 2026-09-28 12:35 UTCGHSA enrichment
  55. 2026-09-28 08:51 UTCVendor advisory
  56. 2026-09-28 08:51 UTCGHSA enrichment
  57. 2026-09-28 05:07 UTCVendor advisory
  58. 2026-09-28 05:07 UTCGHSA enrichment
  59. 2026-09-28 01:23 UTCVendor advisory
  60. 2026-09-28 01:23 UTCGHSA enrichment
  61. 2026-09-27 21:51 UTCCISA KEV update
  62. 2026-09-27 21:39 UTCVendor advisory
  63. 2026-09-27 21:39 UTCGHSA enrichment
  64. 2026-09-27 17:55 UTCVendor advisory
  65. 2026-09-27 17:55 UTCGHSA enrichment
  66. 2026-09-27 14:10 UTCEG score recompute
  67. 2026-09-27 14:10 UTCVendor advisory
  68. 2026-09-27 14:10 UTCGHSA enrichment
  69. 2026-09-27 13:41 UTCEPSS rescore
  70. 2026-09-27 10:27 UTCVendor advisory
  71. 2026-09-27 10:26 UTCGHSA enrichment
  72. 2026-09-27 06:41 UTCVendor advisory
  73. 2026-09-27 06:40 UTCGHSA enrichment
  74. 2026-09-27 02:56 UTCVendor advisory
  75. 2026-09-27 02:56 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.

  • 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 ↗
  • GitHub PoC0xmaximus/Home-Demolisher
    First seen Nov 22, 2022

    PoC for CVE-2021-31166 and CVE-2022-21907

    Open source ↗
  • GitHub PoCmauricelambert/CVE-2021-31166
    First seen Mar 7, 2022

    CVE-2021-31166: exploitation with Powershell, Python, Ruby, NMAP and Metasploit.

    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 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 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 ↗

Related CVEs(same CWE)

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 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 attention. A fix is available. Apply it.
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.
How do I remediate CVE-2021-31166?
A fix for CVE-2021-31166 is available: update to the fixed version the vendor names in its advisory. The vendor advisories EchelonGraph has for CVE-2021-31166 are linked in the Vendor Advisories panel on this page.

Dependency Blast Radius

Explore the affected products and dependency analysis for CVE-2021-31166

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