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
9.8
EchelonGraph verdictPatch nowTreat as an emergency — this is being exploited.
  • Actively exploited in the wild (CISA-KEV)
CISA-KEV: ExploitedEPSS: 100%CVSS: 9.8Exploit: NoneExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

HTTP Protocol Stack Remote Code Execution Vulnerability

CVSS v3
9.8
EG Score
9.8(high)
EG Risk
99(Act)
EG Risk 99/100SSVC: Act

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%
Action: Fix now — active exploitation, automatable, high impact.
EPSS
99.9%
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.

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.

Data Freshness Timeline

(refreshed 58× in last 7d / 365× 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 704 total refreshes for this CVE.

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

Related CVEs(same vendor + 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 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

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

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