Vulnerability report

Exploitation attempts observed in the wild Confirmed confidence In CISA KEV

CVE-2021-31166

HTTP Protocol Stack Remote Code Execution Vulnerability

Microsoft / Windows 10 Version 2004 · affected before 10.0.19041.982

Watch CVE-2021-31166 Browse KEV feed
Severity
CVSS 9.8 · Critical
Confidence
Confirmed
Exploit status
Exploitation attempts observed in the wild
EPSS
99.9%
First observed
—
Last observed
—

Decision summary

What security teams need to know first

Direct answers before the deeper technical record.

What it is

CVE-2021-31166 is an unauthenticated HTTP Protocol Stack Remote Code Execution Vulnerability affecting Microsoft Windows 10 Version 2004 and 3 other products.

Have exploitation attempts been observed?

Yes. Exploitation attempts have been reported for this vulnerability. Previdian lists it as a known exploited vulnerability. Listed in CISA KEV. Confidence is confirmed.

Who is affected?

Microsoft / Windows 10 Version 2004 affected before 10.0.19041.982.

What should we do?

Prioritize remediation. Validate affected assets and apply vendor fixes on an accelerated timeline.

Overview

HTTP Protocol Stack Remote Code Execution Vulnerability

HTTP Protocol Stack Remote Code Execution Vulnerability

Microsoft Affected
Windows 10 Version 2004
Before 10.0.19041.982
Microsoft Affected
Windows Server version 2004
Before 10.0.19041.982
Microsoft Affected
Windows 10 Version 20H2
Before 10.0.19042.982
Microsoft Affected
Windows Server version 20H2
Before 10.0.19042.982
Published
11 May 2021
Exploitation Attempts Reported
06 Apr 2022
Attack vector
Remote
Complexity
Low
Privileges
None
User interaction
None

Tags

microsoft cisa windows metasploit_scanner

CVE References

Exploitation evidence

Evidence of exploitation attempts

Third-party attestation and first-party sensor observation are shown separately so teams can judge the evidence chain.

Exploitation attempts observed in the wild

CISA

Recorded 06 Apr 2022

A trusted third party reported exploitation attempts.

Proof of concept available

GitHub

Recorded 16 May 2021

Public scanner or PoC coverage increases practical exploitability.

Known exploited vulnerability sources

Per-source evidence links for KEV attestations are available through the Previdian Pro API.

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Source Added
CISA First 2022-04-06 00:00 UTC

Detection

Operational artifacts and observed signals

Make the evidence actionable in scanner, SOC, and edge-control workflows.

Virtual patch status

No Previdian virtual patch is currently available. Future rules ship for ModSecurity, Cloudflare, and AWS WAF.

Learn about virtual patches →

Scanner Artifacts

Scanner and exploit-framework references linked to this CVE.

Scanner Reference Detected
Metasploit https://github.com/rapid7/metasploit-framework/blob/master/modules/auxiliary/dos/windows/http/http_sys_accept_encoding_dos_cve_2021_31166.rb 11 May 2021

Risk and context

Severity, weaknesses, and research context

CVSS v3.1

9.8 Critical

Potential damage if exploited. Separate from whether attackers are using it.

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

EPSS

99.9%

Estimated chance of exploitation in the next 30 days. Previdian's warning comes from evidence, not this score.

Weaknesses

Use After Free CWE-416

Potential Proof of Concepts

These PoCs are unverified and could contain malware. Use at your own risk.

0vercl0k/CVE-2021-31166

github · Created 2021-05-16 16:15:56 UTC · 823 stars · AI assessment 90%

Proof of concept for CVE-2021-31166, a remote HTTP.sys use-after-free triggered remotely.

Timeline

From disclosure to observed exploitation attempts

  1. 00:00 UTC

    Added to CISA KEV

    Listed in the CISA Known Exploited Vulnerabilities catalog

  2. 16:15 UTC

    Public PoC available

    Public proof-of-concept code published

  3. 19:11 UTC

    CVE published

    Vulnerability disclosed publicly

  4. 00:00 UTC

    Metasploit module available

    Exploit module available

  5. 00:00 UTC

    CVE ID reserved

    Identifier reserved by the CNA

Pro API

Automate this intelligence

Confidence, exploit status, sensor telemetry, PoCs, scanner integrations, mentions, and tags are available programmatically for VM, SOC, and CTI workflows.

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  • Evidence confidence and provenance
  • First-party sensor telemetry
  • PoC and scanner references
  • Affected versions and enrichment
  • Automation-ready JSON delivery

GET /api/v2/pro/kevs/CVE-2021-31166

Sample API response 
{
  "cve_id": "CVE-2021-31166",
  "confidence": "Confirmed",
  "cvss_score": 9.8,
  "cvss_estimated": false,
  "epss_score": 0.99867,
  "exploit_status": {
    "exploited_in_the_wild": true,
    "active_exploitation_observed": false
  },
  "sensor_telemetry": { "attempts": 0, "sensors": 0 }
}