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Fencer Intel
critical 9.8

CVE-2021-31166 kev in the critical feed json

HTTP Protocol Stack Remote Code Execution Vulnerability

published 5y ago modified 4mo ago also EUVD-2021-18079

cvss
9.8
version 3.1, CNA
cisa kev
Exploited
added 2022-04-06
epss
99.9%
EPSS 99.9% chance of exploitation in 30 days, percentile 99.9%
public exploits
1
Metasploit

Summary

HTTP Protocol Stack Remote Code Execution Vulnerability

Patch / fix

cisa required action Apply updates per vendor instructions.

  • Microsoft Windows 10 Version 2004 10.0.19041.982
  • Microsoft Windows Server version 2004 10.0.19041.982
  • Microsoft Windows 10 Version 20H2 10.0.19042.982
  • Microsoft Windows Server version 20H2 10.0.19042.982

Is CVE-2021-31166 in your stack? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.

Check with Fencer

Timeline

  1. First public exploit (Metasploit auxiliary/dos/windows/http/http_sys_accept_encoding_dos_cve_2021_31166)
  2. Published
  3. Added to CISA KEV
  4. CISA patch deadline
  5. CISA SSVC: exploitation active

Known exploited

Added
4y ago
Patch by
2022-04-27 (4y ago)
Ransomware
Not known
Required action
Apply updates per vendor instructions.
Vendor, product
Microsoft, HTTP Protocol Stack
CISA name
Microsoft HTTP Protocol Stack Remote Code Execution Vulnerability

ATT&CK techniques

How it is exploited and what it leads to, in MITRE ATT&CK terms.

  • T1190 Exploit Public-Facing Application exploitation
  • T1059 Command and Scripting Interpreter primary impact

kev release 2025-07-28, att&ck v16.1. KEV to ATT&CK mappings by the MITRE Center for Threat-Informed Defense (Mappings Explorer, Apache-2.0). © 2026 The MITRE Corporation. This work is reproduced and distributed with the permission of The MITRE Corporation. Terms of use

CISA SSVC

Exploitation
Active, exploited in the wild
Automatable
yes
Technical impact
total
Assessed
last yr.

Exploit prediction

99.87% chance of exploitation in the next 30 days, higher than 99.9% of all scored CVEs (FIRST EPSS, 2026-11-09).

Exploits and checks

CVSS scores

assigned by version score vector
CNA 3.1 9.8 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
CNA 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
NVD 2.0 7.5 AV:N/AC:L/Au:N/C:P/I:P/A:P

Affected products

  • Microsoft Windows 10 Version 2004
  • Microsoft Windows Server version 2004
  • Microsoft Windows 10 Version 20H2
  • Microsoft Windows Server version 20H2

Classification

Weakness

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.