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CVE-2021-31166

HTTP Protocol Stack Remote Code Execution Vulnerability

CRITICAL
9.8
CVSS Score
Published: May 11, 2021 at 07:11 PM UTC
Updated: June 17, 2026 at 03:51 AM UTC
5 products affected across 1 vendorWindows 10 1809Windows 10 20H2Windows Server 20H2

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Severity
CRITICAL9.8
Exploitation
CISA KEV
Exploit prediction
EPSS 100%
SSVC
Active

Actively Exploited

CISA Known Exploited Vulnerabilities (KEV) Catalog

EPSS Score99.9%

EPSS (Exploit Prediction Scoring System) estimates a 99.9% chance this CVE is exploited in the wild within the next 30 days — higher than 100% of all CVEs. It complements CVSS: a high EPSS on a mid-severity CVE can still be urgent.

Microsoft HTTP Protocol Stack contains a vulnerability in http.sys that allows for remote code execution.

Required Action
Apply updates per vendor instructions.
Ransomware Use
Unknown
Added to KEV
Apr 6, 2022
Remediation Due
Apr 27, 2022
1649 days overdue
Notes
https://nvd.nist.gov/vuln/detail/CVE-2021-31166

Description

HTTP Protocol Stack Remote Code Execution Vulnerability

Affected

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

Windows 10 1809· Microsoft
95%
10.0.19041.982
Windows 10 2004· Microsoft
95%
v < 10.0.19041.98210.0.0 ≤ v < 10.0.19041.98210.0.0
Affected range stops below 10.0.19041.982, so it looks safe — our inference, not a vendor statement
Windows 10 20H2· Microsoft
95%
v < 10.0.19042.98210.0.0 ≤ v < 10.0.19042.98210.0.0
Affected range stops below 10.0.19042.982, so it looks safe — our inference, not a vendor statement
Windows Server 2004· Microsoft
95%
v < 10.0.19041.98210.0.0 ≤ v < 10.0.19041.98210.0.010.0.19041.982
Affected range stops below 10.0.19041.982, so it looks safe — our inference, not a vendor statement
Windows Server 20H2· Microsoft
95%
v < 10.0.19042.98210.0.0 ≤ v < 10.0.19042.98210.0.0
Affected range stops below 10.0.19042.982, so it looks safe — our inference, not a vendor statement

Remediation

No guide yet

No remediation guidance has been written for this CVE yet. What the record does show:

Patch and advisory references

References (3)

x_refsource_MISCThird Party AdvisoryVDB Entry
Source: [email protected]
http://packetstormsecurity.com/files/162722/Microsoft-HTTP-Protocol-Stack-Remote-Code-Execution.html
x_refsource_MISCPatchVendor Advisory
Source: [email protected]
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31166
US Government Resource
Source: 134c704f-9b21-4f2e-91b3-4a467353bcc0
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-31166

SSVC — Action Prioritization

SSVCActiveAutomatableTotal impact

SSVC signals action urgency (what to patch first), separate from CVSS which measures severity.

CVSS

9.8CRITICALmicrosoft
Vector?

Quick Info

StatusANALYZED
CWE
CWE-416

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