OnlyFm252
Windows HTTP.sys

CVE-2021-31166 — HTTP Protocol Stack Remote Code Execution Vulnerability

Critical CVSS 9.8 EPSS 0.99657 2021-05 archive

Executive Summary

None

Overview

9.8
CVSS CRITICAL
Critical
MS Severity
Not Exploited
MS Exploit Status
More Likely
MS Exploit Likelihood
Category Remote Code Execution
Released May 11 2021
Last Updated May 11 2021
Publicly Disclosed No
CISA KEV Not Listed
Known Exploits None Known
EPSS Score 0.99657 — 0.99947 percentile
NVD CVSS 9.8 CRITICAL — matches MSRC

CVSS Vector

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
ATTACK VECTOR
Network
ATTACK COMPLEXITY
Low
PRIVILEGES REQUIRED
None
USER INTERACTION
None
SCOPE
Unchanged
CONFIDENTIALITY
High
INTEGRITY
High
AVAILABILITY
High
EXPLOIT CODE MATURITY
Unproven
REMEDIATION LEVEL
Official Fix
REPORT CONFIDENCE
Confirmed
Temporal Score: 8.5

EPSS Score

0.99657
probability of exploitation in the next 30 days
0.99947 percentile - updated 2026-06-21
View on FIRST.org

Affected Products

7 affected products
Product KB Article Severity Impact Restart Required
Windows 10 Version 2004 for 32-bit Systems 5003173 (Security Update) Critical Remote Code Execution Yes
Windows 10 Version 2004 for ARM64-based Systems 5003173 (Security Update) Critical Remote Code Execution Yes
Windows 10 Version 2004 for x64-based Systems 5003173 (Security Update) Critical Remote Code Execution Yes
Windows 10 Version 20H2 for 32-bit Systems 5003173 (Security Update) Critical Remote Code Execution Yes
Windows 10 Version 20H2 for ARM64-based Systems 5003173 (Security Update) Critical Remote Code Execution Yes
Windows Server, version 2004 (Server Core installation) 5003173 (Security Update) Critical Remote Code Execution Yes
Windows Server, version 20H2 (Server Core Installation) 5003173 (Security Update) Critical Remote Code Execution Yes

Patches

1 patch
Article Type Restart
5003173 Security Update Yes

Known Exploits

Acknowledgments

Microsoft Platform Security & Vulnerability Research

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