CVE-2021-31166Known exploitationPublic exploitNetwork attack vector

Unauthenticated code execution via HTTP.sys use-after-free

Published May 11, 2021 · Updated Oct 21, 2025

Use-after-free in HTTP.sys on affected Windows 10 and Windows Server builds allows remote attackers to execute code via a crafted HTTP request. UlpParseContentCoding moves a local encoding-entry list into the request but leaves the local list linked, allowing an error path to free entries still referenced by the request. Any unauthenticated network path to a service using HTTP.sys can reach the parser; successful exploitation executes code, while the published proof of concept causes a kernel crash.

CVSS severity9.8
Critical
EPSS probability99.77%
Next 30 days · Sep 16, 2026
Known exploitationReported
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Use-after-free in HTTP.sys on affected Windows 10 and Windows Server builds allows remote attackers to execute code via a crafted HTTP request. UlpParseContentCoding moves a local encoding-entry list into the request but leaves the local list linked, allowing an error path to free entries still referenced by the request. Any unauthenticated network path to a service using HTTP.sys can reach the parser; successful exploitation executes code, while the published proof of concept causes a kernel crash.

The record

CVE
CVE-2021-31166
Published
May 11, 2021
Updated
Oct 21, 2025
Vendor
Microsoft
Product
Windows Server, version 2004
Classifications
CWE-416, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 11, 2021CVE publishedPublication date reported by the CVE source.
  2. Oct 21, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=10.0.0 <10.0.19041.982

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Microsoft · Windows Server, version 2004version=10.0.0 <10.0.19041.982
Microsoft · Windows 10 Version 20H2version=10.0.0 <10.0.19042.982
Microsoft · Windows Server version 20H2version=10.0.0 <10.0.19042.982
Microsoft · Windows 10 Version 2004version=10.0.0 <10.0.19041.982

Published CVSS scores

9.8MicrosoftCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationReported
Public exploitPublished

Weakness, pattern, technique

CWE-416Use After Free
T1190Exploit Public-Facing Application

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Reported exploitation

Technologies

Your stack

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Your stack

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