CVE-2021-33781Network attack vector

Unauthorized device authentication via legacy KDFv1

Published Jul 14, 2021 · Updated Aug 10, 2026

Security feature bypass in Microsoft Windows before July 2021 allows remote authenticated users to perform unauthorized device authentication. The affected authentication path accepts the legacy KDFv1 algorithm, whose device-authentication design permits unauthorized authentication. Exploitation requires Entra-joined or hybrid-joined device authentication and can expose or alter resources available to the impersonated identity.

CVSS severity8.1
High
EPSS probability2.41%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Security feature bypass in Microsoft Windows before July 2021 allows remote authenticated users to perform unauthorized device authentication. The affected authentication path accepts the legacy KDFv1 algorithm, whose device-authentication design permits unauthorized authentication. Exploitation requires Entra-joined or hybrid-joined device authentication and can expose or alter resources available to the impersonated identity.

The record

CVE
CVE-2021-33781
Published
Jul 14, 2021
Updated
Aug 10, 2026
Vendor
Microsoft
Product
Windows Server 2019
Classifications
CWE-254
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Jul 14, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 10, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

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

  1. Affected versionversion=10.0.17763.0 <10.0.17763.2061

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Microsoft · Windows Server 2019version=10.0.17763.0 <10.0.17763.2061
Microsoft · Windows 10 Version 21H1version=10.0.0 <10.0.19043.1110
Microsoft · Windows 10 Version 1909version=10.0.0 <10.0.18363.1679
Microsoft · Windows Server 2016 (Server Core installation)version=10.0.14393.0 <10.0.14393.4704
Microsoft · Windows 11 version 21H2version=10.0.22000.0 <10.0.22000.258
Microsoft · Windows Server, version 2004version=10.0.0 <10.0.19041.1110
Microsoft · Windows Server 2016version=10.0.14393.0 <10.0.14393.4704
Microsoft · Windows 10 Version 20H2version=10.0.0 <10.0.19042.1110
Microsoft · Windows Server 2019 (Server Core installation)version=10.0.17763.0 <10.0.17763.2061
Microsoft · Windows Server version 20H2version=10.0.0 <10.0.19042.1110
Microsoft · Windows 10 Version 1607version=10.0.14393.0 <10.0.14393.4704
Microsoft · Windows 10 Version 2004version=10.0.0 <10.0.19041.1110
Microsoft · Windows 10 Version 1809version=10.0.0 <10.0.17763.2061; version=10.0.17763.0 <10.0.17763.2061

Published CVSS scores

5.5NIST NVDAV:N/AC:L/Au:S/C:P/I:P/A:N
8.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
8.1MicrosoftCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

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
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-254Security Features

Public exploit references

No public exploit references are available in this record.

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

Technologies

Your stack

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Your stack

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