CVE-2021-26443

Guest-to-host code execution via crafted VMBus messages

Published Nov 10, 2021 · Updated Aug 19, 2026

Remote code execution in Microsoft Windows VMBus allows guest OS users to execute arbitrary code on the Hyper-V host. The host-side VMBus handling improperly processes specially crafted communications sent through a channel by an authenticated guest. The attacker must already control a guest VM account; successful exploitation crosses the virtualization boundary and runs code on the underlying host operating system.

CVSS severity9.0
Critical
EPSS probability1.68%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Remote code execution in Microsoft Windows VMBus allows guest OS users to execute arbitrary code on the Hyper-V host. The host-side VMBus handling improperly processes specially crafted communications sent through a channel by an authenticated guest. The attacker must already control a guest VM account; successful exploitation crosses the virtualization boundary and runs code on the underlying host operating system.

The record

CVE
CVE-2021-26443
Published
Nov 10, 2021
Updated
Aug 19, 2026
Vendor
Microsoft
Product
Windows Server 2019
Classifications
T1068
Attack vector
adjacent
Privileges
authenticated

Timeline

How it unfolded

  1. Nov 10, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 19, 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 <10.0.17763.2300

What conditions does exploitation require?

Attack vectoradjacent
Required privilegesauthenticated

What is affected?

Microsoft · Windows Server 2019version=10.0.17763.0 <10.0.17763.2300
Microsoft · Windows 10 Version 21H1version=10.0.0 <10.0.19043.1348
Microsoft · Windows 10 Version 1909version=10.0.0 <10.0.18363.1916
Microsoft · Windows 11 version 21H2version=10.0.22000.0 <10.0.22000.318
Microsoft · Windows Server, version 2004version=10.0.0 <10.0.19041.1348
Microsoft · Windows Server 2019 (Server Core installation)version=10.0.17763.0 <10.0.17763.2300
Microsoft · Windows Server version 20H2version=10.0.0 <10.0.19041.1348
Microsoft · Windows Server 2022version=10.0.20348.0 <10.0.20348.350
Microsoft · Windows 10 Version 2004version=10.0.0 <10.0.19041.1348
Microsoft · Windows 10 Version 1809version=10.0.17763.0 <10.0.17763.2300

Published CVSS scores

7.7NVDAV:A/AC:L/Au:S/C:C/I:C/A:C
9.0MicrosoftCVSS:3.1/AV:A/AC:L/PR:L/UI:N/S:C/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.

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)
Vectoradjacent
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1068Exploitation for Privilege Escalation

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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