Evidence dossier

CVE-2021-40444

Microsoft MSHTML Remote Code Execution Vulnerability

Exploited in the wild (CISA KEV since Nov 3, 2021). microsoft reports CVSS 3.1 8.8. Severity assessments differ within at least one CVSS version. EPSS estimates 97.8% exploit likelihood as of Oct 16, 2026.

79.3–95.1Priority evidence range

Evidence through Oct 22, 2026, 6:07 PM UTC

As of Oct 22, 2026

Normalized restatement

Microsoft is investigating reports of a remote code execution vulnerability in MSHTML that affects Microsoft Windows. Microsoft is aware of targeted attacks that attempt to exploit this vulnerability by using specially-crafted Microsoft Office documents. An attacker could craft a malicious ActiveX control to be used by a Microsoft Office document that hosts the browser rendering engine. The attacker would then have to convince the user to open the malicious document. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights. Microsoft Defender Antivirus and Microsoft Defender for Endpoint both provide detection and protections for the known vulnerability. Customers should keep antimalware products up to date. Customers who utilize automatic updates do not need to take additional action. Enterprise customers who manage updates should select the detection build 1.349.22.0 or newer and deploy it across their environments. Microsoft Defender for Endpoint alerts will be displayed as: “Suspicious Cpl File Execution”. Upon completion of this investigation, Microsoft will take the appropriate action to help protect our customers. This may include providing a security update through our monthly release process or providing an out-of-cycle security update, depending on customer needs. Please see the Mitigations and Workaround sections for important information about steps you can take to protect your system from this vulnerability. UPDATE September 14, 2021: Microsoft has released security updates to address this vulnerability. Please see the Security Updates table for the applicable update for your system. We recommend that you install these updates immediately. Please see the FAQ for important information about which updates are applicable to your system.

State: PUBLISHED
Published: Sep 15, 2021
Updated: Aug 10, 2026
Evidence coverage: 72%

Evidence chronology

What was known when

Download this view's receipt →

Source dateSource date omittedFirst observed by CASCAAug 10, 2026
CISA-ADP
Record text: CISA ADP Vulnrichment
Inspect raw assertion
Field
container
Value
CISA ADP Vulnrichment
Original evidence ↗
Source dateSource date omittedFirst observed by CASCAAug 10, 2026
CVE Program
Record text: CVE Program Container
Inspect raw assertion
Field
container
Value
CVE Program Container
Original evidence ↗
Source dateSource date omittedFirst observed by CASCAAug 10, 2026
microsoft
Record text: Microsoft MSHTML Remote Code Execution Vulnerability
Inspect raw assertion
Field
container
Value
Microsoft MSHTML Remote Code Execution Vulnerability
Original evidence ↗
Source dateSep 1, 2026First observed by CASCASep 1, 2026
CISA KEV
Exploitation cataloged: Microsoft MSHTML Remote Code Execution Vulnerability
Inspect raw assertion
Field
observed_exploitation
Value
Microsoft MSHTML Remote Code Execution Vulnerability
Original evidence ↗
Source dateOct 16, 2026First observed by CASCAOct 16, 2026
FIRST EPSS
EPSS score: 97.84% probability · 99.91th percentile
Inspect raw assertion
Field
model_probability
Value
Probability 0.978400000000; percentile 0.999090000000
Original evidence ↗

Assessments differ

NVD6.8CVSS 2.0 · source date omitted

NVD7.8CVSS 3.1 · source date omitted

secure@microsoft.com8.8CVSS 3.1 · source date omitted

Values are shown separately by source and CVSS version.

ExploitationCatalog member

CISA KEV · catalog date Nov 3, 2021 · first observed Sep 1, 2026

Exploit likelihood97.84%

FIRST EPSS · score date Oct 16, 2026 · 99.9th percentile · first observed Oct 16, 2026

SeverityAssessments differ

microsoft · CVSS 3.1 · first observed Aug 10, 2026 · values shown separately below

Evidence detail

Source limits and decisions

Why each evidence state has this value

casca-unknown-reasons-v1

Exploitation statusLast verified source snapshot retained

The last verified source boundary remains in force for this claim.

Revision: evidence-policy-v1.1.0
Cutoff: Oct 22, 2026
Resolution: Review source contract

Exploit likelihoodEvidence supported

Eligible evidence is present for this bounded claim.

Revision: evidence-policy-v1.1.0
Cutoff: Oct 22, 2026
Resolution: None

Severity assessmentAssessments differ

Eligible assertions materially conflict and remain visible side by side.

Revision: casca-direct-cvss-eligibility-v1
Cutoff: Oct 22, 2026
Resolution: Resolve conflict

Affected productsSource-reported scope

The cited source assertion is retained while canonical product linkage remains open.

Revision: casca-factor-d-obligations-v1
Cutoff: Oct 22, 2026
Resolution: Resolve identity

Source comparison

Who said what

CISA-ADPIndependent enrichment

Record text

CISA ADP Vulnrichment

Inspect raw assertion

Field: container
Value: CISA ADP Vulnrichment

Source dateSource date omittedFirst observed by CASCAAug 10, 2026Original evidence ↗

CVE ProgramDerivative copy

Record text

CVE Program Container

Inspect raw assertion

Field: container
Value: CVE Program Container

Source dateSource date omittedFirst observed by CASCAAug 10, 2026Original evidence ↗

microsoftOriginal assertion

Record text

Microsoft MSHTML Remote Code Execution Vulnerability

Inspect raw assertion

Field: container
Value: Microsoft MSHTML Remote Code Execution Vulnerability

Source dateSource date omittedFirst observed by CASCAAug 10, 2026Original evidence ↗

CISA KEVOriginal assertion

Exploitation cataloged

Microsoft MSHTML Remote Code Execution Vulnerability

Inspect raw assertion

Field: observed_exploitation
Value: Microsoft MSHTML Remote Code Execution Vulnerability

Source dateSep 1, 2026First observed by CASCASep 1, 2026Original evidence ↗

FIRST EPSSOriginal assertion

EPSS score

97.84% probability · 99.91th percentile

Inspect raw assertion

Field: model_probability
Value: Probability 0.978400000000; percentile 0.999090000000

Source dateOct 16, 2026First observed by CASCAOct 16, 2026Original evidence ↗

Applicability

Cited product scope

Trace impact →

19Underlying assertions

17Canonical products

19Target assertions

0Constraint assertions

Grouped from 1 configuration nodes in this exact snapshot. Visual grouping is navigational; asset exposure and root cause require cited evidence.

Identity source boundaries

Cpe dictionary1,775,266 records · observed through 2026-07-21T06:45:29.809Z27d65b0f-b718-4b4f-bb79-c47c68d09dfa
Cpe match643,502 records · observed through 2026-07-21T08:13:17.697Z955dae73-7302-438b-aee1-058d7cc5d48e

Search scopeRoleOfficial link state

43 scope groups

microsoft · source assertedMicrosoftWindows 10 Version 1507Direct source scope

Affected: 10.0.10240.0 to before 10.0.10240.19060 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "10.0.10240.0", "lessThan": "10.0.10240.19060", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows 10 Version 1607Direct source scope

Affected: 10.0.14393.0 to before 10.0.14393.4651 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "10.0.14393.0", "lessThan": "10.0.14393.4651", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows 10 Version 1809Direct source scope

Affected: 10.0.17763.0 to before 10.0.17763.2183 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "10.0.17763.0", "lessThan": "10.0.17763.2183", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows 10 Version 1809Direct source scope

Affected: 10.0.0 to before 10.0.17763.2183 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "10.0.0", "lessThan": "10.0.17763.2183", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows 10 Version 1909Direct source scope

Affected: 10.0.0 to before 10.0.18363.1801 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "10.0.0", "lessThan": "10.0.18363.1801", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows 10 Version 2004Direct source scope

Affected: 10.0.0 to before 10.0.19041.1237 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "10.0.0", "lessThan": "10.0.19041.1237", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows 10 Version 20H2Direct source scope

Affected: 10.0.0 to before 10.0.19042.1237 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "10.0.0", "lessThan": "10.0.19042.1237", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows 10 Version 21H1Direct source scope

Affected: 10.0.0 to before 10.0.19043.1237 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "10.0.0", "lessThan": "10.0.19043.1237", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows 7Direct source scope

Affected: 6.1.0 to before 6.1.7601.25712 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "6.1.0", "lessThan": "6.1.7601.25712", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows 7 Service Pack 1Direct source scope

Affected: 6.1.0 to before 6.1.7601.25712 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "6.1.0", "lessThan": "6.1.7601.25712", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows 8.1Direct source scope

Affected: 6.3.0 to before 6.3.9600.20120 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "6.3.0", "lessThan": "6.3.9600.20120", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows Server 2008 R2 Service Pack 1Direct source scope

Affected: 6.1.7601.0 to before 6.1.7601.25712 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "6.1.7601.0", "lessThan": "6.1.7601.25712", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows Server 2008 R2 Service Pack 1 (Server Core installation)Direct source scope

Affected: 6.1.7601.0 to before 6.1.7601.25712 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "6.1.7601.0", "lessThan": "6.1.7601.25712", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows Server 2008 Service Pack 2Direct source scope

Affected: 6.0.6003.0 to before 6.0.6003.21218 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "6.0.6003.0", "lessThan": "6.0.6003.21218", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows Server 2008 Service Pack 2 (Server Core installation)Direct source scope

Affected: 6.0.6003.0 to before 6.0.6003.21218 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "6.0.6003.0", "lessThan": "6.0.6003.21218", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows Server 2012Direct source scope

Affected: 6.2.9200.0 to before 6.2.9200.23462 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "6.2.9200.0", "lessThan": "6.2.9200.23462", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows Server 2012 R2Direct source scope

Affected: 6.3.9600.0 to before 6.3.9600.20120 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "6.3.9600.0", "lessThan": "6.3.9600.20120", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows Server 2012 R2 (Server Core installation)Direct source scope

Affected: 6.3.9600.0 to before 6.3.9600.20120 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "6.3.9600.0", "lessThan": "6.3.9600.20120", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows Server 2012 (Server Core installation)Direct source scope

Affected: 6.2.9200.0 to before 6.2.9200.23462 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "6.2.9200.0", "lessThan": "6.2.9200.23462", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows Server 2016Direct source scope

Affected: 10.0.14393.0 to before 10.0.14393.4651 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "10.0.14393.0", "lessThan": "10.0.14393.4651", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows Server 2016 (Server Core installation)Direct source scope

Affected: 10.0.14393.0 to before 10.0.14393.4651 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "10.0.14393.0", "lessThan": "10.0.14393.4651", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows Server 2019Direct source scope

Affected: 10.0.17763.0 to before 10.0.17763.2183 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "10.0.17763.0", "lessThan": "10.0.17763.2183", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows Server 2019 (Server Core installation)Direct source scope

Affected: 10.0.17763.0 to before 10.0.17763.2183 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "10.0.17763.0", "lessThan": "10.0.17763.2183", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows Server 2022Direct source scope

Affected: 10.0.20348.0 to before 10.0.20348.230 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "10.0.20348.0", "lessThan": "10.0.20348.230", "versionType": "custom"}]

microsoft · source assertedMicrosoftWindows Server version 2004Direct source scope

Affected: 10.0.0 to before 10.0.19041.1237 (custom comparison)

Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion

[{"status": "affected", "version": "10.0.0", "lessThan": "10.0.19041.1237", "versionType": "custom"}]

Affected-product evidence

Accepted scope and product mapping

0 canonical links · 2 source-reported links

Source-reported scopeSource-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Vendor specified only by source · Product specified only by source

Source class: Direct cve affected
Assertions: 26
Mapping revision: cpe23-exact-mapping-v1
Observed cutoff: Oct 22, 2026

Retained assertion IDs

01fb9f20-6147-4c09-b927-ce0060eed4f6055634d0-ee2e-46ca-a733-1bd9dd93ca4408188fe4-2083-47ba-bb9d-71f6efee120c0a320aa3-f1a8-4e63-9497-d70be969238d15edeada-1bf5-4eef-b39b-d1ecca0e2f8e18f3133e-61a4-4dbd-bde2-b80dfdac253726444c44-18c9-4f48-afd8-edd9de15e61928cf8ce9-ba8a-4bcb-8f07-42240924d15b3d392f57-890c-4b5b-929c-ed2891fa3a794a768dd1-ee7d-447a-9cc4-f12d6853a59b4ba3568b-0b58-4740-8210-0225ef875b405be235aa-da71-454b-9cd7-860adefc8b0574b9e4e0-21b5-4248-8143-3fa6830e55807752d422-58ce-4b82-9892-4217e7e1cf9878be611d-cdc1-4ca6-82c4-ead338e119717a13df96-4116-447e-b922-7edc486da7097d95c86f-d3ab-4144-b191-85942558a12b8e799cae-ea3c-46e9-87c0-f1c1dbef7b3e9e700a6a-c61c-437e-8c37-7c8704e916fea865e273-0242-4cfe-9365-a37beb15ca10ae79d58a-48b4-44a5-a05e-aa7f163e5fb0b92f8094-9964-4200-9457-d2e2a1437a57c6ae7079-d566-434e-8aed-2f264f494c8bd43c713f-bf6b-4dd6-912f-b3470dd2f0aadef65bb7-6105-4e49-a7d0-969717c31c80f6b2aa56-0938-49d7-b5d6-9cffc9f8c65d

Source-reported scopeSource-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Vendor specified only by source · Product specified only by source

Source class: Nvd cpe vulnerable target
Assertions: 19
Mapping revision: cpe23-exact-mapping-v1
Observed cutoff: Oct 22, 2026

Retained assertion IDs

0fd0fc4b-6807-4c00-ab59-16d37790828e2483f5b6-5dda-4de5-b834-ca39c69e21e02c595ec9-2455-4e1d-93de-fac90ed415ce2e1b59d3-1196-46d5-8d5f-de51a0f041d54a456918-31ca-457e-98c7-5caa5b989d264d336e9d-c60a-44e6-ad06-9f54d1ea1711558d3d47-f4e2-42ca-a502-cb2146aa5b3f7060313c-3879-4e5c-9816-79ed58d924c975250c25-f02b-4bff-8cd7-0074e46209b47aa40e72-ecb1-4c2b-9b28-6ab9c488c94e8826e07e-645c-4414-85af-0bd405a26f99976119e2-d53c-4d8a-b3ad-abb798b923bba75f24ea-70e6-4c78-8b97-383a9243162bb2d1f9a8-30d9-4c30-ba35-e2afe89cf835cb94a055-d3e9-4a87-bfb9-028c28de7b60d2427f97-cc0c-431f-a634-64c7dcc0d8a7dbd87c00-0a6c-46ae-977b-fa4845092d70f509e9b0-d6f2-40cd-840b-064aa46f68f9fd1ec6a0-2869-46a1-9c3f-17ad10ce438a

Assessments

CVSS by origin

6.8

NVDCVSS 2.0 · role Primary · priority eligiblevalid_matchAV:N/AC:M/Au:N/C:P/I:P/A:P

7.8

NVDCVSS 3.1 · role Secondary · priority eligiblevalid_matchCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

8.8

secure@microsoft.comCVSS 3.1 · role Secondary · priority eligiblevalid_matchCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L

8.8

microsoftCVSS 3.1 · role unknown · display onlyDirect record assessment retained outside normalized eligibilityCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L/E:P/RL:O/RC:C

Direct CVE/CNA normalized decisions

8.8Priority eligible

microsoft

CVSS 3.1 · Primary · Original assertion · rank 1

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L/E:P/RL:O/RC:C

Validation: Valid match
Recomputed: 8.8
Decision reason: Evidence supported
Policy: casca-direct-cvss-eligibility-v1

Assessments are retained side by side under closed precedence. Cascade never averages CVSS.

Evidence boundaries

KEV membership is authoritative for the catalog, not proof of exposure in any environment.
EPSS is a dated model probability and not an individual-environment prediction.
Affected or fixed status applies only to the exact cited product and version scope; remaining scope stays source-scoped.
NVD-carried upstream facts remain derivative; independent corroboration requires a separately authored source.
Only NVD metrics validated under the generation-bound calculator are Public Priority eligible; direct CVE record metrics remain display-only.
NVD CVSS source eligibility is closed: NVD-authored, exact record-source, or registered same-CVE container origin; unmapped sources remain display-only.
Core replay supports the active and immediately prior generation; retrospective valid-at replay is deferred.
OSV aggregation and OSV-converted NVD material are not independent corroboration.
Red Hat facts are vendor assertions for the exact supplied products.
Nonmembership and not-yet-observed states are not proof of safety.