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Full vulnerability report · 2021
CVE-2021-40444High confidence

Microsoft MSHTML Remote Code Execution Vulnerability

Microsoft · MSHTML

8.8HighCVSS 3.1
Recommended action
Patch now

CISA confirms exploitation in the wild and lists 2021-11-17 as the remediation due date.

Patch available
R
Operational reassessment

Published severity in operational context

Open reassessment dashboard →
Published severityHighOperational priority:Critical, raised one band.upgradedsince 3 Nov 2021

Evidence used

  • CISA confirms exploitation in the wild.
  • A structured source references public exploit or proof-of-concept material.
  • EPSS is 97.45% for the current model date.

Compensating controls

  • Validate the affected product branch and deploy the verified fixed release.
  • Restrict the affected network interface to trusted sources where business-safe.
  • Increase monitoring for the attack path and post-exploitation behaviour described in the report.

Verification

  1. Confirm that the asset runs Microsoft MSHTML and falls inside the recorded affected range.
  2. Verify the installed build against the product-specific fixed version after deployment.
  3. Validate exposure, authentication requirements and compensating controls in the actual environment.
  4. Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Mitigation target: Within 3 daysRemediation target: Within 90 days

This automated reassessment organises public evidence. It does not know asset exposure, business impact or control effectiveness and does not replace CVSS or a human risk decision.

Optional official sources

National CERT insights
?CERT means Computer Emergency Response Team; CSIRT is the closely related term Computer Security Incident Response Team.

Choose official national sources for this report. Each advisory shows its original language. Your selection is remembered on this device and included in shared links.

Official European source

ENISA European Vulnerability Database

Official EUVD identifiers, advisory evidence and known-exploited context. Missing fields are not treated as evidence of low risk.

1 current
ENISA EUVD identifier

EUVD-2021-27621

CISA KEV mirrored by ENISA

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.

EUVD state
Present in the current official mapping
Known exploitation
Recorded by ENISA since 3 Nov 2021. Evidence sources: cisa_kev.
ENISA score
8.8 · CVSS 3.1
Advisory evidence
1 linked advisory record
Recommended actionPatch now

CISA confirms exploitation in the wild and lists 2021-11-17 as the remediation due date.

Patch available
01

What, why and how

Microsoft MSHTML contains a unspecified vulnerability that allows for remote code execution.

What

Microsoft MSHTML contains a unspecified vulnerability that allows for remote code execution.

Why

Attacker-controlled path data is not constrained to the intended directory before the application accesses a file.

How

An attacker operating through a network path may attempt exploitation without authentication after a user interaction. If successful, the issue may execute code or commands in the affected security context.

What

Microsoft MSHTML contains a unspecified vulnerability that allows for remote code execution.

Why

Attacker-controlled path data is not constrained to the intended directory before the application accesses a file.

How

An attacker operating through a network path may attempt exploitation without authentication after a user interaction. If successful, the issue may execute code or commands in the affected security context.

02

Exploit reality and attack path

CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.

Observed exploitation
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
Confirmed in the wild

CISA added this CVE to its Known Exploited Vulnerabilities catalogue on 2021-11-03. Known ransomware campaign use is recorded.

Public PoC / exploit material
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
Reference recorded

A structured CVE source labels at least one public reference as exploit material. BlackTree has not independently validated that it is safe, reliable or weaponised.

Likely attack path
a network path → Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') → execute code or commands in the affected security context
Attack surface
Network
Privileges required
None: unauthenticated exploitation is possible
User interaction
Required interaction required
Attack complexity
Low: no specialised conditions are recorded
Security boundary
Changed: exploitation can affect a different security authority
Weakness
?CWE means Common Weakness Enumeration: a standard category for the underlying weakness.
CWE-22 ↗

CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal'). The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.

CVSS vector
?CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity.
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

Common Vulnerability Scoring System 3.1: the compact vector below is decoded into plain language.

AVNetworkAttack vector: The vulnerable component can be reached over a network.ACLowAttack complexity: No specialised conditions are required beyond attacker-controlled input.PRNonePrivileges required: The attacker does not need an account or existing privileges.UIRequiredUser interaction: Another user must perform an action for exploitation to succeed.SChangedScope: The attack can affect a component governed by a different security authority.CLowConfidentiality impact: A successful attack can cause a limited loss.IHighIntegrity impact: A successful attack can cause a major loss.ALowAvailability impact: A successful attack can cause a limited loss.
Post-exploitation / living off the land
After compromise, an attacker may use built-in shells, scripting engines, scheduled tasks and native network utilities for discovery, persistence or movement. This is a plausible LoTL path, not evidence that it has occurred for every attack.
NetworkUnauthenticatedRemote code executionCWE-22CISA KEVPublic exploit reference
A

Official authority intelligence

Only matched European and national findings are included. Language selectors and unavailable sources are omitted.

ENISA EUVD · EUVD-2021-27621Known-exploited evidence recorded

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.

Official EUVD record ↗
Canadian Centre for Cyber Security · English · AL21-018Active Exploitation of Microsoft MSHTML Remote Code Execution Vulnerability - update 1

The Cyber Centre is aware of reported ongoing scanning and exploitation of an unpatched remote code execution vulnerability in MSHTML affecting Microsoft Windows [ 1 ]. Microsoft has released guidance to mitigate this vulnerability.

Official advisory ↗
Cyber Security Agency of Singapore · English · CSA-SB-20210922Security Bulletin 22 Sep 2021

The Cyber Security Agency of Singapore included this CVE in its official Security Bulletin 22 Sep 2021, published on 22 September 2021. Open the linked bulletin for the product, severity and reference information published in that issue.

Official advisory ↗
CERT-FR · French · CERTFR-2021-AVI-710Multiples vulnérabilités dans Microsoft Windows

d?id=CVE-2021-38633 Référence CVE CVE-2021-38634 https://www.cve.org/CVERecord?id=CVE-2021-38634 Référence CVE CVE-2021-38635 https://www.cve.org/CVERecord?id=CVE-2021-38635 Référence CVE CVE-2021-38636 https://www.cve.org/CVERecord?id=CVE-2021-38636 Référence CVE CVE-2021-38637 https://www.cve.org/CVERecord?id=CVE-2021-38637 Référence CVE CVE-2021-38638 https://www.cve.org/CVERecord?id=CVE-2021-38638 Référence CVE CVE-2021-38639 https://www.cve.org/CVERecord?id=CVE-2021-38639 Référence CVE CVE-2021-38667 https://www.cve.org/CVERecord?id=CVE-2021-38667 Référence CVE CVE-2021-38671 https://www.cve.org/CVERecord?id=CVE-2021-38671 Référence CVE CVE-2021-40444 https://www.cve.org/CVERecord?id=CVE-2021-40444 Référence CVE CVE-2021-40447 https://www.cve.org/CVERecord?id=CVE-2021-40447 Gestion détaillée du document le 15 septembre 2021 Version initiale Alertes Avis Bulletins d’actualités Mentions légales Conditions générales À propos Contact cyber.gouv.fr service-public.fr legifrance.gouv.fr info.gouv.fr france.fr info.gouv.fr/risques Premier Ministre / Secrétariat Général de la Défense et de la Sécurité Nationale / Agence nationale de la sécurité des systèmes d'information

Official advisory ↗
JVN iPedia · Japanese · JVNDB-2021-002681複数の Microsoft Windows 製品におけるリモートでコードを実行される脆弱性

複数の Microsoft Windows 製品には、Microsoft MSHTML に不備があるため、リモートでコードを実行される脆弱性が存在します。

Official advisory ↗
KISA KrCERT/CC · Korean · KNVD-5439MS MSHTML 보안 업데이트 권고

MSHTML에서 입력값 검증이 미흡하여 발생하는 원격코드실행 취약점(CVE-2021-40444) [3]

Official advisory ↗
KISA KrCERT/CC · Korean · KNVD-5392추석 연휴기간 사이버 공격 대비 보안 권고 사항 (상세)

AD 인프라를 운영 중인 기업의 경우, 관리자 그룹 계정의 최소화 및 관련 PC의 인터넷망 분리 운영 ⑥ 주요 관리자 PC에 대한 주기적인 보안 점검 및 인터넷망 분리 운영 ☞ 특히 아래의 취약점을 이용하여 해킹사고가 발생 할 수 있으니 반드시 보안 업데이트 등 조치 필요 (1) MS 윈도우 Exchange 서버 취약점 보안 업데이트 권고 (CVE-2021-26855, 26857, 27065, 26585, 27065, 31207, 34473, 34523) [6] (2) MS 윈도우 프린터 스풀러 취약점 보안 업데이트 권고 (CVE-2021-34481, 34527, 1657) [7] (3) 아틀라시안 컨플루언스 제품 취약점 보안 업데이트 권고 (CVE-2021-26084) [8] o PC 보안 강화 방안 ① 피싱 메일에 주의하고 본문 링크 클릭, 첨부파일 다운로드, 실행에 주의 ② 매월 운영체제 및 주요 프로그램(웹브라우저, Flash, Java 등)의 보안 업데이트 확인 적용 ③ 상용 메일을 통한 주요 업무 자료 송수신 금지 ※ 불가피한 경우, OTP 설정 및 허가된 사용자 단말기 추가 등을 통해 인증 강화 ☞ 특히 아래의 취약점을 이용하여 감염되는 사례가 발생할 수 있으니 임시 대응 방안 조치 필요 (1) MS MSHTML 취약점 주의 권고 (CVE-2021-40444) [9] ※ `21.9.14 현재 자동 보안패치없음 o NAS 보안 강화 방안 ① 최초 설치 시 기본 관리자 패스워드는 반드시 변경 후 사용 ② 자동 업데이트를 활성화하여 최신 펌웨어 유지 ③ 인터넷을 통한 직접 접속은 차단하고, 사내망에서 운영 권고 ※ 불가피한 경우, 장비의 비밀번호 관리 및 백업, 보안 업데이트 등 철저한 관리 필요 ☞ 특히 아래의 취약점을 이용하여 감염되는 사례가 있으니 반드시 보안 업데이트 등 조치 필요 (1) QNAP社 NAS 제품 보안 업데이트 권고 (CVE-2021-28799) [10] o 공통 보안 강화 방안 ① 사용하지 않는 시스템은 전원을 종료하여 해킹 경로로 활용되는 것을 사전 방지 ② 중요 파일 및 문서 등은 네트워크와 분리된 오프라인 백업 권고 ③ 유추하기 어려운 패스워드(숫자, 대소문자, 특수문자 조합 8자리 이상)사용으로 관리 강화 ④ 사용하지 않는 네트워크 서비스는 비활성화하고, 인가된 관리자만 접속할 수 있도록 방화벽 등에서 접근제어 설정 ⑤ 신뢰할 수 있는 백신을 설치(최신 버전 유지, 실시간 감지 적용 등)하고 정기적으로 검사 진행 o 이상 징후 포착 및 침해사고 발생 시, 한국인터넷진흥원으로 즉시 신고 ※ 'KISA 인터넷보호나라&KrCERT' 홈페이지(www.boho.or.kr) - 상담

Official advisory ↗
KISA KrCERT/CC · Korean · KNVD-5382MS MSHTML 취약점 주의 권고

MSHTML에서 입력값 검증이 미흡하여 발생하는 원격코드실행 취약점(CVE-2021-40444) [1]

Official advisory ↗
03

Patch and workaround

Operational remediation based on structured source evidence.

Status
?Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change.
Patch available
Affected
Fixed
An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
Action
Review the linked authoritative reference and apply the recorded fixed release appropriate to the affected product branch.
Workaround
A mitigation or workaround reference is available from the source linked below; validate it against the affected product and version.
04

Evidence and provenance

Published 15 Sept 2021 · Last source change 10 Aug 2026, 15:10 UTC · CWE-22 · Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

CVE recordCVE.org · 5.2
CVSS sourceCNA
EPSS source
?The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.
FIRST · tracked since 2026-08-14
European sourceENISA EUVD · EUVD-2021-27621
Product sourceCISA KEV
Remediation sourceCVE/CNA references
CWE sourceCISA ADP
NVD statusNVD enriched

Core structured fields are present and their contributing authorities are shown above.

Material change intelligence

What changed after publication

View recent updates ↗
  1. Affected versionsThe structured affected or fixed version information changed.
    Before
    10.0.10240.0 < 10.0.10240.19060; 10.0.14393.0 < 10.0.14393.4651; 10.0.17763.0 < 10.0.17763.2183; 10.0.0 < 10.0.17763.2183; 10.0.0 < 10.0.18363.1801; 10.0.0 < 10.0.19041.1237; 10.0.0 < 10.0.19042.1237; 10.0.0 < 10.0.19043.1237 · Fixed: An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
    After
    Windows 10 Version 1507: 10.0.10240.0 < 10.0.10240.19060; Windows 10 Version 1607: 10.0.14393.0 < 10.0.14393.4651; Windows 10 Version 1809: 10.0.17763.0 < 10.0.17763.2183, 10.0.0 < 10.0.17763.2183; Windows 10 Version 1909: 10.0.0 < 10.0.18363.1801; Windows 10 Version 2004: 10.0.0 < 10.0.19041.1237; Windows 10 Version 20H2: 10.0.0 < 10.0.19042.1237; Windows 10 Version 21H1: 10.0.0 < 10.0.19043.1237; Windows 7: 6.1.0 < 6.1.7601.25712; Windows 7 Service Pack 1: 6.1.0 < 6.1.7601.25712; Windows 8.1: 6.3.0 < 6.3.9600.20120; Windows Server 2008 R2 Service Pack 1: 6.1.7601.0 < 6.1.7601.25712; Windows Server 2008 R2 Service Pack 1 (Server Core installation): 6.1.7601.0 < 6.1.7601.25712; Windows Server 2008 Service Pack 2: 6.0.6003.0 < 6.0.6003.21218; Windows Server 2008 Service Pack 2 (Server Core installation): 6.0.6003.0 < 6.0.6003.21218; Windows Server 2012: 6.2.9200.0 < 6.2.9200.23462; Windows Server 2012 (Server Core installation): 6.2.9200.0 < 6.2.9200.23462; Windows Server 2012 R2: 6.3.9600.0 < 6.3.9600.20120; Windows Server 2012 R2 (Server Core installation): 6.3.9600.0 < 6.3.9600.20120; Windows Server 2016: 10.0.14393.0 < 10.0.14393.4651; Windows Server 2016 (Server Core installation): 10.0.14393.0 < 10.0.14393.4651; Windows Server 2019: 10.0.17763.0 < 10.0.17763.2183; Windows Server 2019 (Server Core installation): 10.0.17763.0 < 10.0.17763.2183; Windows Server 2022: 10.0.20348.0 < 10.0.20348.230; Windows Server version 2004: 10.0.0 < 10.0.19041.1237; Windows Server version 20H2: 10.0.0 < 10.0.19042.1237 · Fixed: An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
    CNA ↗
Material fields only · duplicate refreshes suppressed · history retained for the configured operational retention period
CVE published
Added to CISA KEV
Fixed release or patch reference recorded
ENISA EUVD · EUVD-2021-27621↗CISA KEV record↗CVE authority or vendor reference↗Patch or update reference↗Public exploit reference↗Public exploit reference↗Official CVE record (CNA and ADP data)↗NVD record↗
Technical terms and abbreviations used in this report
CVE
Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
CVSS
Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
EPSS
Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
CWE
Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
CNA
CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
CISA ADP
Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
NVD
National Vulnerability Database: NIST's enrichment service for CVE records.
CERT / CSIRT
A computer security incident response team that publishes warnings or coordinates incident response.
PoC
Proof of concept: public material that demonstrates or helps reproduce exploitation.
CSAF
Common Security Advisory Framework: a machine-readable format for security advisories.
LoTL
Living off the land: abuse of legitimate tools or system functions during an attack.
Free version - for non-commercial use only.CVE-2021-40444 · cve.blacktree.nl