CriticalCVE-2021-33742

    Windows MSHTML Platform Remote Code Execution Vulnerability (CVE-2021-33742)

    Published In CISA KEV since Sources: SingCERT, CVE.org

    What is affected, what to do

    Windows MSHTML Platform Remote Code Execution Vulnerability

    The vulnerability is being actively exploited; treat it as a priority. Update Microsoft Windows 10 Version 1809 to version 10.0.17763.1999, Microsoft Windows Server 2019 to version 10.0.17763.1999, Microsoft Windows 10 Version 1909 to version 10.0.18363.1621. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft Windows 10 Version 1809 lt 10.0.0..10.0.17763.1999
    • Microsoft Windows Server 2019 lt 10.0.0..10.0.17763.1999
    • Microsoft Windows 10 Version 1909 lt 10.0.0..10.0.18363.1621
    • Microsoft Windows 10 Version 21H1 lt 10.0.0..10.0.19043.1052
    • Microsoft Windows 10 Version 2004 lt 10.0.0..10.0.19041.1052
    • Microsoft Windows 10 Version 20H2 lt 10.0.0..10.0.19042.1052
    • Microsoft Windows 10 Version 1507 lt 10.0.0..10.0.10240.18967
    • Microsoft Windows 10 Version 1607 lt 10.0.0..10.0.14393.4467
    • Microsoft Windows Server 2016 lt 10.0.0..10.0.14393.4467
    • Microsoft Windows 7 lt 6.1.0..6.1.7601.25632
    • Microsoft Windows 7 gte 6.1.0..*
    • Microsoft Windows 7 Service Pack 1 lt 6.1.0..6.1.7601.25632
    • Microsoft Windows 7 Service Pack 1 gte 6.1.0..*
    • Microsoft Windows 8.1 lt 6.3.0..6.3.9600.20045
    • Microsoft Windows 8.1 lt 6.3.0..1.0.0.0
    • Microsoft Windows Server 2008 Service Pack 2 lt 6.0.0..6.0.6003.21137
    • Microsoft Windows Server 2008 Service Pack 2 gte 6.0.0..*
    • Microsoft Windows Server 2008 R2 Service Pack 1 lt 6.1.0..6.1.7601.25632
    • Microsoft Windows Server 2008 R2 Service Pack 1 gte 6.1.0..*
    • Microsoft Windows Server 2012 lt 6.2.0..6.2.9200.23372
    • Microsoft Windows Server 2012 lt 6.2.0..1.0.0.0
    • Microsoft Windows Server 2012 R2 lt 6.3.0..6.3.9600.20045
    • Microsoft Windows Server 2012 R2 lt 6.3.0..1.0.0.0
    Fixed in
    • Microsoft Windows 10 Version 1809 10.0.17763.1999
    • Microsoft Windows Server 2019 10.0.17763.1999
    • Microsoft Windows 10 Version 1909 10.0.18363.1621
    • Microsoft Windows 10 Version 21H1 10.0.19043.1052
    • Microsoft Windows 10 Version 2004 10.0.19041.1052
    • Microsoft Windows 10 Version 20H2 10.0.19042.1052
    • Microsoft Windows 10 Version 1507 10.0.10240.18967
    • Microsoft Windows 10 Version 1607 10.0.14393.4467
    • Microsoft Windows Server 2016 10.0.14393.4467
    • Microsoft Windows 7 6.1.7601.25632
    • Microsoft Windows 7 Service Pack 1 6.1.7601.25632
    • Microsoft Windows 8.1 6.3.9600.20045
    • Microsoft Windows 8.1 1.0.0.0
    • Microsoft Windows Server 2008 Service Pack 2 6.0.6003.21137
    • Microsoft Windows Server 2008 R2 Service Pack 1 6.1.7601.25632
    • Microsoft Windows Server 2012 6.2.9200.23372
    • Microsoft Windows Server 2012 1.0.0.0
    • Microsoft Windows Server 2012 R2 6.3.9600.20045
    • Microsoft Windows Server 2012 R2 1.0.0.0

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    7.5

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    7.5

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: SingCERT, CVE.org

    59.4 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.12

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    216.10 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    actively exploitedSource: ANSSI / CERT-FR How we classify
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    7.5 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    7.5
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    59.4 %
    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    99.12
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    CISA assessment (SSVC)
    Exploitation: active · not automatable · Technical impact: total
    Source: CISA Vulnrichment, as of 4 February 2025
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-787

    Response and mitigation

    Why act now?

    Prioritisation rationale

    The flaw gives code execution through the MSHTML rendering engine, which Internet Explorer, Office and any application with an embedded web view all use. An attacker has to lure a user into opening crafted web content or a document; attack complexity is elevated, but no authentication is needed. In the wild it has been used for targeted attacks on individuals via links and lure documents. CISA does not flag ransomware use, but exploitation likelihood is still rated very high. The fix has been available since June 2021, so any affected host points to a basic patch-management gap. Rank it high for exposed user groups (executives, procurement, press office) and for unsupported Windows 7 legacy systems in KRITIS and OT environments.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Check the patch level of all Windows clients and terminal servers and deploy the June 2021 (or later) cumulative security updates. Minimum builds: 1507 → 10.0.10240.18967, 1607 → 10.0.14393.4467, 1809 → 10.0.17763.1999, 1909 → 10.0.18363.1621, 2004/20H2/21H1 → 10.0.19041/19042/19043.1052. KB numbers are listed in the Microsoft advisory; look them up there.
      • Inventory Windows 7 hosts separately (minimum 6.1.7601.25632). Without an ESU licence there is no update. Cut these hosts off from the internet now or restrict them to the line-of-business application only; they typically sit in OT and control-room environments.
      • Pull build data from your inventory tooling (Intune, SCCM, Defender TVM) or run Get-ItemProperty 'HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion' | Select CurrentBuild,UBR and compare against the minimum builds. Treat any gap as an emergency change.
      • On unpatched hosts, stop Internet Explorer 11 and embedded MSHTML use from Office. Block iexplore.exe via AppLocker/WDAC and apply the "Disable Internet Explorer 11" GPO.
      • Triage EDR hits on suspicious child processes of browsers or Office on unpatched hosts first (see Detection). On a positive hit, isolate the endpoint and reset the logged-on user's credentials.

    2. Runbook step 2

      Mitigation layers

      • Endpoint hardening: Set Defender ASR rules to block mode, in particular "Block all Office applications from creating child processes" (D4F940AB-401B-4EFC-AADC-AD5F3C50688A) and "Block execution of potentially obfuscated scripts".
      • Configuration: Disable ActiveX in all internet zones via GPO (Zone 3 "Run ActiveX controls and plug-ins" = Disable). Enforce Protected View for internet-sourced Office documents, and make sure archivers and file shares do not strip Mark-of-the-Web.
      • Mail/proxy: Quarantine .html, .htm, .mht, .mhtml and .hta attachments, plus Office documents with external OLE or template references, at the mail gateway. Block uncategorised and newly registered domains at the web proxy.
      • Network segmentation: Move legacy hosts (Windows 7, frozen builds) into dedicated VLANs and allow outbound HTTP/HTTPS only through an allow-list proxy. Drop direct internet egress from these segments at the firewall.
      • IAM/least privilege: Run end users without local admin rights (use LAPS for local admin accounts), so that successful user-context code execution cannot immediately escalate.

    3. Runbook step 3

      Detection rules

      • Sysmon EID 1 / Defender DeviceProcessEvents: iexplore.exe, winword.exe, excel.exe, outlook.exe or mshta.exe spawning cmd.exe, powershell.exe, rundll32.exe, regsvr32.exe or wscript.exe. KQL: DeviceProcessEvents | where InitiatingProcessFileName in~ ("iexplore.exe","winword.exe","excel.exe","mshta.exe") and FileName in~ ("cmd.exe","powershell.exe","rundll32.exe","regsvr32.exe")
      • Sysmon EID 7 (ImageLoad): Office processes loading mshtml.dll or jscript9.dll on hosts below the minimum build. Sigma shape: ImageLoaded|endswith: '\mshtml.dll' AND Image|endswith: ['\winword.exe','\excel.exe','\powerpnt.exe'].
      • Sysmon EID 11 (FileCreate): New .dll, .exe or .cpl files in %LOCALAPPDATA%\Microsoft\Windows\INetCache\ or %TEMP% shortly after a browser or Office launch. Correlate with a subsequent process start from the same path.
      • Defender/ASR events: Collect Event ID 1121 (block) and 1122 (audit) from Microsoft-Windows-Windows Defender/Operational. Spikes following phishing waves point to exploitation attempts.
      • Proxy/Zeek: HTTP requests with MSIE or Trident/7.0 user agents from clients that policy says should no longer use IE, and .cab or .inf downloads immediately after an HTML page load.

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    Windows MSHTML Platform Remote Code Execution Vulnerability

    Source: CSA/SingCERT, NVD (NIST), cvelistv5, CISA KEV Catalog, CVE.org (MITRE)

    Vendors and products

    Affected operating systems

    Affected operating systems
    WindowsMicrosoftWindows 10 Version 150710.0.10240.18967
    WindowsMicrosoftWindows 10 Version 160710.0.14393.4467
    WindowsMicrosoftWindows 10 Version 180910.0.17763.1999
    WindowsMicrosoftWindows 10 Version 190910.0.18363.1621
    WindowsMicrosoftWindows 10 Version 200410.0.19041.1052
    WindowsMicrosoftWindows 10 Version 20H210.0.19042.1052
    WindowsMicrosoftWindows 10 Version 21H110.0.19043.1052
    WindowsMicrosoftWindows 76.1.7601.25632
    WindowsMicrosoftWindows 7 Service Pack 16.1.7601.25632
    WindowsMicrosoftWindows 8.16.3.9600.20045
    WindowsMicrosoftWindows Server 2008 R2 Service Pack 16.1.7601.25632
    WindowsMicrosoftWindows Server 2008 Service Pack 26.0.6003.21137
    WindowsMicrosoftWindows Server 20126.2.9200.23372
    WindowsMicrosoftWindows Server 2012 R26.3.9600.20045
    WindowsMicrosoftWindows Server 201610.0.14393.4467
    WindowsMicrosoftWindows Server 201910.0.17763.1999
    Windowsmicrosoftwindows_10_150710.0.10240.18967
    Windowsmicrosoftwindows_10_1507—
    Windowsmicrosoftwindows_10_160710.0.14393.4467
    Windowsmicrosoftwindows_10_1607—
    Windowsmicrosoftwindows_10_180910.0.17763.1999
    Windowsmicrosoftwindows_10_180910.0.18363.1621
    Windowsmicrosoftwindows_10_180910.0.19041.1052
    Windowsmicrosoftwindows_10_1809—

    Weakness classes (CWE)

    • CWE-787Out-of-bounds Write

      Base

      The product writes data past the end, or before the beginning, of the intended buffer.

      Entry at MITRE CWE-787

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited
    ANSSI / CERT-FRLicence Ouverte 2.0, as of CERTFR-2021-ALE-012 – Multiples vulnérabilités dans Microsoft Windows (clôturée 2021-10-19) (opens in a new tab)cert.cyber.gouv.fractively exploited

    References & sources

    • portal.msrc.microsoft.comen-US/security-guidance/advisory/CVE-2021-33742x_refsource_MISC, Patch, Vendor advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-33742government-resource, US government resource
    • cert.cyber.gouv.frANSSI / CERT-FR: CERTFR-2021-ALE-012 – Multiples vulnérabilités dans Microsoft Windows (clôturée 2021-10-19)cert-fr
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    <a href="https://intel.neosec.eu/en/cve/CVE-2021-33742"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-33742.svg?lang=en" alt="CVE-2021-33742 – NEOSEC Intel" height="20"></a>