{"id":"CVE-2021-40444","published":"2021-09-15T12:15:16Z","modified":"2026-08-10T20:19:30Z","title":"Microsoft MSHTML Remote Code Execution Vulnerability","vendor":"Microsoft","product":"MSHTML","tier":"core","eol":null,"description":{"text":"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.\nAn 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.\nMicrosoft 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”.\nUpon 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.\nPlease see the Mitigations and Workaround sections for important information about steps you can take to protect your system from this vulnerability.\nUPDATE 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.","provenance":"source","from":"NVD"},"summary":{"text":"The MSHTML browser rendering engine in Microsoft Windows contains a remote code execution flaw, which is hosted by Microsoft Office documents that embed the browser rendering engine. An attacker who can get a user to open a specially crafted Office document containing a malicious ActiveX control can execute code, and the attack requires no prior authentication from the attacker, only that the victim open the document; users with fewer user rights are less affected than those with administrative rights.","provenance":"ai","model":"claude-haiku-5-5","generatedAt":"2026-10-29T07:53:13Z"},"cvss":{"score":8.8,"severity":"HIGH","version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L","provenance":"source","from":"NVD"},"epss":{"score":0.9784,"percentile":0.99907,"date":"2026-10-30","provenance":"source"},"kev":{"dateAdded":"2021-11-03","dueDate":"2021-11-17","ransomware":true,"requiredAction":"Apply updates per vendor instructions.","notes":"https://nvd.nist.gov/vuln/detail/CVE-2021-40444","provenance":"source"},"vulncheckKev":{"dateAdded":"2021-09-07","exploitedFirstSeen":"2021-09-07","ransomware":true,"evidence":[{"url":"https://api.msrc.microsoft.com/cvrf/v3.0/cvrf/2021-Sep","date":"2021-09-07"},{"url":"https://www.fortinet.com/blog/threat-research/microsoft-mshtml-remote-code-execution-vulnerability-exploited-in-wild-cve-2021-40444","date":"2021-09-09"},{"url":"https://docs.google.com/spreadsheets/d/1lkNJ0uQwbeC1ZTRrxdtuPLCIl7mlUreoKfSIgajnSyY/edit","date":"2021-09-14"},{"url":"https://www.microsoft.com/en-us/security/blog/2021/09/15/analyzing-attacks-that-exploit-the-mshtml-cve-2021-40444-vulnerability/","date":"2021-09-15"},{"url":"https://www.microsoft.com/security/blog/2021/09/15/analyzing-attacks-that-exploit-the-mshtml-cve-2021-40444-vulnerability/","date":"2021-09-15"},{"url":"https://www.riskiq.com/blog/external-threat-management/wizard-spider-windows-0day-exploit/","date":"2021-09-15"},{"url":"https://www.cisa.gov/sites/default/files/feeds/known_exploited_vulnerabilities.json","date":"2021-11-03"},{"url":"https://www.trellix.com/en-gb/about/newsroom/stories/research/prime-ministers-office-compromised.html","date":"2022-01-25"},{"url":"https://cybersecurityworks.com/howdymanage/uploads/file/ransomware-_-2022-spotlight-report_compressed.pdf","date":"2022-01-26"},{"url":"https://www.prevailion.com/what-wicked-webs-we-unweave/","date":"2022-03-15"}],"evidenceTotal":41,"provenance":"source"},"exploited":true,"otx":{"pulseCount":50,"pulses":[{"name":"Actively_Exploited_Kev | Oct 29, 2026","url":"https://otx.alienvault.com/pulse/6ae20c1db17e7057ae7d9ad1","created":"2026-10-28"},{"name":"Actively_Exploited_Kev | Oct 28, 2026","url":"https://otx.alienvault.com/pulse/6ae0ba7e0e2a97ff7b218819","created":"2026-10-27"},{"name":"Actively_Exploited_Kev | Oct 27, 2026","url":"https://otx.alienvault.com/pulse/6adf68b85c3e852f3c111d18","created":"2026-10-26"},{"name":"Actively_Exploited_Kev | Oct 24, 2026","url":"https://otx.alienvault.com/pulse/6adb740e397c3ad4799aa4cd","created":"2026-10-23"},{"name":"Actively_Exploited_Kev | Oct 23, 2026","url":"https://otx.alienvault.com/pulse/6ada236fcfba2dfa730ea482","created":"2026-10-22"},{"name":"Actively_Exploited_Kev | Oct 22, 2026","url":"https://otx.alienvault.com/pulse/6ad8d33a077cbb5cab524d80","created":"2026-10-21"},{"name":"Actively_Exploited_Kev | Oct 21, 2026","url":"https://otx.alienvault.com/pulse/6ad78130c4d1ac2be83a8b6d","created":"2026-10-20"},{"name":"Actively_Exploited_Kev | Oct 20, 2026","url":"https://otx.alienvault.com/pulse/6ad62f63c24a405a3423cc13","created":"2026-10-19"},{"name":"Actively_Exploited_Kev | Oct 19, 2026","url":"https://otx.alienvault.com/pulse/6ad4dda174e5797d0bd238b7","created":"2026-10-18"},{"name":"Actively_Exploited_Kev | Oct 18, 2026","url":"https://otx.alienvault.com/pulse/6ad38c22e1848f8189d1c121","created":"2026-10-17"}],"provenance":"machine"},"cwes":[{"id":"CWE-22","name":"Improper Limitation of a Pathname to a Restricted Directory"}],"references":[{"url":"https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-40444","tags":["Patch","Vendor Advisory"]},{"url":"http://packetstormsecurity.com/files/164210/Microsoft-Windows-MSHTML-Overview.html","tags":["Third Party Advisory","VDB Entry"]},{"url":"http://packetstormsecurity.com/files/165214/Microsoft-Office-Word-MSHTML-Remote-Code-Execution.html","tags":["Exploit","Third Party Advisory","VDB Entry"]},{"url":"http://packetstormsecurity.com/files/167317/Microsoft-Office-MSDT-Follina-Proof-Of-Concept.html","tags":["Exploit","Third Party Advisory","VDB Entry"]},{"url":"https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-40444","tags":["Mitigation","Patch","Vendor Advisory"]},{"url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-40444","tags":["US Government Resource"]}],"techniques":[{"id":"T1204.002","name":"Malicious File","tactics":["execution"],"provenance":"ai","basis":"ai-mapping","bases":["ai-mapping"],"basisDetail":"claude-haiku-5-5: User opens malicious Office document","mappingType":"exploitation_technique","evidence":[{"from":"description","quote":"The attacker would then have to convince the user to open the malicious document","start":390,"end":470}],"model":"claude-haiku-5-5","confidence":"medium","confidenceNote":"AI-mapped from the description (quote-validated, not expert-reviewed)"},{"id":"T1203","name":"Exploitation for Client Execution","tactics":["execution"],"provenance":"ai","basis":"ai-mapping","bases":["ai-mapping"],"basisDetail":"claude-haiku-5-5: Malicious ActiveX control executes via MSHTML","mappingType":"primary_impact","evidence":[{"from":"description","quote":"An attacker could craft a malicious ActiveX control to be used by a Microsoft Office document that hosts the browser rendering engine","start":255,"end":388}],"model":"claude-haiku-5-5","confidence":"medium","confidenceNote":"AI-mapped from the description (quote-validated, not expert-reviewed)"}],"reports":[{"id":"SRC-f144807c3c05fbc98a1a","title":"Ransomware as a service: Understanding the cybercrime gig economy and how to protect yourself","publisher":"microsoft.com","url":"https://www.microsoft.com/en-us/security/blog/2022/05/09/ransomware-as-a-service-understanding-the-cybercrime-gig-economy-and-how-to-protect-yourself/","date":"2022-05-09","mentions":[{"quote":"DEV-0193 infrastructure has also been implicated in attacks deploying novel techniques, including exploitation of CVE-2021-40444.","start":26963,"end":27092}]},{"id":"SRC-e9f330cbfcc2f8587bc7","title":"Exposing initial access broker with ties to Conti","publisher":"blog.google","url":"https://blog.google/threat-analysis-group/exposing-initial-access-broker-ties-conti/","date":"2022-03-17","mentions":[{"quote":"In early September 2021, Threat Analysis Group (TAG) observed a financially motivated threat actor we refer to as EXOTIC LILY, exploiting a 0day in Microsoft MSHTML (CVE-2021-40444).","start":262,"end":444},{"quote":"Although the group came to our attention initially due to its use of documents containing an exploit for CVE-2021-40444, they later switched to the delivery of ISO files with hidden BazarLoader DLLs and LNK shortcuts.","start":4925,"end":5142},{"quote":"In March, the group continued delivering ISO files, but with a DLL containing a custom loader which is a more advanced variant of a first-stage payload previously seen during CVE-2021-40444 exploitation.","start":5555,"end":5758},{"quote":"Earlier reports of attacks exploiting CVE-2021-40444 (by Microsoft and other members of the security community) have also indicated overlaps between domains involved in the delivery chain of an exploit and infrastructure used for BazarLoader and Trickbot distribution.","start":6465,"end":6733}]},{"id":"SRC-9b3493e7c1602964294c","title":"Trellix Threat Report, April 2022","publisher":"trellix.com","url":"https://www.trellix.com/assets/threat-reports/trellix-threat-report-apr-2022.pdf","date":null,"mentions":[{"quote":"vulnerability (CVE-2021-40444).","start":8998,"end":9029},{"quote":"CVE-\r\n2021-40444","start":9812,"end":9828}]}],"affected":[],"advisories":[],"euvd":{"id":"EUVD-2021-27621","url":"https://euvd.enisa.europa.eu/vulnerability/EUVD-2021-27621","exploited":true,"exploitedSince":"2021-11-03","provenance":"source"},"exploits":[],"exploitSources":{"pocGithub":{"count":26,"skipped":9,"earliest":"2021-09-08","earliestUrl":"https://github.com/ozergoker/CVE-2021-40444","repos":[{"name":"lockedbyte/CVE-2021-40444","url":"https://github.com/lockedbyte/CVE-2021-40444","stars":1851,"created":"2021-09-10"},{"name":"klezVirus/CVE-2021-40444","url":"https://github.com/klezVirus/CVE-2021-40444","stars":838,"created":"2021-09-15"},{"name":"aslitsecurity/CVE-2021-40444_builders","url":"https://github.com/aslitsecurity/CVE-2021-40444_builders","stars":175,"created":"2021-09-12"},{"name":"Edubr2020/CVE-2021-40444--CABless","url":"https://github.com/Edubr2020/CVE-2021-40444--CABless","stars":105,"created":"2021-09-19"},{"name":"34zY/Microsoft-Office-Word-MSHTML-Remote-Code-Execution-Exploit","url":"https://github.com/34zY/Microsoft-Office-Word-MSHTML-Remote-Code-Execution-Exploit","stars":65,"created":"2021-12-19"}]},"metasploit":[{"name":"Microsoft Office Word Malicious MSHTML RCE","module":"exploit/windows/fileformat/word_mshtml_rce","type":"exploit","disclosureDate":"2021-09-23","url":"https://github.com/rapid7/metasploit-framework/blob/master/modules/exploits/windows/fileformat/word_mshtml_rce.rb"}],"first":{"date":"2021-09-08","source":"PoC-in-GitHub","url":"https://github.com/ozergoker/CVE-2021-40444"},"provenance":"source"},"detections":{"cveRules":[{"title":"Potential CVE-2021-40444 Exploitation Attempt","id":"894397c6-da03-425c-a589-3d09e7d1f750","level":"high","status":"test","path":"rules-emerging-threats/2021/Exploits/CVE-2021-40444/proc_creation_win_exploit_cve_2021_40444.yml","url":"https://github.com/SigmaHQ/sigma/blob/master/rules-emerging-threats/2021/Exploits/CVE-2021-40444/proc_creation_win_exploit_cve_2021_40444.yml","techniques":["T1059"]},{"title":"Potential Exploitation Attempt From Office Application","id":"868955d9-697e-45d4-a3da-360cefd7c216","level":"high","status":"test","path":"rules-emerging-threats/2021/Exploits/CVE-2021-40444/proc_creation_win_exploit_cve_2021_40444_office_directory_traversal.yml","url":"https://github.com/SigmaHQ/sigma/blob/master/rules-emerging-threats/2021/Exploits/CVE-2021-40444/proc_creation_win_exploit_cve_2021_40444_office_directory_traversal.yml","techniques":[]}]},"exploitRefs":["http://packetstormsecurity.com/files/165214/Microsoft-Office-Word-MSHTML-Remote-Code-Execution.html","http://packetstormsecurity.com/files/167317/Microsoft-Office-MSDT-Follina-Proof-Of-Concept.html","http://packetstormsecurity.com/files/164210/Microsoft-Windows-MSHTML-Overview.html"],"flow":[{"stage":"exploitation","techniques":["T1204.002"],"provenance":"ai"},{"stage":"primary","techniques":["T1203"],"provenance":"ai"}],"timeline":[{"date":"2021-09-07","kind":"exploited-reported","label":"Exploitation first reported (VulnCheck KEV evidence)","url":"https://api.msrc.microsoft.com/cvrf/v3.0/cvrf/2021-Sep","provenance":"source"},{"date":"2021-09-08","kind":"exploit-public","label":"First public exploit: PoC-in-GitHub; later: Metasploit 2021-09-23","url":"https://github.com/ozergoker/CVE-2021-40444","provenance":"source"},{"date":"2021-09-15","kind":"published","label":"Published (NVD)","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-40444","provenance":"source"},{"date":"2021-11-03","kind":"kev","label":"Added to CISA KEV (known ransomware use)","url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog","provenance":"source"},{"date":"2021-11-03","kind":"euvd-exploited","label":"ENISA EUVD: exploited","url":"https://euvd.enisa.europa.eu/vulnerability/EUVD-2021-27621","provenance":"source"},{"date":"2022-03-17","kind":"report","label":"Named in blog.google: Exposing initial access broker with ties to Conti","url":"https://blog.google/threat-analysis-group/exposing-initial-access-broker-ties-conti/","provenance":"machine"},{"date":"2022-05-09","kind":"report","label":"Named in microsoft.com: Ransomware as a service: Understanding the cybercrime gig economy and how to protect yourself","url":"https://www.microsoft.com/en-us/security/blog/2022/05/09/ransomware-as-a-service-understanding-the-cybercrime-gig-economy-and-how-to-protect-yourself/","provenance":"machine"}],"speed":{"daysToPublicExploit":null,"firstExploitFrom":"PoC-in-GitHub","daysToKev":49},"priority":{"score":86.9,"factors":[{"name":"In a KEV catalog","points":35.0,"detail":"listed in CISA KEV and VulnCheck KEV"},{"name":"Recent KEV listing","points":0.0,"detail":"added 1880 days ago"},{"name":"Known ransomware use","points":10.0,"detail":"CISA KEV and VulnCheck KEV: Known"},{"name":"EPSS percentile","points":19.98,"detail":"percentile 0.9991"},{"name":"Named in threat reports","points":7.74,"detail":"3 reports"},{"name":"Public exploit code","points":5.0,"detail":"2 NVD references tagged Exploit; 1 NVD exploit link; 26 PoC-in-GitHub repos; 1 Metasploit module"},{"name":"Mentioned in OTX pulses","points":3.0,"detail":"50 pulses"},{"name":"CVSS severity","points":6.16,"detail":"base score 8.8"},{"name":"End-of-life product","points":0.0,"detail":"supported or unknown"}]}}