{"id":"CVE-2021-40444","kind":"vulnerability","aliases":["CVE-2021-40444"],"related":[],"title":"Microsoft MSHTML: Microsoft MSHTML Remote Code Execution Vulnerability","description":"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.","summary":"Microsoft MSHTML, the Windows browser rendering engine, has a remote code execution flaw that is actively exploited in the wild. An attacker who gets a user to open a crafted Office document or web page can run code on the machine.","severity":"critical","cvss":8.8,"epss":0.9784,"exploited":true,"exploited_at":"2021-11-03T00:00:00+00:00","products":[{"name":"Microsoft MSHTML","fixed":[],"affected":"","ecosystem":"vendor"}],"ecosystems":["vendor"],"tags":["exploited","infrastructure","microsoft-office","ransomware","self-hosted","windows"],"not_affected_if":["patched-windows"],"how_to_check":null,"fix":"Apply the Microsoft security update for CVE-2021-40444 per vendor instructions (CISA KEV required action).","status":"open","published_at":"2021-09-15T12:15:16+00:00","first_seen_at":"2026-10-23T19:11:53+00:00","triaged_at":"2026-10-23T21:41:44+00:00","updated_at":"2026-10-23T21:57:34+00:00","category":"infrastructure","confidence":0.6,"triage":{"at":"2026-10-23T21:41:44Z","model":"claude-haiku-5-5","output":{"fix":"Apply the Microsoft security update for CVE-2021-40444 per vendor instructions (CISA KEV required action).","reach":"internet","impact":"high","summary":"Microsoft MSHTML, the Windows browser rendering engine, has a remote code execution flaw that is actively exploited in the wild. An attacker who gets a user to open a crafted Office document or web page can run code on the machine.","category":"infrastructure","who_cares":["windows","microsoft-office","self-hosted"],"confidence":0.6,"needs_auth":"no","worst_case":"Attacker runs code on the Windows machine after the user opens a crafted document","analyst_note":"Find the affected Windows build list and KB article for the August 2021 update at the Microsoft Security Response Center entry for CVE-2021-40444 to fill in how_to_check and exact fixed versions.","how_to_check":"unknown","needs_analyst":true,"not_affected_if":["patched-windows"],"user_interaction":"yes","needs_special_setup":"unknown","severity_if_unknown":"critical"},"cost_usd":0.000467,"input_tokens":2821,"output_tokens":369,"severity_before":"unknown"},"analyst_status":"queued","analyst_note":"Find the affected Windows build list and KB article for the August 2021 update at the Microsoft Security Response Center entry for CVE-2021-40444 to fill in how_to_check and exact fixed versions.","search":"'-2021':10A,13A '-40444':11A,14A '1.349.22.0':212C '14':299C '2021':300C 'account':142C 'across':218C 'action':201C,244C 'activ':42B 'activex':110C 'addit':200C 'address':307C 'administr':162C 'affect':81C 'alert':225C 'antimalwar':185C 'antivirus':167C 'applic':318C,342C 'appropri':243C 'attack':48B,89C,105C,127C 'attempt':91C 'automat':193C 'awar':86C 'browser':31B,123C 'build':211C 'code':6A,37B,64B,75C 'complet':235C 'configur':144C 'control':111C 'convinc':132C 'could':106C,153C 'cpl':231C 'craft':56B,100C,107C 'custom':182C,190C,203C,249C,273C 'cve':9A,12A 'cycl':268C 'date':189C 'defend':166C,170C,222C 'depend':271C 'deploy':216C 'detect':175C,210C 'display':228C 'document':58B,103C,119C,139C 'endpoint':172C,224C 'engin':33B,125C 'enterpris':202C 'environ':220C 'execut':7A,38B,76C,233C 'exploit':17B,43B,93C 'faq':334C 'fewer':147C 'file':232C 'flaw':39B 'get':50B 'help':246C 'host':25B,121C 'immedi':330C 'impact':156C 'import':283C,336C 'includ':252C 'inform':284C,337C 'infrastructur':18B 'instal':327C 'investig':70C,238C 'keep':184C 'known':180C 'less':155C 'machin':67B 'malici':109C,138C 'manag':205C 'may':251C 'microsoft':1A,3A,15A,20B,27B,68C,82C,84C,101C,117C,165C,169C,221C,239C,301C 'microsoft-offic':19B 'mitig':278C 'month':259C 'mshtml':2A,4A,16A,28B,79C 'need':197C,274C 'newer':214C 'offic':21B,57B,102C,118C 'open':54B,136C 'oper':160C 'out-of-cycl':265C 'page':61B 'pleas':275C,310C,331C 'process':261C 'product':186C 'protect':177C,247C,291C 'provid':174C,253C,263C 'ransomwar':22B 'recommend':324C 'releas':260C,303C 'remot':5A,36B,74C 'render':32B,124C 'report':71C 'right':149C,164C 'run':63B 'section':281C 'secur':255C,269C,304C,313C 'see':276C,311C,332C 'select':208C 'self':24B 'self-host':23B 'septemb':298C 'special':99C 'specially-craft':98C 'step':286C 'suspici':230C 'system':152C,293C,322C,345C 'tabl':315C 'take':199C,241C,289C 'target':88C 'updat':194C,206C,256C,270C,297C,305C,314C,319C,329C,340C 'upon':234C 'use':97C,114C 'user':52B,134C,140C,148C,158C,163C 'util':192C 'vulner':8A,77C,95C,181C,296C,309C 'web':60B 'whose':141C 'wild':46B 'window':26B,30B,83C 'workaround':280C 'would':128C","nicknames":[],"mitigations":[],"who_affected":null,"tier":null,"verified":{},"analyst_revisit_at":null,"first_public_at":"2021-09-15T12:15:16+00:00","kev":{"dateAdded":"2021-11-03","dueDate":"2021-11-17","requiredAction":"Apply updates per vendor instructions.","knownRansomwareCampaignUse":"Known"},"cvss_vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L","cvss_by":"secure@microsoft.com","poc_count":0,"sources":[{"source":"kev","external_id":"CVE-2021-40444","url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-40444","severity":null,"published_at":"2021-11-03T00:00:00+00:00","modified_at":null,"first_seen_at":"2026-10-23T19:11:53+00:00","fetched_at":"2026-10-25T15:39:44+00:00","note":null},{"source":"epss","external_id":"CVE-2021-40444","url":"https://api.first.org/data/v1/epss?cve=CVE-2021-40444","severity":null,"published_at":null,"modified_at":null,"first_seen_at":"2026-10-23T19:13:43+00:00","fetched_at":"2026-10-23T19:13:43+00:00","note":null},{"source":"nvd","external_id":"CVE-2021-40444","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-40444","severity":null,"published_at":"2021-09-15T12:15:16+00:00","modified_at":"2026-08-10T20:19:30+00:00","first_seen_at":"2026-10-23T21:57:34+00:00","fetched_at":"2026-10-23T21:57:34+00:00","note":null}],"documents":[],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}