MSHTML Remote Code Execution via Malicious Office Documents
Actively exploited in the wild. Patch or mitigate immediately — don't wait for a maintenance window.
An unauthenticated attacker, remotely over the network, if a user is tricked into opening a malicious file or link, can tamper with data.
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.
Inferred from the weakness type (CWE) and how the flaw is reached — the techniques an attacker would chain to exploit this. Real-world corroboration appears here as threat intel is folded in.
Corroborated threat activity referencing this CVE — with source and confidence, not inference.
Named threat actors and observed ATT&CK techniques appear here as intel feeds are folded in — the ATT&CK chain above stays inferred until then.
Generated by Wavense from the CVE record — an immediate mitigation and the permanent fix.
Until patches are applied, ensure Microsoft Defender Antivirus is updated to detection build 1.349.22.0 or newer to leverage existing protections. Educate users about the risks of opening unsolicited or suspicious Microsoft Office documents from untrusted sources. Additionally, enforcing least privilege for user accounts can limit the potential impact of a successful exploit.
The permanent resolution is to immediately apply the security updates released by Microsoft on September 14, 2021, to address this vulnerability. Consult the official Microsoft Security Update Guide for CVE-2021-40444 to identify and install the applicable updates for all affected Windows systems. Prompt patching is crucial to eliminate the remote code execution risk posed by this flaw.
An attacker can achieve remote code execution on a vulnerable system simply by convincing a user to open a specially crafted Microsoft Office document. This can lead to full system compromise, allowing the attacker to install programs, view, change, or delete data, and create new accounts with full user rights. Such compromise can result in significant data loss, operational disruption, and potential lateral movement within the network.
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Microsoft Windows XP SP3, Windows XP x64 XP2, Windows Server 2003 SP2, Windows Vista, Windows 7 SP1, Windows Server 2008 SP2 and R2 SP1, Windows 8, Windows 8.1 and Windows RT 8.1, Windows Server 2012 and R2, Windows 10 Gold, 1511, 1607, and 1703, and Windows Server 2016 allow an attacker to take control of the affected system when Windows Search fails to handle objects in memory, aka "Windows Search Remote Code Execution Vulnerability".
HTTP.sys in Microsoft Windows 7 SP1, Windows Server 2008 R2 SP1, Windows 8, Windows 8.1, and Windows Server 2012 Gold and R2 allows remote attackers to execute arbitrary code via crafted HTTP requests, aka "HTTP.sys Remote Code Execution Vulnerability."