AlertsActive Exploitation of Microsoft MSHTML Remote Code Execution Vulnerability - Update 1

Number: AL21-018 UPDATE 1
Date: 8 September 2021
Updated: 14 September 2021


This Alert is intended for IT professionals and managers of notified organizations.


An Alert is used to raise awareness of a recently identified cyber threat that may impact cyber information assets, and to provide additional detection and mitigation advice to recipients. The Canadian Centre for Cyber Security ("Cyber Centre") is also available to provide additional assistance regarding the content of this Alert to recipients as requested.


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.


On 7 September 2021 Microsoft released a KB article [1] detailing an unpatched remote code execution vulnerability, tracked as CVE-2021-40444, in MSHTML affecting multiple versions of Microsoft Windows. Exploitation of this vulnerability could involve the creation of a malicious ActiveX control to be used by a Microsoft Office document that hosts the browser rendering engine.

Successful exploitation requires user interaction to open a malicious document and then enable editing. By default, Microsoft Office prevents exploitation of this vulnerability by opening documents from the Internet in Protected View or Application Guard for Office.

Microsoft reports that this vulnerability has been publicly disclosed and that it is aware of targeted attempts at exploitation using specially crafted Microsoft Office documents [1][2].


On 14 September 2021, a patch for CVE-2021-40444 was released as part of the September 2021 Security Updates [1].

Microsoft outlines a workaround to mitigate this vulnerability as part of the KB article released on 7 September 2021.

The Cyber Centre encourages organizations to review the Microsoft KB article [1] for detailed information.


Microsoft indicates that both Microsoft Defender Antivirus and Microsoft Defender for Endpoint can detect and protect against CVE-2021-40444 [1].

The Cyber Centre encourages organizations to ensure their anti-virus software is up to date [3].


[1] Windows KB Article – CVE-2021-40444

[2] CISA - Microsoft Releases Mitigations and Workarounds for CVE-2021-40444  

[3] Protect Your Organization From Malware (ITSAP.00.057)


The Canadian Centre for Cyber Security (Cyber Centre) operates as part of the Communications Security Establishment. We are Canada's national authority on cyber security and we lead the government's response to cyber security events. As Canada's national computer security incident response team, the Cyber Centre works in close collaboration with government departments, critical infrastructure, Canadian businesses and international partners to prepare for, respond to, mitigate, and recover from cyber events. We do this by providing authoritative advice and support, and coordinating information sharing and incident response. The Cyber Centre is outward-facing, welcoming partnerships that help build a stronger, more resilient cyber space in Canada.


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