Microsoft MSHTML Remote Code Execution Vulnerability

Exploitation signals 5 / 5

• KEV catalog (CISA)03 Nov 2021
• Exploited in EUVD (ENISA)03 Nov 2021
• Used by ransomwareyes
• EPSS above 10 %97.8%
• No account, over the networkyes

What to do

Description ENISA EUVD

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.

Affected products

• 6.1.7601.0 <6.1.7601.25712
• 6.3.0 <6.3.9600.20120
• 10.0.17763.0 <10.0.17763.2183
• 10.0.0 <10.0.19042.1237
• 6.2.9200.0 <6.2.9200.23462
• 10.0.0 <10.0.19041.1237
• 6.3.9600.0 <6.3.9600.20120
• 6.0.6003.0 <6.0.6003.21218
• 10.0.0 <10.0.19043.1237
• 10.0.14393.0 <10.0.14393.4651
• 10.0.20348.0 <10.0.20348.230
• 10.0.10240.0 <10.0.10240.19060

Open source packages OSV

Affected libraries according to OSV, with fixed versions.

Timeline

1. 15 Sept 2021

   Published

2. 03 Nov 2021

   Exploitation reported (ENISA)

3. 03 Nov 2021

   Added to the KEV catalog

4. 17 Nov 2021

   CISA remediation due date

5. 14 Oct 2026

   EPSS peak: 97.8%

References

• cve.org · CVE-2021-40444
• ENISA EUVD · EUVD-2021-27621
• CISA KEV
• FIRST EPSS
• msrc.microsoft.com

Other exploited vulnerabilities from the same vendor

• CVE-2026-96951Microsoft Windows Command Injection Vulnerability
  5.6 · Actively exploited
• CVE-2026-65660Microsoft SharePoint Code Injection Vulnerability
  5.0 · Actively exploited
• CVE-2026-85880Microsoft Windows Heap-Based Buffer Overflow Vulnerability
  4.3 · Actively exploited
• CVE-2026-81963Microsoft Windows Link Following Vulnerability
  4.3 · Actively exploited
• CVE-2019-1068Microsoft SQL Server Remote Code Execution Vulnerability
  7.0 · Actively exploited
• CVE-2026-69836Microsoft Entra
  5.3 · Actively exploited