Background
On 7th September 2021, Microsoft released an advisory for CVE-2021-40444. The CVE has a CVSS score of 8.8.
What is the issue?
Microsoft MSHTML Remote Code Execution Vulnerability (CVE-2021-40444)
Microsoft has released an advisory for CVE-2021-40444, a Remote Code Execution Vulnerability in MSHTML that affects Microsoft Windows. The exploit for this vulnberability has been detected in the wild.
Systems impacted
- Windows 7 for x64-based Systems Service Pack 1
- Windows 7 for 32-bit Systems Service Pack 1
- Windows Server 2012 R2 (Server Core installation)
- Windows Server 2012 R2
- Windows Server 2012 (Server Core installation)
- Windows Server 2012
- Windows Server 2008 R2 for x64-based Systems Service Pack 1 (Server Core installation)
- Windows Server 2008 R2 for x64-based Systems Service Pack 1
- Windows Server 2008 for x64-based Systems Service Pack 2 (Server Core installation)
- Windows Server 2008 for x64-based Systems Service Pack 2
- Windows Server 2008 for 32-bit Systems Service Pack 2 (Server Core installation)
- Windows Server 2008 for 32-bit Systems Service Pack 2
- Windows RT 8.1
- Windows 8.1 for x64-based systems
- Windows 8.1 for 32-bit systems
- Windows Server 2016 (Server Core installation)
- Windows Server 2016
- Windows 10 Version 1607 for x64-based Systems
- Windows 10 Version 1607 for 32-bit Systems
- Windows 10 for x64-based Systems
- Windows 10 for 32-bit Systems
- Windows Server, version 20H2 (Server Core Installation)
- Windows 10 Version 20H2 for ARM64-based Systems
- Windows 10 Version 20H2 for 32-bit Systems
- Windows 10 Version 20H2 for x64-based Systems
- Windows Server, version 2004 (Server Core installation)
- Windows 10 Version 2004 for x64-based Systems
- Windows 10 Version 2004 for ARM64-based Systems
- Windows 10 Version 2004 for 32-bit Systems
- Windows Server 2022 (Server Core installation)
- Windows Server 2022
- Windows 10 Version 21H1 for 32-bit Systems
- Windows 10 Version 21H1 for ARM64-based Systems
- Windows 10 Version 21H1 for x64-based Systems
- Windows 10 Version 1909 for ARM64-based Systems
- Windows 10 Version 1909 for x64-based Systems
- Windows 10 Version 1909 for 32-bit Systems
- Windows Server 2019 (Server Core installation)
- Windows Server 2019
- Windows 10 Version 1809 for ARM64-based Systems
- Windows 10 Version 1809 for x64-based Systems
- Windows 10 Version 1809 for 32-bit Systems
What can you do to protect yourself?
Microsoft has suggested a workaround of disabling installation of all ActiveX controls in Internet Explorer which will allow previously installed ActiveX controls to run but will not expose this vulnerability. By default, Microsoft Office opens documents from the internet in Protected View or Application Guard for Office. These safeguards will prevent the current exploitation of CVE-2021-40444.
Zscaler coverage
Zscaler ThreatLabZ has added detection signatures for exploitation of this vulnerability through our Advanced Threat Protection and Advanced Cloud Sandbox Protection.
- Advanced Cloud Sandbox Signature
DOC.Exploit.CVE-2021-40444 - Advanced Threat Protection Signature
HTML.Exploit.CVE-2021-40444
Details related to these signatures can be found in the Zscaler Threat Library.
Reference
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-40444
https://us-cert.cisa.gov/ncas/current-activity/2021/09/07/microsoft-releases-mitigations-and-workarounds-cve-2021-40444
