Assessment
Why it matters
CISA lists this vulnerability as known to be exploited; exploitation is confirmed; EPSS is 97.5%; technical severity is CVSS 8.8.
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.
Who should care
- Teams operating MSHTML, Windows 10 1507, Windows 10 1607 +15 more
- Vulnerability and exposure management teams
- Security operations teams monitoring exploitation activity
Response plan
What I would do
- Confirm whether MSHTML, Windows 10 1507, Windows 10 1607 +15 more is present in the environment.
- Identify affected versions and establish whether vulnerable services are exposed or reachable.
- Apply updates per vendor instructions.
- Review relevant security telemetry for evidence of attempted or successful exploitation.
- Document the remediation decision and track it to verified completion.
Vendor remediation
Patch and remediation links
Use the vendor source below to confirm the correct fixed version, package or mitigation for your affected product.
Technical details
- CVSS vector
- CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L
- CWE
- CWE-22
- Affected versions
- Microsoft MSHTML; Microsoft Windows 10 1507 (End Excluding: 10.0.10240.19060); Microsoft Windows 10 1607 (End Excluding: 10.0.14393.4651); Microsoft Windows 10 1809 (End Excluding: 10.0.17763.2183); Microsoft Windows 10 1909 (End Excluding: 10.0.18363.1801); Microsoft Windows 10 2004 (End Excluding: 10.0.19041.1237); Microsoft Windows 10 20h2 (End Excluding: 10.0.19042.1237); Microsoft Windows 10 21h1 (End Excluding: 10.0.19043.1237)
- Published
- 15 September 2021
- Attack vector
- Network
- Privileges required
- None
- User interaction
- Required
Signal timeline
- CVE published
- Added to CISA KEV
- Latest EPSS score: 97.5%
- Signal calculated at 87