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PatchSiren cyber security CVE debrief

CVE-2021-40444 Microsoft CVE debrief

CVE-2021-40444 is a Microsoft MSHTML remote code execution vulnerability that CISA added to its Known Exploited Vulnerabilities catalog on 2021-11-03. Because it is listed as actively exploited and marked as known ransomware campaign use, it should be treated as an urgent remediation item. Follow vendor guidance and prioritize affected Microsoft systems immediately.

Vendor
Microsoft
Product
MSHTML
CVSS
Unknown
CISA KEV
Listed
Original CVE published
2021-11-03
Original CVE updated
2021-11-03
Advisory published
2021-11-03
Advisory updated
2021-11-03

Who should care

Security teams, IT operations, and endpoint administrators responsible for Microsoft Windows and applications that rely on MSHTML. Organizations that track CISA KEV items or have exposure to internet-facing user endpoints should prioritize this CVE first.

Technical summary

The vulnerability is described by Microsoft and CISA as a Microsoft MSHTML remote code execution issue. The supplied sources do not include deeper technical detail, so the safest evidence-based summary is that successful exploitation could allow an attacker to execute code through MSHTML. CISA’s KEV listing indicates the issue was being actively exploited in the wild.

Defensive priority

Critical. This CVE is in CISA’s Known Exploited Vulnerabilities catalog and has known ransomware campaign use, with a remediation due date of 2021-11-17 in the supplied timeline.

Recommended defensive actions

  • Apply Microsoft updates per vendor instructions as soon as possible.
  • Prioritize assets that use or expose MSHTML-related functionality.
  • Treat this CVE as an active-exploitation response item rather than a routine patch.
  • Verify remediation status across endpoints and servers, and close gaps before the KEV due date.
  • Use the CISA KEV catalog and vendor guidance to drive internal escalation and exception handling.

Evidence notes

Evidence is limited to the supplied CVE record and CISA KEV metadata. The CVE title/description identify Microsoft MSHTML remote code execution. The CISA KEV source marks the vulnerability as known exploited, with dateAdded 2021-11-03, dueDate 2021-11-17, and knownRansomwareCampaignUse set to Known. No additional technical details were inferred beyond those sources.

Official resources

CVE published and modified on 2021-11-03. CISA KEV dateAdded is 2021-11-03 and dueDate is 2021-11-17. The timeline dates in this debrief reflect the supplied CVE and source metadata, not publication or review timing of this summary.