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DEV-0193 infrastructure has also been implicated in attacks deploying novel techniques, including exploitation of CVE-2021-40444.
CVE-2021-40444 KEVRansomwarePublic exploit High · 8.8 EPSS 97.8%
Microsoft MSHTML Remote Code Execution Vulnerability
Microsoft · MSHTML
Summary
The MSHTML browser rendering engine in Microsoft Windows contains a remote code execution flaw, which is hosted by Microsoft Office documents that embed the browser rendering engine. An attacker who can get a user to open a specially crafted Office document containing a malicious ActiveX control can execute code, and the attack requires no prior authentication from the attacker, only that the victim open the document; users with fewer user rights are less affected than those with administrative rights.
Official description · NVD
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.
Priority scorePatchfirst’s 0 to 100 ranking. It adds points for CISA KEV listing, ransomware use, EPSS, threat reports, public exploit code and CVSS. Every CVE page shows the breakdown. Breakdown
86.9 / 100
- Exploited
- Yes, in CISA KEV
- Reports
- 3
- Tactics
- 1/15
How an attack unfolds
Based on MITRE ATT&CKA public catalogue of the techniques attackers use, maintained by MITRE. Patchfirst uses it to describe what attackers do with a vulnerability. mappings. Select a step to see the evidence.Exploitation timelineDated milestones for a CVE: when it was disclosed, when exploit code became public, when exploitation was confirmed, and when threat reports named it.
First seen exploited 8 days before it was disclosed. Public exploit code (PoC-in-GitHub) was available 7 days before disclosure.
- First seen exploitedVulnCheck KEV evidence8 days before
- Public exploit codePoC-in-GitHub7 days before
- Disclosedday 0
- Added to CISA KEVknown ransomware use; EU database agrees+49 days
- First dated threat reportblog.google+6 mo
CISA KEV listing is when exploitation was confirmed, not necessarily the first attack.
MITRE ATT&CKA public catalogue of the techniques attackers use, maintained by MITRE. Patchfirst uses it to describe what attackers do with a vulnerability. techniques
2 TechniqueOne specific attacker behaviour in MITRE ATT&CK, such as T1190 “Exploit Public-Facing Application”. IDs with a dot, like T1059.004, are sub-techniques., grouped by TacticThe attacker’s goal at a stage of an attack in MITRE ATT&CK, such as Initial Access or Persistence. Each technique belongs to one or more tactics. in kill-chain order, 2 cited passages 2ExecutionTA0002
Exploitation for Client Execution T1203 Medium Confidence (technique links)How sure Patchfirst is that an ATT&CK technique applies to this CVE. High and medium links are shown by default; low ones come from early or generic mappings, text matching, or contradict the CVE’s details.
AI mapping from the description · primary impact
Malicious ActiveX control executes via MSHTML
An attacker could craft a malicious ActiveX control to be used by a Microsoft Office document that hosts the browser rendering engine
Malicious File T1204.002 Medium Confidence (technique links)How sure Patchfirst is that an ATT&CK technique applies to this CVE. High and medium links are shown by default; low ones come from early or generic mappings, text matching, or contradict the CVE’s details.
AI mapping from the description · how it is exploited
User opens malicious Office document
The attacker would then have to convince the user to open the malicious document
Detection
2 rules for this CVE · 74 general rules for its techniquesRules for this CVE
2Sigma ruleAn open, vendor-neutral detection rule for log data. Security teams convert Sigma rules into searches for their own SIEM. that reference CVE-2021-40444 directly.General rules for this CVE’s techniques
These detect the technique in general, not this specific vulnerability.
Malicious FileT1204.00239 general rules
- Droppers Exploiting CVE-2017-11882criticalstable
- Ursnif Malware C2 URL Patterncriticalstable
- Exploit for CVE-2017-8759criticaltest
- Potential Maze Ransomware Activitycriticaltest
- File With Uncommon Extension Created By An Office Applicationhightest
- Flash Player Update from Suspicious Locationhightest
- GAC DLL Loaded Via Office Applicationshightest
- HackTool - LittleCorporal Generated Maldoc Injectionhightest
Exploitation for Client ExecutionT120335 general rules
- Antivirus - Exploitation Framework Signaturecriticalstable
- Droppers Exploiting CVE-2017-11882criticalstable
- Audit CVE Eventcriticaltest
- CVE-2021-31979 CVE-2021-33771 Exploitscriticaltest
- CVE-2021-31979 CVE-2021-33771 Exploits by Sourgumcriticaltest
- Exploit for CVE-2017-8759criticaltest
- Antivirus - APT Malware Signaturecriticalexperimental
- Antivirus - Remote Access Tools Signaturecriticalexperimental
Sigma rules © SigmaHQ, Detection Rule License 1.1; linked, not reproduced.
What security researchers reported
Named in 3 threat reports3 reportsMicrosoftGoogletrellix.comMar 2022 – May 2022 · 1 undated
Reports are matched by finding the exact id CVE-2021-40444 in their text; nobody has reviewed the match. One passage is shown per report, at most 320 characters, linked to the original.
Exposing initial access broker with ties to Conti
Earlier reports of attacks exploiting CVE-2021-40444 (by Microsoft and other members of the security community) have also indicated overlaps between domains involved in the delivery chain of an exploit and infrastructure used for BazarLoader and Trickbot distribution.
4 passages name itOriginal (opens in a new tab) ↗
Trellix Threat Report, April 2022
Mentions the CVE; no further detail.
Also mentioned in 50 community threat pulses (AlienVault OTX)
Weaker evidencePulses are indicator collections that anyone can publish. A mention is not confirmation of exploitation.
- Actively_Exploited_Kev | Oct 29, 202628 Oct 2026
- Actively_Exploited_Kev | Oct 28, 202627 Oct 2026
- Actively_Exploited_Kev | Oct 27, 202626 Oct 2026
- Actively_Exploited_Kev | Oct 24, 202623 Oct 2026
- Actively_Exploited_Kev | Oct 23, 202622 Oct 2026
- Actively_Exploited_Kev | Oct 22, 202621 Oct 2026
- 44 more on OTX