CVE-2021-40444: Microsoft MSHTML Remote Code Execution Vulnerability
Noroxi assessment
Priority: Now (action score 94/100)
- CISA has confirmed exploitation in the wild (Nov 3, 2021); it is known to be used in ransomware campaigns. Remediation deadline for US federal agencies: Nov 17, 2021.
- A ready-made attack module is public (Metasploit); exploiting it takes no special expertise.
- Probability of exploitation in the next 30 days: 97.8% (FIRST EPSS); higher than 99.9% of all records.
- Anyone who can reach it over the internet can trigger it. No account or password is required. A user must open a link or visit a page. No special conditions are required; it is repeatable.
- Fixed version: python-oletools (openSUSE:Tumbleweed) 0.60.2-2.4.
- Mentioned 6 times in Turkish national cyber security advisories (siberguvenlik.gov.tr).
Patch today or shut off access. Upgrade to python-oletools (openSUSE:Tumbleweed) 0.60.2-2.4 or later.
Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it
Primary sources: NVD · CVE.org · CISA KEV · FIRST EPSS
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.
- Published
- Sep 15, 2021
- Updated
- Aug 10, 2026
- EPSS
- 97.8% · 99th percentile
- CWE
- CWE-22
Report tools
Action score
94
Now
Patch today or shut off access.
- CVSS
- 35 / 40 · 8.8 / 10
- CISA KEV
- 30 / 30 · Listed
- EPSS
- 29 / 30 · 97.8%
CISA SSVC decision
- Exploitation
- active
- Automatable
- no
- Technical impact
- total
Vulnrichment: CISA's decision-tree inputs.
CNA vs NVD score
- NVD
- —
- CNA · microsoft
- 8.8
- NVD has not scored this yet; the score shown is the CNA’s.
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L/E:P/RL:O/RC:C
The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.
Actively exploited in the wild
CISA added this to its Known Exploited Vulnerabilities catalog on Nov 3, 2021. That signals real attacks, not theory.
Remediation deadline for US federal agencies: Nov 17, 2021.
Known to be used in ransomware campaigns.
Get told when this record changes
- In CISA KEV: exploitation in the wild confirmed
- Weaponized exploit exists
- A fix has been released
If any of these changes, we'll tell you in the daily email digest.
Is your version affected? Check itNoroxi analysis
What does it mean in your environment?
The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.
We use this product, ask for helpAffected systems
Is your version affected?
Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.
| Vendor | Product | CPE |
|---|---|---|
| microsoft | windows 10 1507 | cpe:2.3:o:microsoft:windows_10_1507 |
| microsoft | windows 10 1607 | cpe:2.3:o:microsoft:windows_10_1607 |
| microsoft | windows 10 1809 | cpe:2.3:o:microsoft:windows_10_1809 |
| microsoft | windows 10 1909 | cpe:2.3:o:microsoft:windows_10_1909 |
| microsoft | windows 10 2004 | cpe:2.3:o:microsoft:windows_10_2004 |
| microsoft | windows 10 20h2 | cpe:2.3:o:microsoft:windows_10_20h2 |
| microsoft | windows 10 21h1 | cpe:2.3:o:microsoft:windows_10_21h1 |
| microsoft | windows 7 | cpe:2.3:o:microsoft:windows_7 |
| microsoft | windows 8.1 | cpe:2.3:o:microsoft:windows_8.1 |
| microsoft | windows rt 8.1 | cpe:2.3:o:microsoft:windows_rt_8.1 |
| microsoft | windows server 2004 | cpe:2.3:o:microsoft:windows_server_2004 |
| microsoft | windows server 2008 | cpe:2.3:o:microsoft:windows_server_2008 |
and 5 more
Affected versions
NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.
- microsoft windows server 2004before 10.0.19041.1237
- microsoft windows server 2008all versions
- microsoft windows server 2008r2
- microsoft windows server 2012all versions
- microsoft windows server 2016before 10.0.14393.4651
- microsoft windows server 2019before 10.0.17763.2183
- microsoft windows server 2022before 10.0.20348.230
- microsoft windows server 20h2before 10.0.19042.1237
Versions reported by the vendor
Affected version ranges reported by the assigning authority (microsoft). Independent of NVD's CPE analysis and usually ahead of it.
Microsoft Windows 10 Version 1507
- 10.0.10240.0 and later · before 10.0.10240.19060affected
Microsoft Windows 10 Version 1607
- 10.0.14393.0 and later · before 10.0.14393.4651affected
Microsoft Windows 10 Version 1809
- 10.0.17763.0 and later · before 10.0.17763.2183affected
- 10.0.0 and later · before 10.0.17763.2183affected
Microsoft Windows 10 Version 1909
- 10.0.0 and later · before 10.0.18363.1801affected
Microsoft Windows 10 Version 2004
- 10.0.0 and later · before 10.0.19041.1237affected
Microsoft Windows 10 Version 20H2
- 10.0.0 and later · before 10.0.19042.1237affected
Microsoft Windows 10 Version 21H1
- 10.0.0 and later · before 10.0.19043.1237affected
Microsoft Windows 7
- 6.1.0 and later · before 6.1.7601.25712affected
Microsoft Windows 7 Service Pack 1
- 6.1.0 and later · before 6.1.7601.25712affected
Microsoft Windows 8.1
- 6.3.0 and later · before 6.3.9600.20120affected
Microsoft Windows Server 2008 R2 Service Pack 1
- 6.1.7601.0 and later · before 6.1.7601.25712affected
Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation)
- 6.1.7601.0 and later · before 6.1.7601.25712affected
Package-level exposure
OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.
| Ecosystem | Package | Affected range | Fix |
|---|---|---|---|
| openSUSE:Tumbleweed | python-oletools | before 0.60.2-2.4 | 0.60.2-2.4 |
Same product
microsoft: all recordsOther highest-scoring records for the same primary product.
- CVE-2021-34527Windows Print Spooler Remote Code Execution VulnerabilityKEV95Now
- CVE-2025-33053Internet Shortcut Files Remote Code Execution VulnerabilityKEV91Now
- CVE-2023-36025Windows SmartScreen Security Feature Bypass VulnerabilityKEV91Now
Published before and after this one in the same product
- ← EarlierCVE-2021-34484Aug 12, 2021 · Windows User Profile Service Elevation of Privilege VulnerabilityKEV68This week
- ← EarlierCVE-2021-36955Sep 15, 2021 · Windows Common Log File System Driver Elevation of Privilege VulnerabilityKEV62This week
- Later →CVE-2021-40449Oct 12, 2021 · Win32k Elevation of Privilege VulnerabilityKEV86Now
Remediation
Which version to upgrade to
Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.
| Product / package | Fixed version | Source |
|---|---|---|
| opensuse:python-oletools | 0.60.2-2.4 · openSUSE:Tumbleweed | Package registry (OSV) |
Exploit status
Weaponized
A ready exploit tool or active in-the-wild use is known for this vulnerability. This should be your top priority.
Evidence
- Metasploit module ×1
- Proof-of-concept repository on GitHub ×35
Noroxi does not host or link to exploit code. Metasploit module names and Nuclei template ids are shown (names, not code); Exploit-DB entries and PoC repositories are given as counts only.
| Source | Kind | Date |
|---|---|---|
| Metasploit module | exploit/windows/fileformat/word_mshtml_rceMicrosoft Office Word Malicious MSHTML RCEexploit | 2021-09-23 |
| Proof-of-concept repository on GitHub | stars:20 | 2021-09-08 |
| Proof-of-concept repository on GitHub | stars:10 | 2021-09-09 |
| Proof-of-concept repository on GitHub | stars:20 | 2021-09-09 |
| Proof-of-concept repository on GitHub | stars:4 | 2021-09-09 |
| Proof-of-concept repository on GitHub | stars:0 | 2021-09-09 |
| Proof-of-concept repository on GitHub | stars:3 | 2021-09-09 |
| Proof-of-concept repository on GitHub | stars:4 | 2021-09-10 |
| Proof-of-concept repository on GitHub | stars:1858 | 2021-09-10 |
| Proof-of-concept repository on GitHub | stars:7 | 2021-09-11 |
| Proof-of-concept repository on GitHub | stars:0 | 2021-09-11 |
| Proof-of-concept repository on GitHub | stars:11 | 2021-09-11 |
Research context
For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.
Timeline
From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.
- First PoC-7 d
- Published
- Metasploit+8 d
- CISA KEV+49 d
- TodayNov 12, 2026+1884 d
No history yet (it accumulates with the daily EPSS job).
Patch and commit links
Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.
No commit or PR link among the references.
Credits
All researchersFinders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.
No credits in the CNA record.
Variant candidates
Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.
No nightly-computed relations.
Chain candidates
An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.
—
Bug bounty scope
No known public program.
Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).
National notice (Türkiye Cybersecurity Directorate / USOM)
Official security notices citing this record; remediation advice is on the agency's page.
- TR-22-0612 · Sep 14, 2022(Microsoft Ürünleri Güvenlik Zafiyeti)
- TR-22-0394 · Jun 1, 2022(Microsoft Sıfırıncı Gün Zafiyeti)
- TR-22-0030 · Jan 12, 2022(Microsoft Güvenlik Güncellemesi)
- TR-21-0765 · Sep 22, 2021(Netgear Güvenlik Güncellemesi)
- TR-21-0739 · Sep 15, 2021(Microsoft Güvenlik Bildirimi)
- TR-21-0735 · Sep 14, 2021(Google Chrome Güvenlik Güncellemesi)
Technical details
Attack conditions
- Anyone who can reach it over the internet can trigger it.
- No account or password is required.
- A user must open a link or visit a page.
- No special conditions are required; it is repeatable.
If successful
- Confidentiality
- low · data can be read
- Integrity
- high · data or configuration can be modified
- Availability
- low · the service can be disrupted
Impact can extend beyond the vulnerable component (scope changes).
- Attack vector
- Network
- Attack complexity
- Low
- Privileges required
- None
- User interaction
- Required
- Scope
- Changed
- Confidentiality impact
- Low
- Integrity impact
- High
- Availability impact
- Low
Weakness class (CWE)
CWE-22 · Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:L
nvd-secondary
Root cause
A user-supplied file name is joined with the allowed directory, but the result is never checked to confirm it is still inside that directory.
The bug in code
The whole CWE-22 classA representative example of this vulnerability class. Not the vendor's source code; it shows the faulty pattern and its fix.
Vulnerable
const file = path.join(ROOT, req.query.name);
res.sendFile(file);Fixed
const file = path.resolve(ROOT, req.query.name);
if (!file.startsWith(ROOT + path.sep)) {
return res.sendStatus(400);
}
res.sendFile(file);Attack context
MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.
Attack patterns (CAPEC)
- Path TraversalCAPEC-126likelihood: High · Very High
- Using Slashes and URL Encoding Combined to Bypass Validation LogicCAPEC-64likelihood: High · High
- Manipulating Web Input to File System CallsCAPEC-76likelihood: High · Very High
- Using Escaped Slashes in Alternate EncodingCAPEC-78likelihood: High · High
- Using Slashes in Alternate EncodingCAPEC-79likelihood: High · High
ATT&CK techniques
—
Change log
- Fix✗ → ✓
For records you follow, these changes also arrive as notifications. →
References
Show all references (2 more)Show less
Vendor advisories and official records. Exploit/PoC links are deliberately left out.