Microsoft Windows

CVE-2021-36955: Microsoft Windows Common Log File System (CLFS) Driver Privilege Escalation Vulnerability

Is it exploited?

Yes. CISA added it to the Known Exploited Vulnerabilities catalog on 3 Nov 2021, 49 days after the CVE record was published.

How likely is exploitation in the next 30 days?

4.1%

EPSS score, percentile 90.6 of all scored records

How severe?

7.8 High

CVSS v3.1 base score, given by the CNA

What should I do?

CISA deadline 17 Nov 2021. Apply updates per vendor instructions.

Advisories and patches (1)

Which versions are affected?

Affected

from 10.0.10240.0 to before 10.0.10240.19060
from 10.0.14393.0 to before 10.0.14393.4651
from 10.0.17763.0 to before 10.0.17763.2183
from 10.0.0 to before 10.0.17763.2183
from 10.0.0 to before 10.0.18363.1801
from 10.0.0 to before 10.0.19041.1237

and 14 more in the table below

Versions by product

Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

What to do

Required action, set by CISA

Apply updates per vendor instructions.

CISA due date for this action: 17 November 2021.

Advisories and patches

Windows Common Log File System Driver Elevation of Privilege Vulnerability (Vendor advisory, Patch)

Linked from CISA's catalog entry

https://nvd.nist.gov/vuln/detail/CVE-2021-36955

Affected products and versions

As listed in the CVE record.

Microsoft Windows 10 Version 1507
Status	Versions
Affected	from 10.0.10240.0 to before 10.0.10240.19060

Microsoft Windows 10 Version 1607
Status	Versions
Affected	from 10.0.14393.0 to before 10.0.14393.4651

Microsoft Windows 10 Version 1809
Status	Versions
Affected	from 10.0.17763.0 to before 10.0.17763.2183

Microsoft Windows 10 Version 1809
Status	Versions
Affected	from 10.0.0 to before 10.0.17763.2183

Microsoft Windows 10 Version 1909
Status	Versions
Affected	from 10.0.0 to before 10.0.18363.1801

Microsoft Windows 10 Version 2004
Status	Versions
Affected	from 10.0.0 to before 10.0.19041.1237

Microsoft Windows 10 Version 20H2
Status	Versions
Affected	from 10.0.0 to before 10.0.19042.1237

Microsoft Windows 10 Version 21H1
Status	Versions
Affected	from 10.0.0 to before 10.0.19043.1237

Microsoft Windows 7
Status	Versions
Affected	from 6.1.0 to before 6.1.7601.25712

Microsoft Windows 7 Service Pack 1
Status	Versions
Affected	from 6.1.0 to before 6.1.7601.25712

Microsoft Windows 8.1
Status	Versions
Affected	from 6.3.0 to before 6.3.9600.20120

Microsoft Windows Server 2008 R2 Service Pack 1
Status	Versions
Affected	from 6.1.7601.0 to before 6.1.7601.25712

Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation)
Status	Versions
Affected	from 6.1.7601.0 to before 6.1.7601.25712

Microsoft Windows Server 2008 Service Pack 2
Status	Versions
Affected	from 6.0.6003.0 to before 6.0.6003.21218

Microsoft Windows Server 2008 Service Pack 2 (Server Core installation)
Status	Versions
Affected	from 6.0.6003.0 to before 6.0.6003.21218

Microsoft Windows Server 2012
Status	Versions
Affected	from 6.2.9200.0 to before 6.2.9200.23462

Microsoft Windows Server 2012 (Server Core installation)
Status	Versions
Affected	from 6.2.9200.0 to before 6.2.9200.23462

Microsoft Windows Server 2012 R2
Status	Versions
Affected	from 6.3.9600.0 to before 6.3.9600.20120

Microsoft Windows Server 2012 R2 (Server Core installation)
Status	Versions
Affected	from 6.3.9600.0 to before 6.3.9600.20120

Microsoft Windows Server 2016
Status	Versions
Affected	from 10.0.14393.0 to before 10.0.14393.4651

Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

About this vulnerability

Microsoft Windows Common Log File System (CLFS) driver contains an unspecified vulnerability that allows for privilege escalation.

Summary: CISA's KEV catalog, as of 28 Oct 2026.

CISA added CVE-2021-36955 to its Known Exploited Vulnerabilities (KEV) catalog on 3 Nov 2021.
CISA marks it as known to be used in ransomware campaigns.
The due date CISA sets for the required action is 17 Nov 2021.
The CVE record was published on 15 Sep 2021, 49 days before it was added to KEV.
The CVE record carries a CVSS v3.1 base score of 7.8 (High), given by its CNA.
FIRST's EPSS model puts the probability of exploitation in the next 30 days at 4.1% (percentile 90.6), as of 28 Oct 2026.
CISA's SSVC assessment in the CVE record: exploitation active, automatable no, technical impact total.

Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026. About the data.

Description

Windows Common Log File System Driver Elevation of Privilege Vulnerability

Windows Common Log File System Driver Elevation of Privilege Vulnerability

Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

Timeline

Date	Event
15 Sep 2021	CVE record published
3 Nov 2021	Added to CISA KEV
17 Nov 2021	CISA due date for the required action
10 Aug 2026	CVE record last updated

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