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Full vulnerability report · 2021
CVE-2021-36955High confidence

Microsoft Windows Common Log File System (CLFS) Driver Privilege Escalation Vulnerability

Microsoft · Windows

7.8HighCVSS 3.1
Recommended action
Patch now

CISA confirms exploitation in the wild and lists 2021-11-17 as the remediation due date.

Patch available
R
Operational reassessment

Published severity in operational context

Open reassessment dashboard →
Published severityHighOperational priority:Critical, raised one band.upgradedsince 3 Nov 2021

Evidence used

  • CISA confirms exploitation in the wild.
  • Exploitation requires an existing local or physical foothold with privileges.
  • EPSS is 4.07% for the current model date.

Compensating controls

  • Validate the affected product branch and deploy the verified fixed release.
  • Restrict local access and enforce least privilege on affected hosts.
  • Increase monitoring for the attack path and post-exploitation behaviour described in the report.

Verification

  1. Confirm that the asset runs Microsoft Windows and falls inside the recorded affected range.
  2. Verify the installed build against the product-specific fixed version after deployment.
  3. Validate exposure, authentication requirements and compensating controls in the actual environment.
  4. Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Mitigation target: Within 3 daysRemediation target: Within 90 days

This automated reassessment organises public evidence. It does not know asset exposure, business impact or control effectiveness and does not replace CVSS or a human risk decision.

Optional official sources

National CERT insights
?CERT means Computer Emergency Response Team; CSIRT is the closely related term Computer Security Incident Response Team.

Choose official national sources for this report. Each advisory shows its original language. Your selection is remembered on this device and included in shared links.

Official European source

ENISA European Vulnerability Database

Official EUVD identifiers, advisory evidence and known-exploited context. Missing fields are not treated as evidence of low risk.

1 current
ENISA EUVD identifier

EUVD-2021-23531

CISA KEV mirrored by ENISA

Windows Common Log File System Driver Elevation of Privilege Vulnerability

EUVD state
Present in the current official mapping
Known exploitation
Recorded by ENISA since 3 Nov 2021. Evidence sources: cisa_kev.
ENISA score
7.8 · CVSS 3.1
Advisory evidence
No linked advisory details stored yet
Recommended actionPatch now

CISA confirms exploitation in the wild and lists 2021-11-17 as the remediation due date.

Patch available
01

What, why and how

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

What

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

Why

The current structured CVE record identifies a security weakness, but the root cause requires confirmation in the linked vendor material.

How

An attacker operating through local access may attempt exploitation with low privileges. If successful, the issue may gain additional privileges.

What

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

Why

The current structured CVE record identifies a security weakness, but the root cause requires confirmation in the linked vendor material.

How

An attacker operating through local access may attempt exploitation with low privileges. If successful, the issue may gain additional privileges.

02

Exploit reality and attack path

CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.

Observed exploitation
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
Confirmed in the wild

CISA added this CVE to its Known Exploited Vulnerabilities catalogue on 2021-11-03. Known ransomware campaign use is recorded.

Public PoC / exploit material
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
None recorded

No exploit-tagged reference or CISA SSVC proof-of-concept state is currently recorded. Research may still exist outside the structured feeds.

Likely attack path
local access → vulnerable operation → gain additional privileges
Attack surface
Local
Privileges required
Low: a basic authenticated account is required
User interaction
None
Attack complexity
Low: no specialised conditions are recorded
Security boundary
Unchanged: impact remains within the vulnerable component's security authority
Weakness
?CWE means Common Weakness Enumeration.
CWE not yet assigned
CVSS vector
?CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity.
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C

Common Vulnerability Scoring System 3.1: the compact vector below is decoded into plain language.

AVLocalAttack vector: The attacker needs local access to the vulnerable system.ACLowAttack complexity: No specialised conditions are required beyond attacker-controlled input.PRLowPrivileges required: The attacker needs basic user-level privileges.UINoneUser interaction: No action by another user is required.SUnchangedScope: The security impact remains within the vulnerable component's authority.CHighConfidentiality impact: A successful attack can cause a major loss.IHighIntegrity impact: A successful attack can cause a major loss.AHighAvailability impact: A successful attack can cause a major loss.
Post-exploitation / living off the land
The issue can support a local privilege or sandbox boundary transition; normal system utilities may then be available in the gained context.
Privilege escalationCISA KEV
A

Official authority intelligence

Only matched European and national findings are included. Language selectors and unavailable sources are omitted.

ENISA EUVD · EUVD-2021-23531Known-exploited evidence recorded

Windows Common Log File System Driver Elevation of Privilege Vulnerability

Official EUVD record ↗
Cyber Security Agency of Singapore · English · CSA-SB-20210922Security Bulletin 22 Sep 2021

The Cyber Security Agency of Singapore included this CVE in its official Security Bulletin 22 Sep 2021, published on 22 September 2021. Open the linked bulletin for the product, severity and reference information published in that issue.

Official advisory ↗
CERT-FR · French · CERTFR-2021-AVI-710Multiples vulnérabilités dans Microsoft Windows

rabilités ont été découvertes dans Microsoft Windows. Certaines d'entre elles permettent à un attaquant de provoquer une exécution de code arbitraire à distance, un contournement de la politique de sécurité et une atteinte à la confidentialité des données. Solution Se référer au bulletin de sécurité de l'éditeur pour l'obtention des correctifs (cf. section Documentation). Documentation Bulletin de sécurité Microsoft du 14 septembre 2021 https://msrc.microsoft.com/update-guide/ Référence CVE CVE-2021-26435 https://www.cve.org/CVERecord?id=CVE-2021-26435 Référence CVE CVE-2021-36954 https://www.cve.org/CVERecord?id=CVE-2021-36954 Référence CVE CVE-2021-36955 https://www.cve.org/CVERecord?id=CVE-2021-36955 Référence CVE CVE-2021-36958 https://www.cve.org/CVERecord?id=CVE-2021-36958 Référence CVE CVE-2021-36959 https://www.cve.org/CVERecord?id=CVE-2021-36959 Référence CVE CVE-2021-36960 https://www.cve.org/CVERecord?id=CVE-2021-36960 Référence CVE CVE-2021-36961 https://www.cve.org/CVERecord?id=CVE-2021-36961 Référence CVE CVE-2021-36962 https://www.cve.org/CVERecord?id=CVE-2021-36962 Référence CVE CVE-2021-36963 https://www.cve.org/CVERecord?id=CVE-2021-36963 Référence CVE CVE-2021-36964 https://www.cve.org/CVERecord?id=CVE-2021-36964 Référence CVE CVE-2021-36965 https://www.cve.org/CVERecord?id=

Official advisory ↗
JVN iPedia · Japanese · JVNDB-2021-002718複数の Microsoft Windows 製品 における権限を昇格される脆弱性

複数の Microsoft Windows 製品 には、Windows 共通ログファイルシステムドライバに不備があるため、権限を昇格される脆弱性が存在します。 本脆弱性は、CVE-2021-36955 および CVE-2021-36963 とは異なる脆弱性です。

Official advisory ↗
JVN iPedia · Japanese · JVNDB-2021-002702複数の Microsoft Windows 製品における権限を昇格される脆弱性

複数の Microsoft Windows 製品には、Windows 共通ログファイルシステムドライバに不備があるため、権限を昇格される脆弱性が存在します。 本脆弱性は、CVE-2021-36955 および CVE-2021-38633 とは異なる脆弱性です。

Official advisory ↗
JVN iPedia · Japanese · JVNDB-2021-002697複数の Microsoft Windows 製品における権限を昇格される脆弱性

複数の Microsoft Windows 製品には、Windows 共通ログファイルシステムドライバに不備があるため、権限を昇格される脆弱性が存在します。 本脆弱性は、CVE-2021-36963 および CVE-2021-38633 とは異なる脆弱性です。

Official advisory ↗
KISA KrCERT/CC · Korean · KNVD-5393MS 9월 보안 위협에 따른 정기 보안 업데이트 권고

VE-2021-26434 Visual Studio 권한 상승 취약성 Windows Ancillary Function Driver for WinSock CVE-2021-38638 WinSock용 Windows 보조 기능 드라이버 권한 상승 취약성 Windows Ancillary Function Driver for WinSock CVE-2021-38628 WinSock용 Windows 보조 기능 드라이버 권한 상승 취약성 Windows Authenticode CVE-2021-36959 Windows Authenticode 스푸핑 취약성 Windows Bind Filter Driver CVE-2021-36954 Windows Bind Filter 드라이버 권한 상승 취약성 Windows BitLocker CVE-2021-38632 BitLocker 보안 기능 우회 취약성 Windows Common Log File System Driver CVE-2021-38633 Windows 공용 로그 파일 시스템 드라이버 권한 상승 취약성 Windows Common Log File System Driver CVE-2021-36963 Windows 공용 로그 파일 시스템 드라이버 권한 상승 취약성 Windows Common Log File System Driver CVE-2021-36955 Windows 공용 로그 파일 시스템 드라이버 권한 상승 취약성 Windows Event Tracing CVE-2021-38630 Windows 이벤트 추적 권한 상승 취약성 Windows Event Tracing CVE-2021-36964 Windows 이벤트 추적 권한 상승 취약성 Windows Installer CVE-2021-36962 Windows 설치 관리자 정보 유출 취약성 Windows Installer CVE-2021-36961 Windows 설치 관리자 서비스 거부 취약성 Windows Kernel CVE-2021-38626 Windows 커널 권한 상승 취약성 Windows Kernel CVE-2021-38625 Windows 커널 권한 상승 취약성 Windows Key Storage Provider CVE-2021-38624 Windows 키 저장소 공급자 보안 기능 우회 취약성 Windows Print Spooler Components CVE-2021-40447 Windows 인쇄 스풀러 권한 상승 취약성 Windows Print Spooler Components CVE-2021-38671 Windows 인쇄 스풀러 권한 상승 취약성 Windows Print Spooler Components CVE-2021-38667 W

Official advisory ↗
03

Patch and workaround

Operational remediation based on structured source evidence.

Status
?Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change.
Patch available
Affected
Fixed
An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
Action
Review the linked authoritative reference and apply the recorded fixed release appropriate to the affected product branch.
Workaround
No verified workaround is recorded. Limit untrusted access and use least privilege until authoritative guidance is available.
04

Evidence and provenance

Published 15 Sept 2021 · Last source change 10 Aug 2026, 15:10 UTC · CWE not yet assigned

CVE recordCVE.org · 5.2
CVSS sourceCNA
EPSS source
?The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.
FIRST · tracked since 2026-08-14
European sourceENISA EUVD · EUVD-2021-23531
Product sourceCISA KEV
Remediation sourceCVE/CNA references
CWE sourceUnavailable
NVD statusNVD enriched

Missing structured fields: CWE classification. Missing data is not evidence of low risk; review the primary advisory.

Material change intelligence

What changed after publication

View recent updates ↗
  1. Affected versionsThe structured affected or fixed version information changed.
    Before
    10.0.10240.0 < 10.0.10240.19060; 10.0.14393.0 < 10.0.14393.4651; 10.0.17763.0 < 10.0.17763.2183; 10.0.0 < 10.0.17763.2183; 10.0.0 < 10.0.18363.1801; 10.0.0 < 10.0.19041.1237; 10.0.0 < 10.0.19042.1237; 10.0.0 < 10.0.19043.1237 · Fixed: An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
    After
    Windows 10 Version 1507: 10.0.10240.0 < 10.0.10240.19060; Windows 10 Version 1607: 10.0.14393.0 < 10.0.14393.4651; Windows 10 Version 1809: 10.0.17763.0 < 10.0.17763.2183, 10.0.0 < 10.0.17763.2183; Windows 10 Version 1909: 10.0.0 < 10.0.18363.1801; Windows 10 Version 2004: 10.0.0 < 10.0.19041.1237; Windows 10 Version 20H2: 10.0.0 < 10.0.19042.1237; Windows 10 Version 21H1: 10.0.0 < 10.0.19043.1237; Windows 7: 6.1.0 < 6.1.7601.25712; Windows 7 Service Pack 1: 6.1.0 < 6.1.7601.25712; Windows 8.1: 6.3.0 < 6.3.9600.20120; Windows Server 2008 R2 Service Pack 1: 6.1.7601.0 < 6.1.7601.25712; Windows Server 2008 R2 Service Pack 1 (Server Core installation): 6.1.7601.0 < 6.1.7601.25712; Windows Server 2008 Service Pack 2: 6.0.6003.0 < 6.0.6003.21218; Windows Server 2008 Service Pack 2 (Server Core installation): 6.0.6003.0 < 6.0.6003.21218; Windows Server 2012: 6.2.9200.0 < 6.2.9200.23462; Windows Server 2012 (Server Core installation): 6.2.9200.0 < 6.2.9200.23462; Windows Server 2012 R2: 6.3.9600.0 < 6.3.9600.20120; Windows Server 2012 R2 (Server Core installation): 6.3.9600.0 < 6.3.9600.20120; Windows Server 2016: 10.0.14393.0 < 10.0.14393.4651; Windows Server 2016 (Server Core installation): 10.0.14393.0 < 10.0.14393.4651; Windows Server 2019: 10.0.17763.0 < 10.0.17763.2183; Windows Server 2019 (Server Core installation): 10.0.17763.0 < 10.0.17763.2183; Windows Server 2022: 10.0.20348.0 < 10.0.20348.230; Windows Server version 2004: 10.0.0 < 10.0.19041.1237; Windows Server version 20H2: 10.0.0 < 10.0.19042.1237 · Fixed: An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
    CNA ↗
Material fields only · duplicate refreshes suppressed · history retained for the configured operational retention period
CVE published
Added to CISA KEV
Fixed release or patch reference recorded
ENISA EUVD · EUVD-2021-23531↗CISA KEV record↗CVE authority or vendor reference↗Patch or update reference↗Official CVE record (CNA and ADP data)↗NVD record↗
Technical terms and abbreviations used in this report
CVE
Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
CVSS
Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
EPSS
Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
CWE
Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
CNA
CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
CISA ADP
Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
NVD
National Vulnerability Database: NIST's enrichment service for CVE records.
CERT / CSIRT
A computer security incident response team that publishes warnings or coordinates incident response.
PoC
Proof of concept: public material that demonstrates or helps reproduce exploitation.
CSAF
Common Security Advisory Framework: a machine-readable format for security advisories.
LoTL
Living off the land: abuse of legitimate tools or system functions during an attack.
Free version - for non-commercial use only.CVE-2021-36955 · cve.blacktree.nl