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PatchSiren cyber security CVE debrief

CVE-2021-36955 Microsoft CVE debrief

CVE-2021-36955 affects the Microsoft Windows Common Log File System (CLFS) driver and is described as a privilege escalation vulnerability. CISA added it to the Known Exploited Vulnerabilities catalog on 2021-11-03 and marked known ransomware campaign use, indicating it should be treated as a high-priority remediation item. CISA’s required action is to apply updates per vendor instructions.

Vendor
Microsoft
Product
Windows
CVSS
HIGH 7.8
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

Windows administrators, endpoint and server security teams, vulnerability management teams, and incident responders should prioritize this CVE, especially in environments where local privilege escalation would materially increase attacker impact.

Technical summary

The supplied source corpus identifies CVE-2021-36955 as a Microsoft Windows CLFS driver privilege escalation issue. The official CISA KEV entry indicates it is a known exploited vulnerability and notes known ransomware campaign use. No additional exploit mechanics, affected build list, or CVSS score were provided in the supplied data.

Defensive priority

High. This is a known exploited Windows vulnerability with ransomware-campaign relevance and a CISA remediation due date of 2021-11-17.

Recommended defensive actions

  • Apply Microsoft updates per vendor instructions as soon as possible.
  • Prioritize systems exposed to untrusted local users or with elevated privilege pathways.
  • Use vulnerability management and configuration monitoring to confirm remediation across Windows assets.
  • Monitor for signs of local privilege escalation activity and unusual administrative token use.
  • Track CISA KEV guidance and validate closure before the 2021-11-17 due date when using the KEV timeline as a remediation target.

Evidence notes

Supported by the CVE record title/description, CISA KEV metadata, and the official source item. The supplied corpus states: Microsoft Windows Common Log File System (CLFS) Driver Privilege Escalation Vulnerability; dateAdded 2021-11-03; dueDate 2021-11-17; knownRansomwareCampaignUse: Known; requiredAction: Apply updates per vendor instructions. No further technical details, affected versions, or CVSS data were included in the supplied sources.

Sources and references

Verified primary and authoritative sources

  • CVE-2021-36955 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2021-36955

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2021-36955 NVD vulnerability detail

    Publisher, destination, and source semantics verified

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

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

  • CISA Known Exploited Vulnerabilities catalog

    Publisher, destination, and source semantics verified

    URL: https://www.cisa.gov/known-exploited-vulnerabilities-catalog

    Cybersecurity and Infrastructure Security Agency - Official CISA catalog of vulnerabilities known to be exploited in the wild.

Supplemental references

  • Source item URL

    Unverified legacy reference

    URL: https://www.cisa.gov/sites/default/files/feeds/known_exploited_vulnerabilities.json

    cisa_kev

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.