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CVE-2026-20839 Microsoft CVE debrief

AI-assisted PatchSiren debrief based on the supplied source corpus. The CVE record for CVE-2026-20839 was published on 2026-01-13T18:16:12.320Z and has not been modified since then. CVE-2026-20839 is a medium-severity vulnerability in Windows Client-Side Caching (CSC) Service due to improper access control. An authorized attacker can exploit this vulnerability to disclose information locally. The vulnerability has a CVSS score of 5.5 and affects various versions of Windows 10, Windows 11, and Windows Server. Organizations should assess exposure and implement recommended actions. Microsoft has provided a vendor advisory for mitigation.

Vendor
Microsoft
Product
Windows 10 Version 1607
CVSS
MEDIUM 5.5
CISA KEV
Not listed in stored evidence
Original CVE published
2026-01-13
Original CVE updated
2026-07-30
Advisory published
2026-01-13
Advisory updated
2026-07-30

Who should care

Organizations using affected versions of Windows 10, Windows 11, and Windows Server should prioritize patching or mitigating CVE-2026-20839. IT administrators, cybersecurity teams, and vulnerability management professionals should assess exposure and implement recommended actions.

Technical summary

CVE-2026-20839 is a medium-severity vulnerability in Windows Client-Side Caching (CSC) Service due to improper access control. An authorized attacker can exploit this vulnerability to disclose information locally. The vulnerability has a CVSS score of 5.5 and affects various versions of Windows 10, Windows 11, and Windows Server.

Defensive priority

Medium-priority defensive tasks are recommended given the local information disclosure risk and medium CVSS score.

Recommended defensive actions

  • Inventory and assess Windows systems for exposure to CVE-2026-20839, focusing on systems with Client-Side Caching (CSC) Service enabled.
  • Apply vendor-recommended mitigations or patches for CVE-2026-20839 as available.
  • Monitor system logs for suspicious activity related to the CSC Service.
  • Implement compensating controls such as restricting access to sensitive information and enforcing least privilege principles.
  • Consider temporary disabling of the CSC Service if immediate patching is not feasible.

Evidence notes

The CVE-2026-20839 record indicates improper access control in Windows Client-Side Caching (CSC) Service, allowing an authorized attacker to disclose information locally. Microsoft has provided a vendor advisory for mitigation.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-20839 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-20839

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

  • CVE-2026-20839 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-20839

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

Supplemental references

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.