# CVE-2026-20839: Microsoft Windows Client-Side Caching Service improper access control

Source: https://junglewise.ai/threats/cve-2026-20839-microsoft-windows-client-side-caching-service-improper-access  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-01-13. Updated: 2026-07-30  
License: CC BY 4.0 with a link back

CVE-2026-20839 · Severity: medium · CVSS 5.5 · Published 2026-01-13

A vulnerability in the Windows Client-Side Caching (CSC) Service could allow an authorized user to access information they should not be able to see. This service is responsible for making network files available offline; an exploit could lead to the unauthorized disclosure of sensitive local data. To use this flaw, an attacker must already have the ability to log onto the affected system.

Technologies: [Microsoft Windows 11](https://junglewise.ai/threats/technologies/windows-11), [Microsoft Windows Server 2008 R2](https://junglewise.ai/threats/technologies/windows-server-2008-r2), [Microsoft Windows 10](https://junglewise.ai/threats/technologies/windows-10). Vendors: [Microsoft](https://junglewise.ai/threats/vendors/microsoft).

## Quick answers

**Is CVE-2026-20839 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-20839 being exploited in the wild.

**How severe is CVE-2026-20839?** Medium severity, with a CVSS score of 5.5, published 13 January 2026.

**Which products and versions are affected?** Microsoft Windows 11 22H3, 23H2, 24H2, 25H2; Microsoft Windows Server 2008 R2 Service Pack 1; Microsoft Windows 10 1607, 1809, 21H2, 22H2.

**Is there a fix for CVE-2026-20839?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-20839 attacked?** It is attacked with local access, in an operating system.

**Am I affected by CVE-2026-20839?** Check whether you run Microsoft Windows 11, Microsoft Windows Server 2008 R2 or Microsoft Windows 10 at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

An improper access control vulnerability (CWE-284) exists in the Windows Client-Side Caching (CSC) Service. The flaw allows a locally authenticated attacker with low privileges to bypass security restrictions and gain unauthorized access to sensitive information stored within the CSC cache. The attack vector is local, meaning the attacker must have existing credentials and the ability to execute code on the target system. Microsoft has released security updates to address this issue across multiple versions of Windows and Windows Server.

## Affected products

- Microsoft Windows 10 1607, 1809, 21H2, 22H2
- Microsoft Windows 11 22H3, 23H2, 24H2, 25H2
- Microsoft Windows Server 2008 R2 Service Pack 1

## Timeline

- 2026-01-13: disclosed
- 2026-01-13: advisory

## References

- [https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20839](https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20839)

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