CVE-2026-20839
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedImproper access control in Windows Client-Side Caching (CSC) Service allows an authorized attacker to disclose information locally.
In the news
Third-party coverage- Microsoft CVE Summary - Bleeping Computer
- Microsoft January 2026 Patch Tuesday fixes 3 zero-days, 114 flaws
Surfaced from public web coverage — external links open in a new tab.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · moderate confidenceThis is an improper access control vulnerability in Windows Client-Side Caching (CSC) Service that allows a local authorized attacker to bypass intended permission restrictions and disclose sensitive information. The CSC service manages offline file caching and synchronization, and the access control flaw enables an attacker with standard user privileges to access data they should not have permission to read.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data< 10.0.14393.8783< 10.0.17763.8276< 10.0.19044.6809< 10.0.19045.6809< 10.0.22631.6491< 10.0.26100.7623< 10.0.26200.7623= r2CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Local
- Complexity
- Low
- Privileges
- Low
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- None
- Availability
- None
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Check Windows versionRun 'winver' or 'systeminfo | findstr /B /C:"OS Name" /C:"OS Version"' to obtain the exact Windows version and build numberAffected if The build number is lower than any of the following: 14393.8783 (Win10 1607), 17763.8276 (Win10 1809), 19044.6809 (Win10 21h2/22h2), 22631.6491 (Win11 23h2), 26100.7623 (Win11 24h2), 26200.7623 (Win11 25h2), or if running Windows Server 2008 R2
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Identify CSC service statusOpen Services console (services.msc) and locate the 'cscservice' or 'Offline Files' service, or run 'sc query cscservice' from an elevated command promptAffected if The CSC service (cscservice) exists and is running on the system, indicating offline file caching is enabled
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Locate CSC cache directoryThe default CSC cache is typically at %SYSTEMROOT%\CSC (e.g., C:\Windows\CSC). Verify its existence with 'dir C:\Windows\CSC' or 'icacls C:\Windows\CSC'Affected if The CSC cache directory exists on the system (offline files have been used at least once)
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Inspect CSC directory permissionsRun 'icacls C:\Windows\CSC' from a standard user command prompt to check what permissions standard users have on the cache directoryAffected if Standard (non-admin) users have read or write access to the CSC directory beyond what should be granted, indicating the access control flaw is present
A user is affected if they run a Windows version listed in the affected ranges AND the CSC service/cache directory exists on their system, allowing standard users to access cached files they should not have permission to read.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped10.0.14393.878310.0.17763.827610.0.19044.6809
Apply the appropriate Microsoft security update for this vulnerability once released, or manually restrict CSC service permissions to enforce proper access controls. Verify that only authorized SYSTEM and administrative accounts have access to CSC cache directories.
Install the security update from Microsoft Update for your specific Windows version (10.0.14393.8783, 10.0.17763.8276, 10.0.19044.6809, 10.0.19045.6809, 10.0.22631.6491, 10.0.26100.7623, or 10.0.26200.7623 depending on your version)
- Identify your current Windows version and build number (winver command)
- Open Windows Settings > Update & Security > Windows Update
- Click 'Check for updates' to download and install the latest security updates
- Alternatively, manually download the specific security update from the Microsoft Update Catalog using the KB number associated with this CVE (check MSRC for the KB)
- Restart the system after the update is installed
- Verify the installation by checking the installed updates in Control Panel > Programs and Features > View installed updates
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation3.0 h
- Implementation4.0 h
- Testing2.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,168.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-20839 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-20839 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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