CVE-2026-20939 Overview
CVE-2026-20939 is an information disclosure vulnerability in Windows File Explorer that allows an authorized attacker with local access to expose sensitive information to unauthorized actors. This vulnerability is classified under CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor), indicating that the affected component improperly handles sensitive data, potentially allowing it to be accessed by parties who should not have visibility into such information.
Critical Impact
Local attackers with low privileges can exploit this vulnerability to disclose sensitive information with high confidentiality impact, potentially exposing system data, user credentials, or other protected information stored or accessible through Windows File Explorer.
Affected Products
- Windows File Explorer (specific version details pending from Microsoft)
- Windows Operating Systems with vulnerable File Explorer component
Discovery Timeline
- January 13, 2026 - CVE-2026-20939 published to NVD
- January 13, 2026 - Last updated in NVD database
Technical Details for CVE-2026-20939
Vulnerability Analysis
This vulnerability represents an information disclosure flaw within Windows File Explorer. The core issue stems from improper handling of sensitive information, where the component fails to adequately protect data from being accessed by unauthorized actors operating locally on the system.
The vulnerability requires local access and low-privilege authentication, meaning an attacker must first gain some level of access to the target system. Once positioned, the attacker can leverage this flaw to disclose sensitive information without requiring any user interaction. The attack complexity is low, making exploitation relatively straightforward once the prerequisite access is obtained.
The impact is concentrated solely on confidentiality—there is no integrity or availability impact associated with this vulnerability. This suggests the vulnerability allows reading of sensitive data but does not permit modification of files or system disruption.
Root Cause
The root cause is classified under CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor). This indicates that Windows File Explorer contains code paths that inadvertently expose sensitive information to actors who should not have access to such data. This could manifest through improper access controls on file metadata, cached information, or through side-channel information leakage during file operations.
Attack Vector
The attack vector is local, requiring the attacker to have authenticated access to the target system with low privileges. The exploitation does not require user interaction, meaning once an attacker has local access, they can trigger the information disclosure without needing to trick a user into performing any action.
Exploitation would likely involve a malicious local user or process manipulating File Explorer functionality to extract sensitive information that should be protected. The specific mechanism may involve querying file properties, accessing cached data, or exploiting how File Explorer handles certain file types or paths.
Detection Methods for CVE-2026-20939
Indicators of Compromise
- Unusual File Explorer process behavior or unexpected child processes spawned by explorer.exe
- Anomalous access patterns to sensitive file locations or system directories
- Unexpected reads of file metadata or properties by non-administrative users
- Log entries indicating bulk enumeration of file attributes or properties
Detection Strategies
- Monitor explorer.exe for suspicious API calls related to file information queries
- Implement file access auditing on sensitive directories to detect unauthorized information access
- Deploy endpoint detection and response (EDR) solutions to identify anomalous File Explorer behavior
- Utilize Windows Security Event Log monitoring for unusual file access patterns
Monitoring Recommendations
- Enable Windows Security Auditing for file system access events
- Configure SentinelOne agents to monitor for suspicious File Explorer activity patterns
- Implement behavioral analysis rules to detect information exfiltration attempts via File Explorer
- Review security logs for repeated failed or suspicious file access attempts by low-privilege users
How to Mitigate CVE-2026-20939
Immediate Actions Required
- Apply the Microsoft security update as soon as it becomes available
- Review and restrict local access permissions to minimize exposure
- Audit user accounts with local system access for compliance with least-privilege principles
- Enable enhanced monitoring on endpoints until patching is complete
Patch Information
Microsoft has acknowledged this vulnerability and released a security update. Organizations should refer to the Microsoft Security Update for CVE-2026-20939 for official patch information, download links, and deployment guidance. Ensure Windows Update is configured to receive security updates promptly, and prioritize deployment across all affected Windows systems.
Workarounds
- Restrict local access to systems by enforcing strict user authentication policies
- Implement application control policies to limit what processes can interact with File Explorer
- Consider using Windows Defender Application Control (WDAC) to restrict unauthorized executables
- Apply network segmentation to limit lateral movement opportunities for attackers who may exploit this vulnerability
# Enable Windows Security Auditing for file access monitoring
auditpol /set /subcategory:"File System" /success:enable /failure:enable
# Review current audit policy settings
auditpol /get /category:"Object Access"
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
