CVE-2026-20939 is a vulnerability classified under CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor) affecting Microsoft Windows 10 Version 1809 (build 10.0.17763.0). The issue resides in Windows File Explorer, where an authorized attacker with local access and low privileges can disclose sensitive information that should otherwise be protected. The vulnerability does not require user interaction, which increases its risk profile in environments where local access is possible. The CVSS 3.1 base score is 5.5 (medium), reflecting the local attack vector (AV:L), low attack complexity (AC:L), and low privileges required (PR:L). The impact is limited to confidentiality (C:H), with no effect on integrity or availability. The vulnerability was reserved in December 2025 and published in January 2026, with no known exploits in the wild and no patches currently available. This suggests it may be a recently discovered issue or one that has not yet been weaponized. The lack of patches means organizations must rely on compensating controls to mitigate risk. The vulnerability could be exploited by insiders or malware that has gained limited local access, potentially exposing sensitive files or data handled by File Explorer. Since Windows 10 Version 1809 is an older release, many organizations may have already migrated to newer versions, but legacy systems remain at risk. The exposure of sensitive information could lead to data leakage, compliance violations, and increased risk of further attacks if attackers gain intelligence from disclosed data.

For European organizations, the primary impact is the potential unauthorized disclosure of sensitive information on systems running Windows 10 Version 1809. This could include personal data protected under GDPR, intellectual property, or confidential business information. The confidentiality breach could lead to regulatory penalties, reputational damage, and increased risk of targeted attacks leveraging the disclosed information. Sectors such as finance, healthcare, government, and critical infrastructure are particularly sensitive to such leaks. Since the vulnerability requires local access, the threat is higher in environments where endpoint security is weak, or insider threats are a concern. Organizations with legacy systems or those that have not fully migrated to supported Windows versions are more vulnerable. The absence of known exploits reduces immediate risk but does not eliminate it, especially as threat actors may develop exploits over time. The medium severity rating indicates a moderate but non-trivial risk that should be addressed promptly to prevent escalation.

1. Restrict local access to systems running Windows 10 Version 1809 by enforcing strict physical and network access controls. 2. Apply the principle of least privilege to limit user permissions, ensuring users do not have unnecessary access to sensitive files or system components. 3. Monitor and audit local user activities, especially those involving File Explorer and access to sensitive directories, to detect anomalous behavior. 4. Where possible, upgrade affected systems to a supported and patched version of Windows to eliminate the vulnerability. 5. Implement endpoint detection and response (EDR) solutions to identify potential exploitation attempts or suspicious local processes. 6. Educate users and administrators about the risks of local privilege misuse and enforce strong authentication and session management policies. 7. Maintain up-to-date backups and incident response plans to quickly recover from any data exposure incidents. 8. Stay informed about official patches or updates from Microsoft and apply them promptly once available.