CVE-2026-20939 is a vulnerability classified under CWE-200, indicating exposure of sensitive information to unauthorized actors. It affects Microsoft Windows 10 Version 1809 (build 10.0.17763.0), specifically within the Windows File Explorer component. The vulnerability allows an attacker with authorized local access and low privileges to disclose sensitive information stored or processed by the system. The attack vector is local (AV:L), requiring the attacker to have some level of access to the system but not elevated privileges (PR:L). No user interaction is required (UI:N), and the scope remains unchanged (S:U), meaning the vulnerability affects only the vulnerable component without impacting other system components. The confidentiality impact is high (C:H), while integrity and availability impacts are none (I:N, A:N). The CVSS v3.1 base score is 5.5, reflecting a medium severity level. The vulnerability does not currently have known exploits in the wild, and no patches have been linked yet, though Microsoft is likely to release updates. The flaw likely stems from improper access control or information disclosure within File Explorer, allowing sensitive data to be read by unauthorized local users. This could include file metadata, system information, or cached data that should be protected. Since the vulnerability requires local access, it is primarily a risk in environments where multiple users share systems or where attackers can gain initial footholds with limited privileges.

For European organizations, especially those in sectors such as finance, healthcare, government, and critical infrastructure, this vulnerability presents a risk of sensitive data exposure. Although exploitation requires local access, insider threats or attackers who have compromised low-privilege accounts could leverage this flaw to escalate information gathering. The exposure of sensitive information could lead to further attacks, including social engineering, privilege escalation, or targeted data theft. Organizations still running Windows 10 Version 1809, which is an older and out-of-mainstream-support version, are particularly vulnerable. The impact is primarily on confidentiality, potentially exposing personal data, intellectual property, or system configuration details. This could result in regulatory non-compliance under GDPR if personal data is involved, leading to legal and financial repercussions. The lack of known exploits reduces immediate risk but does not eliminate the threat, especially as attackers may develop exploits once details become public. The vulnerability does not affect system integrity or availability, so operational disruption is unlikely.

European organizations should prioritize upgrading systems from Windows 10 Version 1809 to a supported and patched Windows version to eliminate exposure to this vulnerability. Until upgrades are feasible, organizations should implement strict access controls to limit local user privileges and prevent unauthorized local access. Employing endpoint detection and response (EDR) tools to monitor for suspicious local activity can help detect exploitation attempts. Network segmentation and the use of least privilege principles reduce the risk of attackers gaining local access. Organizations should also audit and restrict shared system usage, especially on devices handling sensitive information. Monitoring Windows File Explorer behavior and logs may provide early indicators of exploitation attempts. Once Microsoft releases patches, prompt deployment is critical. Additionally, educating users about the risks of local access and enforcing strong authentication mechanisms can reduce the likelihood of unauthorized local access. Regular vulnerability scanning and asset inventory to identify systems running the affected version will aid in targeted remediation.