CVE-2025-27738 is a vulnerability identified in Microsoft Windows 10 Version 21H2, specifically affecting the Windows Resilient File System (ReFS). The issue is categorized under CWE-284, which pertains to improper access control. This vulnerability allows an authorized attacker—meaning one who already has some level of access privileges—to disclose sensitive information over a network. The flaw lies in the way ReFS enforces access control, potentially permitting unauthorized data exposure without requiring user interaction. The vulnerability has a CVSS 3.1 base score of 6.5, indicating a medium severity level. The attack vector is network-based (AV:N), with low attack complexity (AC:L), requiring privileges (PR:L), no user interaction (UI:N), and impacts confidentiality (C:H) but not integrity or availability. The scope remains unchanged (S:U). No known exploits are currently in the wild, and no patches have been linked yet. The vulnerability was reserved in early March 2025 and published in April 2025. Given that ReFS is a file system designed for high resilience and data integrity, improper access control here could lead to unauthorized disclosure of sensitive files or metadata over network shares or remote access scenarios, potentially exposing confidential organizational data.

For European organizations, this vulnerability poses a significant risk to data confidentiality, especially for entities relying on Windows 10 Version 21H2 with ReFS-formatted volumes. Sectors such as finance, healthcare, government, and critical infrastructure that handle sensitive personal or proprietary data could face unauthorized data disclosure if attackers exploit this flaw. Since the vulnerability requires an attacker to have some level of authorized access, the threat is particularly relevant in environments where internal threat actors or compromised accounts exist. The network-based attack vector means that attackers could potentially exploit this vulnerability remotely within corporate networks or through VPN connections, increasing the risk of lateral movement and data leakage. Given the GDPR regulations in Europe, unauthorized data disclosure could lead to regulatory penalties and reputational damage. The lack of known exploits in the wild currently reduces immediate risk but does not eliminate the threat, especially as attackers may develop exploits once patches are released or if the vulnerability becomes publicly known.

European organizations should prioritize the following specific mitigations: 1) Conduct an inventory to identify systems running Windows 10 Version 21H2 with ReFS volumes and assess their exposure to network access. 2) Restrict network access to ReFS shares using network segmentation and strict firewall rules, limiting access to only trusted users and systems. 3) Implement the principle of least privilege rigorously to reduce the number of accounts with access rights sufficient to exploit this vulnerability. 4) Monitor network traffic and file system access logs for unusual or unauthorized access patterns to ReFS volumes. 5) Apply any forthcoming security updates from Microsoft promptly once patches are released. 6) Consider disabling or limiting the use of ReFS in environments where it is not essential, or use alternative file systems until the vulnerability is addressed. 7) Enhance multi-factor authentication and account security to prevent unauthorized access that could lead to exploitation. 8) Educate internal users about the risks of credential compromise and insider threats to reduce the likelihood of authorized attackers exploiting this vulnerability.