CVE-2025-21391 is a vulnerability classified under CWE-59 (Improper Link Resolution Before File Access) affecting Microsoft Windows 10 Version 1809 (build 10.0.17763.0). The issue arises from the Windows Storage component improperly handling symbolic links before accessing files, allowing an attacker with limited privileges to manipulate link resolution. This can lead to an elevation of privilege where the attacker can execute code or modify files with higher system privileges than intended. The vulnerability has a CVSS v3.1 base score of 7.1, indicating high severity, with the vector AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H. This means the attack requires local access, low complexity, and limited privileges but no user interaction, and it impacts system integrity and availability without compromising confidentiality. The flaw could be exploited to overwrite critical system files or execute malicious payloads, potentially disrupting system operations or enabling further attacks. No public exploits are currently known, but the vulnerability is publicly disclosed and should be addressed promptly. The lack of patch links suggests that fixes may be pending or in distribution. Given the persistence of Windows 10 Version 1809 in enterprise environments, especially in legacy systems, this vulnerability poses a significant risk if left unmitigated.

For European organizations, the impact of CVE-2025-21391 can be substantial, particularly in sectors relying on legacy Windows 10 Version 1809 systems such as manufacturing, healthcare, government, and critical infrastructure. Successful exploitation can lead to unauthorized modification or disruption of system files, causing integrity and availability issues. This could result in operational downtime, data loss, or the facilitation of further attacks such as ransomware or lateral movement within networks. Since the vulnerability requires local access, insider threats or attackers who have gained initial footholds could leverage this flaw to escalate privileges and compromise entire systems. The absence of confidentiality impact reduces the risk of direct data leakage, but the elevated privileges could indirectly lead to data exposure through subsequent attacks. European organizations with strict regulatory requirements (e.g., GDPR) may face compliance risks if systems are compromised due to unpatched vulnerabilities. The threat is heightened in environments where Windows 10 Version 1809 remains in use due to compatibility or upgrade constraints.

1. Apply official security patches from Microsoft immediately once available to remediate the vulnerability. 2. If patches are not yet released, restrict local user permissions to the minimum necessary to reduce the risk of exploitation. 3. Implement application whitelisting and endpoint protection solutions that can detect and block suspicious symbolic link creation or manipulation activities. 4. Monitor system logs and file system events for unusual symbolic link operations or privilege escalation attempts. 5. Enforce strict access controls and network segmentation to limit local access to critical systems running Windows 10 Version 1809. 6. Plan and accelerate migration away from Windows 10 Version 1809 to supported and updated Windows versions to reduce exposure to legacy vulnerabilities. 7. Conduct regular security awareness training to inform users about the risks of local exploits and insider threats. 8. Employ vulnerability scanning tools to identify systems running the affected version and prioritize remediation efforts accordingly.