CVE-2025-21391 is a vulnerability classified under CWE-59, which involves improper link resolution before file access, commonly known as 'link following'. This flaw exists in Microsoft Windows 10 Version 1809 (build 10.0.17763.0) and allows an attacker with limited privileges on the system to escalate their privileges to a higher level. The vulnerability arises because the Windows Storage component does not correctly handle symbolic links or junction points before accessing files, enabling an attacker to manipulate file system links to redirect file operations to unintended locations. This can result in unauthorized modification or deletion of files, compromising system integrity and availability. The CVSS v3.1 base score is 7.1, reflecting high severity, with attack vector local (AV:L), low attack complexity (AC:L), requiring low privileges (PR:L), no user interaction (UI:N), unchanged scope (S:U), no confidentiality impact (C:N), high integrity (I:H), and high availability (A:H) impacts. The exploitability is functional (E:F), with official remediation (RL:O) and confirmed report confidence (RC:C). Although no exploits are currently known in the wild, the vulnerability poses a significant risk, especially in environments where Windows 10 Version 1809 is still in use. The flaw can be leveraged by local attackers to gain elevated privileges, potentially leading to full system compromise or disruption of critical services.

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, and government agencies. Successful exploitation can lead to unauthorized modification or deletion of critical files, undermining system integrity and availability. This could disrupt business operations, cause data loss, or facilitate further attacks by enabling privilege escalation. The vulnerability's local attack vector means that insider threats or attackers who gain initial access through other means could leverage this flaw to deepen their foothold. Given the high integrity and availability impact, critical infrastructure and services could be at risk, potentially affecting public safety and economic stability. The absence of known exploits in the wild provides a window for proactive mitigation, but organizations must act swiftly to prevent exploitation.

1. Apply official patches or updates from Microsoft as soon as they become available to address this vulnerability directly. 2. If patches are not yet available, restrict local user privileges to the minimum necessary and enforce strict access controls on file system objects, especially those involving symbolic links or junction points. 3. Implement enhanced monitoring and alerting for suspicious file system activities, such as unexpected link creation or modification attempts. 4. Use application whitelisting and endpoint protection solutions capable of detecting and blocking privilege escalation attempts. 5. Conduct regular audits of systems running Windows 10 Version 1809 to identify and remediate unauthorized changes or suspicious behavior. 6. Consider upgrading affected systems to a supported and patched Windows version to reduce exposure to legacy vulnerabilities. 7. Educate IT staff and users about the risks of local privilege escalation and the importance of maintaining strict security hygiene.