CVE-2025-21391 is a vulnerability classified under CWE-59, which involves improper link resolution before file access, commonly referred to as 'link following'. This flaw exists in Microsoft Windows 10 Version 1507 (build 10.0.10240.0), specifically within the Windows Storage subsystem. The vulnerability allows an attacker with limited privileges (local access with some rights) to manipulate symbolic links or junction points to trick the system into accessing or modifying files in unintended locations. Because the system improperly resolves these links before file operations, an attacker can escalate their privileges by overwriting or corrupting critical files, leading to integrity and availability impacts. The vulnerability does not require user interaction, making it easier to exploit once local access is obtained. The CVSS v3.1 score of 7.1 indicates a high severity level, with attack vector local, low attack complexity, low privileges required, no user interaction, and impacts on integrity and availability. Although no public exploits are known at this time and no official patches have been linked, the vulnerability poses a significant risk to systems running this outdated Windows 10 version. The flaw could be leveraged in targeted attacks or combined with other vulnerabilities to gain full system control.

The primary impact of CVE-2025-21391 is elevation of privilege, enabling attackers with limited local access to gain higher privileges on affected Windows 10 Version 1507 systems. This can lead to unauthorized modification or deletion of critical system files, potentially causing system instability, denial of service, or persistence mechanisms for further attacks. Organizations relying on legacy Windows 10 installations are at risk of internal threat actors or malware exploiting this vulnerability to compromise system integrity and availability. The lack of user interaction requirement increases the risk of automated or stealthy exploitation. While confidentiality is not directly impacted, the integrity and availability consequences can disrupt business operations, especially in environments where legacy systems are critical. The absence of known exploits currently limits immediate widespread impact, but the vulnerability remains a significant threat if weaponized.

To mitigate CVE-2025-21391, organizations should prioritize upgrading from Windows 10 Version 1507 to a supported and patched version of Windows 10 or later. Since no patches are currently linked for this specific vulnerability, upgrading the OS is the most effective mitigation. Additionally, restrict local user privileges to the minimum necessary to reduce the risk of exploitation. Implement strict access controls and monitor for suspicious symbolic link or junction point creation activities. Employ endpoint detection and response (EDR) solutions capable of detecting abnormal file system operations related to link following. Regularly audit and harden file system permissions to prevent unauthorized modifications. Network segmentation and limiting local access to critical systems can further reduce exposure. Finally, maintain up-to-date backups to recover from potential integrity or availability impacts.