CVE-2025-21189 is a security vulnerability identified in Microsoft Windows 10 Version 1809 (build 10.0.17763.0) related to improper resolution of path equivalence, classified under CWE-41. Specifically, this vulnerability involves a security feature bypass in the MapUrlToZone function, which is responsible for determining the security zone of a given URL or path. The flaw allows an attacker to manipulate path equivalence resolution, potentially causing the system to incorrectly assign a URL or file path to a less restrictive security zone than intended. This can lead to bypassing security restrictions that are normally enforced based on zone assignments, such as those controlling script execution, file downloads, or other security policies. The vulnerability has a CVSS v3.1 base score of 4.3, indicating a medium severity level. The vector string (AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N/E:P/RL:O/RC:C) shows that the attack can be performed remotely over the network without privileges but requires user interaction. The impact is limited to confidentiality with no direct impact on integrity or availability. No known exploits are currently reported in the wild, and no official patches have been linked yet. The vulnerability was published on January 14, 2025, and was reserved in December 2024. The improper path equivalence resolution could be leveraged by attackers to trick the system into treating malicious content as if it originated from a trusted zone, thereby bypassing security controls and potentially exposing sensitive information or enabling further attacks that rely on zone-based restrictions.

For European organizations, this vulnerability poses a moderate risk primarily in environments where Windows 10 Version 1809 is still in use, particularly in legacy systems or specialized industrial setups that have not been upgraded. The security feature bypass could allow attackers to circumvent zone-based security policies, potentially exposing users to phishing attacks, malicious scripts, or unauthorized data disclosure. While the vulnerability does not directly affect system integrity or availability, the confidentiality impact could lead to leakage of sensitive information, which is critical under European data protection regulations such as GDPR. Organizations handling personal data, intellectual property, or sensitive business information could face compliance risks and reputational damage if exploited. The requirement for user interaction reduces the likelihood of automated widespread exploitation but does not eliminate targeted attacks, especially via social engineering or spear-phishing campaigns. The absence of known exploits in the wild suggests that immediate risk is moderate but vigilance is necessary, especially in sectors with high-value targets or regulatory scrutiny.

European organizations should prioritize upgrading or patching affected Windows 10 Version 1809 systems as soon as official fixes become available from Microsoft. In the interim, organizations should implement strict network segmentation and limit exposure of vulnerable systems to untrusted networks. Employing application whitelisting and enhanced endpoint protection can help detect and block attempts to exploit this vulnerability. User awareness training should be reinforced to reduce the risk of successful social engineering or phishing attacks that require user interaction. Additionally, organizations can audit and tighten zone security settings and policies to minimize the impact of incorrect zone assignments. Monitoring logs for unusual URL or path resolution behavior and deploying intrusion detection systems tuned to detect attempts to exploit path equivalence issues can provide early warning. Finally, planning for migration to supported and updated Windows versions will reduce exposure to this and other legacy vulnerabilities.