CVE-2025-21336 is a cryptographic information disclosure vulnerability identified in Microsoft Windows 10 Version 1809 (build 10.0.17763.0). The vulnerability is classified under CWE-203, which refers to Observable Discrepancy, indicating that an attacker can infer sensitive information by observing differences in system behavior or responses. Specifically, this flaw allows an attacker with low-level privileges (PR:L) and local access (AV:L) to gain unauthorized access to cryptographic information without requiring user interaction (UI:N). The vulnerability has a CVSS v3.1 base score of 5.6, categorized as medium severity. The attack complexity is high (AC:H), meaning exploitation requires specific conditions or expertise, and the scope is changed (S:C), indicating that the vulnerability affects resources beyond the initially vulnerable component. The impact is primarily on confidentiality (C:H), with no impact on integrity or availability. No known exploits are currently in the wild, and no patches have been linked yet. The vulnerability could allow attackers to extract sensitive cryptographic material, potentially undermining encryption protections and exposing confidential data or cryptographic keys. Since the vulnerability requires local access and elevated privileges, it is more likely to be exploited in environments where attackers have already gained some foothold or insider access. The lack of user interaction requirement increases the risk in automated or unattended systems. The observable discrepancy nature suggests side-channel or timing-based attacks that leak information indirectly, complicating detection and mitigation.

For European organizations, this vulnerability poses a moderate risk primarily to confidentiality of sensitive data protected by Windows 10 Version 1809 cryptographic services. Organizations relying on legacy Windows 10 systems, particularly in sectors handling sensitive personal data (e.g., finance, healthcare, government), could face exposure of cryptographic keys or encrypted data. This could lead to data breaches, loss of trust, and regulatory non-compliance under GDPR. Since exploitation requires local access and elevated privileges, the threat is more significant in environments with weak internal access controls or where insider threats are a concern. The vulnerability could also facilitate lateral movement within networks if attackers leverage it after initial compromise. The absence of impact on integrity and availability limits the risk of system disruption or data tampering but does not diminish the seriousness of confidential data exposure. The medium severity rating reflects these factors, but organizations should not underestimate the potential consequences of cryptographic information leakage.

1. Upgrade and Patch: Although no patches are currently linked, organizations should monitor Microsoft security advisories closely and apply any forthcoming updates promptly to Windows 10 Version 1809 systems. 2. Limit Local Access: Enforce strict access controls to minimize the number of users with local access and low-level privileges on affected systems. Use the principle of least privilege to reduce attack surface. 3. Network Segmentation: Isolate legacy Windows 10 systems from critical network segments to prevent lateral movement if compromise occurs. 4. Monitor for Anomalies: Implement enhanced monitoring and logging to detect unusual access patterns or attempts to exploit cryptographic services. 5. Upgrade OS Versions: Plan migration away from Windows 10 Version 1809 to supported, updated Windows versions with improved security posture. 6. Harden Cryptographic Usage: Where possible, use hardware security modules (HSMs) or external cryptographic providers that are not vulnerable to this flaw. 7. Insider Threat Controls: Strengthen insider threat detection and response capabilities, including user behavior analytics and strict privilege management. 8. Incident Response Preparedness: Prepare incident response plans that consider cryptographic key compromise scenarios to minimize damage if exploitation occurs.