CVE-2025-21369 is a heap-based buffer overflow vulnerability identified in the Digest Authentication mechanism of Microsoft Windows 10 Version 1809 (build 10.0.17763.0). This vulnerability arises from improper handling of input data during the authentication process, leading to memory corruption on the heap. Exploitation allows an attacker with low privileges to execute arbitrary code remotely without requiring user interaction, leveraging network access to the vulnerable service. The vulnerability affects confidentiality, integrity, and availability by enabling remote code execution, potentially allowing attackers to take full control of affected systems. The CVSS v3.1 base score is 8.8, reflecting high severity due to network attack vector, low attack complexity, no user interaction, and high impact on system security properties. Although no public exploits are currently known, the vulnerability's characteristics make it a critical concern for organizations still running Windows 10 Version 1809, which is an older but still deployed OS version in some environments. The lack of available patches at the time of disclosure necessitates immediate risk mitigation through network controls and monitoring until official updates are released.

For European organizations, this vulnerability poses a significant threat, especially in sectors where Windows 10 Version 1809 remains operational due to legacy application dependencies or delayed upgrade cycles. Successful exploitation could lead to full system compromise, data breaches, disruption of services, and lateral movement within networks. Critical infrastructure, government agencies, and enterprises with sensitive data are at heightened risk. The remote code execution capability without user interaction facilitates stealthy attacks that can bypass traditional endpoint defenses. The vulnerability could be leveraged in targeted attacks or automated campaigns once exploit code becomes available, potentially impacting confidentiality of personal and corporate data, integrity of systems and applications, and availability of critical services.

Organizations should prioritize upgrading affected systems to a supported and patched Windows version as soon as Microsoft releases an official fix. In the interim, restrict network access to services using Digest Authentication, especially from untrusted networks. Employ network segmentation and firewall rules to limit exposure of vulnerable hosts. Monitor authentication logs and network traffic for anomalies indicative of exploitation attempts. Implement intrusion detection/prevention systems with updated signatures once available. Conduct thorough asset inventories to identify all systems running Windows 10 Version 1809 and assess their exposure. Consider disabling Digest Authentication if feasible or replacing it with more secure authentication methods. Maintain up-to-date backups and incident response plans to mitigate potential impacts of exploitation.