CVE-2025-21418 is a heap-based buffer overflow vulnerability classified under CWE-122, affecting the Windows Ancillary Function Driver for WinSock on Windows 10 Version 1809 (build 10.0.17763.0). This vulnerability allows an attacker with limited privileges (PR:L) and local access (AV:L) to execute a carefully crafted input that overflows a heap buffer, leading to memory corruption. The flaw can be exploited without user interaction (UI:N) and does not require elevated privileges initially, but it enables elevation of privilege (EoP) by corrupting memory structures or function pointers within the WinSock driver. Successful exploitation can compromise confidentiality, integrity, and availability of the affected system, potentially allowing attackers to execute arbitrary code with SYSTEM-level privileges. The CVSS v3.1 base score is 7.8, reflecting high severity due to the combination of local attack vector, low attack complexity, and high impact on all security properties. No public exploits or patches are currently available, but the vulnerability is officially published and reserved since December 2024. The Ancillary Function Driver for WinSock is a core networking component, making this vulnerability critical for systems relying on Windows 10 1809, a version that is out of mainstream support and still in use in some environments. The lack of user interaction requirement and low complexity make it a viable threat for attackers with local access, including malicious insiders or attackers who have gained limited foothold on a system.

For European organizations, this vulnerability poses a significant risk, especially for those still operating legacy Windows 10 Version 1809 systems. The ability to elevate privileges locally can lead to full system compromise, enabling attackers to bypass security controls, access sensitive data, disrupt services, or deploy ransomware. Critical infrastructure sectors such as finance, healthcare, and government agencies, which often have legacy systems, are particularly vulnerable. The confidentiality of sensitive personal and corporate data could be breached, integrity of systems compromised, and availability disrupted through potential denial-of-service conditions or malware deployment. Since the vulnerability requires local access, the threat is heightened in environments with weak internal network segmentation or insufficient endpoint protection. The absence of known exploits currently provides a window for proactive mitigation, but the high severity score and ease of exploitation underscore the urgency for remediation in European enterprises.

1. Upgrade affected systems from Windows 10 Version 1809 to a currently supported Windows version with active security updates, as 1809 is out of mainstream support and unlikely to receive patches promptly. 2. Apply any security updates or patches released by Microsoft addressing this vulnerability as soon as they become available. 3. Implement strict local access controls and limit administrative privileges to reduce the risk of local exploitation. 4. Employ endpoint detection and response (EDR) solutions to monitor for anomalous local activity indicative of privilege escalation attempts. 5. Enforce network segmentation to limit lateral movement opportunities for attackers who gain local access. 6. Conduct regular vulnerability scanning and system audits to identify and remediate legacy systems still running Windows 10 1809. 7. Educate IT staff and users about the risks of local privilege escalation vulnerabilities and the importance of timely patching and system upgrades. 8. Consider application whitelisting and enhanced logging to detect and prevent exploitation attempts targeting the WinSock driver.