CVE-2025-21418 is a high-severity heap-based buffer overflow vulnerability identified in the Windows Ancillary Function Driver for WinSock on Microsoft Windows 10 Version 1809 (build 10.0.17763.0). This vulnerability is classified under CWE-122, indicating a heap-based buffer overflow condition. The flaw allows an attacker with limited privileges (PR:L) and local access (AV:L) to execute a crafted sequence of operations that overflow a heap buffer within the WinSock driver, which is responsible for network socket operations. Exploiting this vulnerability does not require user interaction (UI:N), and the attack complexity is low (AC:L), meaning it can be reliably exploited by an attacker with local access and limited privileges. Successful exploitation results in elevation of privilege, granting the attacker higher system privileges than originally permitted. The impact on confidentiality, integrity, and availability is high (C:H/I:H/A:H), meaning the attacker can fully compromise the affected system, potentially executing arbitrary code, modifying system files, or causing system crashes. The scope remains unchanged (S:U), indicating the vulnerability affects only the vulnerable component and does not extend beyond the compromised system. The exploitability is functional (E:F), and the report is confirmed (RC:C), but currently, there are no known exploits in the wild. No official patch links are provided yet, indicating that mitigation may rely on workarounds or awaiting vendor updates. Given the vulnerability affects Windows 10 Version 1809, which is an older but still in-use version, systems running this version remain at risk until patched.

For European organizations, this vulnerability poses a significant risk, especially in environments where Windows 10 Version 1809 is still operational. The elevation of privilege can allow attackers to bypass security controls, install persistent malware, or move laterally within networks. Critical infrastructure, government agencies, and enterprises relying on legacy Windows 10 systems could face data breaches, operational disruptions, or ransomware attacks leveraging this vulnerability. The local attack vector limits remote exploitation, but insider threats or compromised user accounts could exploit this flaw. The high impact on confidentiality, integrity, and availability means sensitive data could be exposed or altered, and system availability could be compromised, affecting business continuity. Organizations with strict regulatory requirements under GDPR must consider the potential for data breaches and associated legal and financial consequences.

European organizations should prioritize upgrading or patching Windows 10 Version 1809 systems as soon as official patches become available from Microsoft. Until patches are released, organizations should implement strict access controls to limit local user privileges and restrict access to systems running the vulnerable OS version. Employing application whitelisting and endpoint detection and response (EDR) solutions can help detect anomalous behavior indicative of exploitation attempts. Network segmentation should be enforced to limit lateral movement from compromised hosts. Regularly auditing and monitoring local user activities can help identify potential exploitation attempts early. Additionally, organizations should consider migrating legacy systems to supported Windows versions to reduce exposure to unpatched vulnerabilities. If patching is not immediately feasible, disabling or restricting the use of the Windows Ancillary Function Driver for WinSock, if possible, may reduce attack surface, though this may impact network functionality and should be tested carefully.