CVE-2025-54099 is a high-severity stack-based buffer overflow vulnerability identified in the Windows Ancillary Function Driver for WinSock component of Microsoft Windows 10 Version 1809 (build 10.0.17763.0). This vulnerability arises from improper handling of input data within the WinSock driver, leading to a stack buffer overflow condition. An authorized local attacker with low privileges can exploit this flaw to execute arbitrary code in kernel mode, thereby elevating their privileges to SYSTEM level. The vulnerability is classified under CWE-121, indicating a classic stack-based buffer overflow, which can corrupt the stack and control the execution flow. The CVSS v3.1 base score is 7.0, reflecting high severity, with attack vector local (AV:L), attack complexity high (AC:H), privileges required low (PR:L), no user interaction (UI:N), and impacts on confidentiality, integrity, and availability all rated high (C:H/I:H/A:H). No known exploits are currently reported in the wild, and no patches have been linked yet, suggesting that mitigation is pending. The vulnerability requires local access and some level of user privileges but does not require user interaction, making it a significant risk for systems running this specific Windows 10 version, especially in environments where users have low-level access but should not have elevated privileges. The flaw could be leveraged by attackers to gain full control over affected systems, potentially leading to data breaches, system compromise, or lateral movement within networks.

For European organizations, the impact of CVE-2025-54099 can be substantial, particularly in sectors relying on legacy Windows 10 Version 1809 systems. The ability for a low-privileged local user to escalate privileges to SYSTEM level threatens the confidentiality, integrity, and availability of critical systems and sensitive data. This vulnerability could be exploited by malicious insiders or attackers who gain initial foothold through phishing or other means to escalate privileges and deploy ransomware, steal intellectual property, or disrupt operations. Organizations in finance, healthcare, government, and critical infrastructure sectors are especially at risk due to the sensitive nature of their data and regulatory requirements such as GDPR. The lack of a patch at the time of disclosure increases the window of exposure. Additionally, since the vulnerability affects a core networking component (WinSock), exploitation could facilitate further network-based attacks or persistence mechanisms. The high attack complexity somewhat limits exploitation but does not eliminate the risk, especially in environments with skilled attackers or automated exploit development.

Given the absence of an official patch, European organizations should implement immediate compensating controls. First, restrict local user privileges strictly, ensuring users operate with the least privilege necessary and removing unnecessary local accounts. Employ application whitelisting and endpoint protection solutions capable of detecting anomalous behavior indicative of privilege escalation attempts. Network segmentation should be enforced to limit lateral movement if exploitation occurs. Monitoring and logging of local privilege escalation attempts should be enhanced, focusing on unusual WinSock driver activity or system calls. Organizations should plan rapid deployment of the official patch once released by Microsoft. Additionally, consider upgrading affected systems to a supported and patched Windows version, as Windows 10 Version 1809 is an older release with limited support. Conduct user awareness training to reduce the risk of initial compromise vectors that could lead to local access. Finally, implement strict control over software installation and execution policies to minimize the risk of malicious code execution.