CVE-2026-20831 is a vulnerability classified under CWE-367, representing a time-of-check to time-of-use (TOCTOU) race condition in the Windows Ancillary Function Driver for WinSock on Microsoft Windows 10 Version 1809 (build 10.0.17763.0). This flaw arises when the system checks a resource or condition and then uses it, but the state changes between these two operations due to a race condition, allowing an attacker to exploit the timing gap. Specifically, an authorized local attacker can trigger this race condition to elevate their privileges on the affected system. The vulnerability does not require user interaction but does require local privileges (low-level privileges) to initiate the exploit. The CVSS v3.1 base score is 7.8, indicating a high severity with impacts rated as high on confidentiality, integrity, and availability. The attack vector is local (AV:L), with low attack complexity (AC:L), and privileges required are low (PR:L). The scope is unchanged (S:U), meaning the impact is limited to the vulnerable component's privileges. No known exploits are currently reported in the wild, but the vulnerability's nature makes it a significant risk for privilege escalation attacks. The affected product is Windows 10 Version 1809, which, despite being an older release, is still used in various enterprise and industrial environments. The lack of patch links suggests that a fix may be pending or not yet publicly available, emphasizing the need for vigilance and interim mitigations.

The vulnerability allows an authorized local attacker to elevate privileges, potentially gaining SYSTEM-level access on affected Windows 10 Version 1809 systems. This can lead to full compromise of confidentiality, integrity, and availability of the system. Attackers could install persistent malware, manipulate or exfiltrate sensitive data, disrupt services, or use the compromised system as a pivot point for lateral movement within a network. For European organizations, especially those in critical infrastructure, government, finance, and healthcare sectors that may still operate legacy Windows 10 1809 systems, this vulnerability poses a significant risk. Exploitation could result in data breaches, operational disruptions, and compliance violations under regulations such as GDPR. The local attack vector means that attackers need some level of access already, but this is often achievable through phishing, insider threats, or exploitation of other vulnerabilities. The absence of known exploits in the wild currently reduces immediate risk but does not eliminate the threat, as proof-of-concept or weaponized exploits could emerge.

1. Apply security patches from Microsoft immediately once they become available for Windows 10 Version 1809 to address this TOCTOU race condition. 2. Restrict local access to systems running Windows 10 1809 by enforcing strict access controls, limiting administrative privileges, and using endpoint protection solutions to detect suspicious activity. 3. Employ application whitelisting and behavior monitoring to detect anomalous use of the WinSock driver or privilege escalation attempts. 4. Conduct regular audits of user privileges and remove unnecessary local accounts or reduce their permissions. 5. Use virtualization or sandboxing for untrusted applications to limit the impact of potential local exploits. 6. Monitor system and security logs for unusual events related to network driver usage or privilege escalations. 7. Plan and execute migration strategies to newer, supported Windows versions to reduce exposure to legacy vulnerabilities. 8. Educate users and administrators about the risks of local privilege escalation and the importance of maintaining updated systems.