CVE-2026-20919 is a vulnerability classified under CWE-362, indicating a race condition due to improper synchronization in concurrent execution within the Windows SMB Server component on Windows 10 Version 1809 (build 10.0.17763.0). The flaw occurs when multiple threads or processes access shared resources without adequate synchronization, leading to inconsistent or unexpected states. This race condition can be exploited by an attacker with authorized network access to the SMB service to elevate their privileges on the affected system. The vulnerability allows an attacker with low privileges to gain higher privileges, potentially leading to full system compromise. The CVSS v3.1 score is 7.5 (high), with vector metrics indicating network attack vector (AV:N), high attack complexity (AC:H), low privileges required (PR:L), no user interaction (UI:N), unchanged scope (S:U), and high impact on confidentiality, integrity, and availability (C:H/I:H/A:H). Although no public exploits are currently known, the vulnerability poses a significant risk due to the critical nature of SMB services in Windows environments and the potential for lateral movement and privilege escalation within networks. The vulnerability was reserved in December 2025 and published in January 2026, with no patch links currently available, indicating that remediation may still be pending or in progress.

For European organizations, this vulnerability presents a serious threat, especially in environments where Windows 10 Version 1809 is still in use. The SMB protocol is widely used for file sharing and network communications, making it a common attack vector. Successful exploitation could allow attackers to escalate privileges remotely, potentially leading to unauthorized access to sensitive data, disruption of services, and further compromise of internal network resources. Critical sectors such as finance, healthcare, government, and industrial control systems that rely on Windows SMB services could face increased risk of data breaches and operational disruptions. The high impact on confidentiality, integrity, and availability underscores the potential for significant damage, including data theft, ransomware deployment, or persistent footholds within networks. The requirement for network access and low privileges means that internal threat actors or attackers who have gained initial access could leverage this vulnerability to deepen their control over affected systems.

Organizations should prioritize upgrading or patching Windows 10 Version 1809 systems as soon as Microsoft releases an official fix. In the interim, restrict SMB access by implementing network segmentation and firewall rules to limit SMB traffic to trusted hosts only. Disable SMBv1 if still enabled, and consider disabling SMB services on systems where it is not required. Employ strict access controls and monitor SMB traffic for unusual patterns indicative of exploitation attempts. Use endpoint detection and response (EDR) tools to detect suspicious process behavior related to privilege escalation. Regularly audit user privileges and remove unnecessary administrative rights to reduce the attack surface. Additionally, ensure that all systems are running supported Windows versions, as Windows 10 Version 1809 is an older release and may no longer receive mainstream support, increasing exposure to vulnerabilities. Finally, maintain robust incident response plans to quickly address any signs of compromise related to this vulnerability.