CVE-2026-20840 is a heap-based buffer overflow vulnerability classified under CWE-122 affecting the NTFS file system driver in Microsoft Windows 10 Version 1809 (build 10.0.17763.0). This vulnerability arises when the NTFS driver improperly handles certain operations, leading to a buffer overflow on the heap. An attacker with authorized local access and limited privileges can exploit this flaw to execute arbitrary code in kernel mode, potentially escalating privileges and gaining full control over the affected system. The vulnerability does not require user interaction, which increases its risk profile once local access is obtained. The CVSS 3.1 base score is 7.8, indicating high severity, with attack vector local, attack complexity low, privileges required low, and no user interaction needed. The impact on confidentiality, integrity, and availability is high, as successful exploitation can lead to complete system compromise. No known exploits are currently reported in the wild, and no official patches have been linked yet, though the vulnerability was published on January 13, 2026. Given the affected Windows version is relatively old, many organizations may have migrated to newer versions, but legacy systems remain at risk. The vulnerability's exploitation could be leveraged in targeted attacks or lateral movement scenarios within compromised networks.

For European organizations, this vulnerability poses a significant risk particularly to those still operating legacy Windows 10 Version 1809 systems, which may be common in industrial control systems, government agencies, and enterprises with slow upgrade cycles. Exploitation could lead to local privilege escalation, enabling attackers to bypass security controls, deploy malware, or move laterally within networks. The high impact on confidentiality, integrity, and availability means sensitive data could be exposed or altered, and critical services disrupted. In sectors such as finance, healthcare, and critical infrastructure, this could result in severe operational and reputational damage. The lack of known exploits reduces immediate risk but does not eliminate the threat, as attackers could develop exploits given the vulnerability details. European organizations must consider the risk of insider threats or compromised user accounts that could leverage this vulnerability for further compromise.

1. Immediately inventory and identify all systems running Windows 10 Version 1809 (build 10.0.17763.0) within the organization. 2. Monitor Microsoft security advisories closely for official patches or workarounds and apply them promptly once available. 3. Restrict local access to critical systems by enforcing strict access controls and limiting administrative privileges. 4. Employ application whitelisting and endpoint detection and response (EDR) solutions to detect anomalous behavior indicative of exploitation attempts. 5. Use network segmentation to isolate legacy systems and reduce the attack surface. 6. Conduct regular security awareness training to reduce the risk of credential compromise that could enable local exploitation. 7. Consider upgrading affected systems to supported Windows versions where this vulnerability is not present. 8. Implement strict logging and monitoring of NTFS-related system calls and kernel events to detect potential exploitation activities early. 9. Use vulnerability scanning tools to identify unpatched or vulnerable systems proactively. 10. Develop and test incident response plans specifically addressing local privilege escalation scenarios.