CVE-2026-20925 is a vulnerability classified under CWE-73 (External Control of File Name or Path) affecting Microsoft Windows 10 Version 1809 (build 10.0.17763.0). The issue arises from improper handling of file names or paths within the Windows NTLM authentication mechanism, which is widely used for network authentication in Windows environments. An attacker without any privileges can exploit this vulnerability remotely over the network, requiring user interaction, to perform spoofing attacks. Spoofing in this context means the attacker can manipulate the file path or name to deceive the system or users, potentially redirecting authentication attempts or causing disclosure of sensitive information. The vulnerability impacts confidentiality by allowing unauthorized access or information leakage but does not affect system integrity or availability. The CVSS v3.1 score is 6.5 (medium), reflecting the network attack vector, low attack complexity, no privileges required, but requiring user interaction. No known public exploits or patches are currently available, indicating that the vulnerability is newly disclosed and not yet actively exploited. The vulnerability is particularly relevant for environments still running Windows 10 Version 1809, which is an older release with extended support in some enterprise contexts. The lack of patches necessitates interim mitigations to reduce exposure. The vulnerability's exploitation could facilitate lateral movement or credential theft within enterprise networks, especially where NTLM authentication is prevalent and legacy protocols are enabled.

For European organizations, this vulnerability poses a significant risk to confidentiality, especially in sectors relying on legacy Windows 10 Version 1809 systems and NTLM authentication. Attackers could leverage this flaw to spoof authentication requests, potentially gaining unauthorized access to sensitive data or network resources. This could lead to data breaches, exposure of intellectual property, or compromise of personal data protected under GDPR. The lack of impact on integrity and availability reduces the risk of system downtime or data manipulation but does not diminish the threat to information confidentiality. Organizations with extensive internal networks using NTLM for authentication are particularly vulnerable to lateral movement attacks facilitated by this flaw. The requirement for user interaction means phishing or social engineering could be vectors for exploitation, increasing risk in environments with less mature security awareness. The absence of known exploits currently provides a window for proactive defense, but the medium severity score indicates that exploitation could have meaningful consequences if leveraged by skilled attackers. The threat is heightened in critical infrastructure, government, and financial sectors where confidentiality breaches can have severe operational and reputational impacts.

1. Disable or restrict NTLM authentication where possible, replacing it with more secure protocols like Kerberos. 2. Implement strict network segmentation to limit the spread of attacks leveraging NTLM spoofing. 3. Enforce least privilege access controls to minimize the impact of compromised credentials. 4. Educate users to recognize and avoid suspicious files or links that could trigger user interaction-based exploits. 5. Monitor network traffic for unusual NTLM authentication attempts or anomalies indicative of spoofing. 6. Apply all available Windows updates and security patches promptly once Microsoft releases a fix for this vulnerability. 7. Use endpoint detection and response (EDR) tools to detect and respond to suspicious activities related to NTLM authentication. 8. Consider deploying application whitelisting to prevent execution of unauthorized or malicious files that could exploit the vulnerability. 9. Review and harden Group Policy settings related to NTLM usage and authentication protocols. 10. Conduct regular vulnerability assessments and penetration testing focusing on authentication mechanisms and legacy systems.