CVE-2026-20857 is a vulnerability identified in Microsoft Windows 11 Version 25H2 (build 10.0.26200.0) affecting the Windows Cloud Files Mini Filter Driver. The root cause is an untrusted pointer dereference (classified under CWE-822), which occurs when the driver improperly handles pointers that can be influenced by an attacker. This flaw allows an authorized local attacker—someone with limited privileges on the system—to dereference a pointer that has not been properly validated or sanitized, leading to memory corruption. Such corruption can be exploited to execute arbitrary code in kernel mode, thereby escalating privileges from a low-privilege user to SYSTEM level. The vulnerability does not require user interaction (UI:N) and has a low attack complexity (AC:L), but it does require local access (AV:L) and privileges (PR:L). The scope is unchanged (S:U), but the impact on confidentiality, integrity, and availability is high (C:H/I:H/A:H), meaning an attacker can fully compromise the system. No public exploits are known yet, but the vulnerability is rated high severity with a CVSS v3.1 score of 7.8. The Windows Cloud Files Mini Filter Driver is responsible for managing cloud file synchronization and caching, making this vulnerability particularly relevant in environments leveraging cloud storage integration. The flaw could be leveraged to bypass security controls and gain elevated privileges, potentially allowing attackers to install persistent malware, access sensitive data, or disrupt system operations.

For European organizations, this vulnerability poses a significant risk due to the widespread adoption of Windows 11 in enterprise environments. Successful exploitation could lead to full system compromise, allowing attackers to access sensitive corporate data, intellectual property, or disrupt critical services. Organizations relying on cloud file synchronization features are particularly vulnerable, as the flaw resides in the Cloud Files Mini Filter Driver. This could impact sectors such as finance, healthcare, government, and critical infrastructure, where confidentiality and integrity of data are paramount. The local privilege escalation aspect means that attackers who gain initial foothold through phishing, insider threats, or compromised credentials can leverage this vulnerability to escalate privileges and move laterally within networks. This could facilitate ransomware deployment or espionage activities. The absence of known exploits currently provides a window for proactive mitigation, but the high severity score underscores the urgency of addressing this issue to prevent future exploitation.

1. Apply official Microsoft security updates and patches for Windows 11 Version 25H2 as soon as they become available to remediate the vulnerability. 2. Until patches are released, restrict local access to critical systems by enforcing strict access controls and limiting administrative privileges. 3. Implement application whitelisting and endpoint detection and response (EDR) solutions to monitor and block suspicious activities related to cloud file operations and kernel-mode driver interactions. 4. Conduct regular audits of user accounts and privileges to minimize the number of users with local access rights. 5. Educate users about the risks of phishing and social engineering to reduce the likelihood of initial compromise. 6. Employ network segmentation to limit lateral movement opportunities if an attacker gains local access. 7. Monitor system logs and security alerts for unusual behavior indicative of privilege escalation attempts, focusing on events related to the Cloud Files Mini Filter Driver. 8. Consider disabling or restricting cloud file synchronization features in environments where they are not essential, reducing the attack surface.