Critical Windows Vulnerability Exploited: Immediate Action Required
Microsoft has urgently released security updates to address a zero-day vulnerability in the Windows Cloud Files Mini Filter Driver (cldflt.sys), identified as CVE-2025-62221. This elevation of privilege flaw is actively being exploited in the wild, posing a significant risk to a broad spectrum of Windows operating systems, including Windows 10 Version 1809 through the latest Windows 11 Version 25H2, as well as Windows Server 2025.
Understanding the Vulnerability
CVE-2025-62221 is classified as a Use-After-Free vulnerability within the Cloud Files Mini Filter Driver, a critical kernel component responsible for managing placeholders and synchronization for cloud storage services like OneDrive. This driver enables the operating system to treat cloud-stored files as local entries without downloading their full content, hydrating them only upon access.
The flaw allows a locally authenticated, low-privilege attacker to trigger a memory corruption state, subsequently enabling the execution of arbitrary code with the highest system privileges. This means that an attacker who has already gained local access can escalate their privileges to SYSTEM level, granting them complete control over the affected machine.
Severity and Exploitation
Microsoft has rated this vulnerability as Important, assigning it a CVSS v3.1 base score of 7.8. The Microsoft Threat Intelligence Center (MSTIC) and the Microsoft Security Response Center (MSRC) have confirmed that functional exploit code is being used in the wild to gain SYSTEM privileges on compromised machines.
While the attack complexity is low and requires no user interaction, the attacker must have established local access to the target machine. This suggests that the vulnerability is likely being utilized as a secondary stage in attack chains, where adversaries have already gained a foothold and seek to escalate their privileges to persist or disable security controls.
Affected Versions and Security Updates
The following table outlines the affected Windows versions and the corresponding Knowledge Base (KB) articles released on December 9, 2025. Administrators should prioritize patching these systems immediately, given the confirmed active exploitation status.
| Product Family | Version / Edition | KB Article (Security Update) | Build Number |
|————————–|—————————–|——————————|——————–|
| Windows 11 & Server 2025 | Version 25H2 (x64/ARM64) | KB5072033 / KB5072014 | 10.0.26200.7462 |
| | Version 24H2 (x64/ARM64) | KB5072033 / KB5072014 | 10.0.26100.7462 |
| | Version 23H2 (x64/ARM64) | KB5071417 | 10.0.22631.6345 |
| | Server 2025 (Core) | KB5072033 | 10.0.26100.7462 |
| Windows 10 | Version 22H2 (x64/ARM64/32-bit) | KB5071546 | 10.0.19045.6691 |
| | Version 21H2 (x64/ARM64/32-bit) | KB5071546 | 10.0.19044.6691 |
| | Version 1809 (x64/32-bit) | KB5071544 | 10.0.17763.8146 |
| Windows Server | Server 2022 (Standard & Core) | KB5071547 / KB5071413 | 10.0.20348.4529 |
| | Server 2022, 23H2 Edition | KB5071542 | 10.0.25398.2025 |
| | Server 2019 (Standard & Core) | KB5071544 | 10.0.17763.8146 |
Implications for Organizations
This zero-day vulnerability presents a significant risk to organizations relying on Windows infrastructure, particularly given the confirmed exploitation in the wild. The Official Fix remediation level indicates that standard security updates are sufficient to resolve the issue, and no temporary workarounds have been published.
Security teams should verify that the specific build numbers listed above are reflected on their endpoints after the update deployment to ensure successful mitigation. The absence of required user interaction makes this an attractive vector for automated malware and advanced persistent threats (APTs) operating within a network.
Recommendations
1. Immediate Patching: Apply the security updates provided by Microsoft without delay to mitigate the risk associated with this vulnerability.
2. Verify Deployment: Ensure that the updates have been successfully deployed by checking the build numbers against those listed in the table above.
3. Monitor Systems: Continuously monitor systems for unusual activity that may indicate exploitation attempts, such as unexpected privilege escalations or unauthorized access.
4. Limit Local Access: Restrict local access to systems to trusted personnel only, reducing the risk of an attacker gaining the initial foothold necessary to exploit this vulnerability.
5. Educate Users: Inform users about the importance of security updates and the risks associated with delaying their application.
Conclusion
The discovery and active exploitation of CVE-2025-62221 underscore the critical importance of timely security updates and vigilant system monitoring. Organizations must act swiftly to apply the necessary patches and implement robust security measures to protect their systems from potential compromise.
