CVE-2025-62221 Overview
CVE-2025-62221 is a use-after-free vulnerability [CWE-416] in the Windows Cloud Files Mini Filter Driver (cldflt.sys). An authenticated local attacker can exploit the flaw to elevate privileges to SYSTEM on affected Windows endpoints and servers. Microsoft published the advisory on December 9, 2025, and CISA has added the vulnerability to its Known Exploited Vulnerabilities (KEV) catalog, confirming active in-the-wild exploitation. The flaw affects every supported Windows 10, Windows 11, and Windows Server release. The Exploit Prediction Scoring System (EPSS) places the vulnerability in the 85th percentile, reflecting an elevated likelihood of broader exploitation.
Critical Impact
A local attacker holding standard user privileges can obtain SYSTEM-level code execution by triggering the use-after-free condition in the Cloud Files Mini Filter Driver.
Affected Products
- Microsoft Windows 10 (1809, 21H2, 22H2)
- Microsoft Windows 11 (23H2, 24H2, 25H2)
- Microsoft Windows Server 2019, 2022, 2022 23H2, and 2025
Discovery Timeline
- 2025-12-09 - CVE-2025-62221 published to NVD and Microsoft releases security patch
- 2025-12-10 - Last updated in NVD database
- CISA KEV - Added to the Known Exploited Vulnerabilities catalog
Technical Details for CVE-2025-62221
Vulnerability Analysis
The Cloud Files Mini Filter Driver (cldflt.sys) implements the kernel-mode component of the Windows Cloud Files API, which underpins on-demand file sync features used by OneDrive and other cloud storage providers. The driver mediates placeholder file operations between user-mode sync engines and the NTFS filesystem.
The vulnerability is a use-after-free condition [CWE-416] in this driver. The component frees a kernel object while a separate code path retains a dangling reference. Subsequent operations that dereference the stale pointer allow an attacker to manipulate freed memory before reuse.
Because cldflt.sys runs in kernel context, controlled reuse of the freed allocation enables arbitrary kernel-mode writes or function pointer hijacking. The result is privilege escalation from a standard authenticated user to NT AUTHORITY\SYSTEM.
Root Cause
The defect originates in object lifetime management within cldflt.sys. A reference count or synchronization primitive is released prematurely, leaving one or more handles pointing to memory that the kernel pool allocator can reassign to attacker-influenced data.
Attack Vector
Exploitation requires local access and low-privileged authentication. No user interaction is needed. An attacker invokes Cloud Files API operations from a sandboxed or standard user context, races the driver's object lifecycle, and grooms the kernel pool to land controlled data in the freed slot. The vulnerability is listed in the CISA KEV catalog, indicating confirmed exploitation in the wild.
No public proof-of-concept code has been released. Technical specifics are described in the Microsoft CVE-2025-62221 Advisory.
Detection Methods for CVE-2025-62221
Indicators of Compromise
- Unexpected SYSTEM-level processes spawned from user-context parent processes shortly after Cloud Files API activity.
- Kernel bugchecks or stack traces referencing cldflt.sys on hosts where OneDrive or other cloud sync clients are present.
- Newly created services, scheduled tasks, or local administrators following anomalous use of placeholder file operations.
Detection Strategies
- Hunt for non-OneDrive processes interacting heavily with cldflt.sys reparse points and placeholder files.
- Correlate Windows Error Reporting (WER) crash dumps that implicate cldflt.sys with subsequent privilege escalation events such as Event ID 4672 for unexpected accounts.
- Apply behavioral detections for token impersonation or parent-child process mismatches originating from standard user sessions.
Monitoring Recommendations
- Track patch deployment status against the Microsoft December 2025 security update for every Windows 10, Windows 11, and Windows Server SKU listed above.
- Monitor endpoints classified in the CISA KEV catalog and prioritize them for accelerated remediation.
- Alert on driver-level crashes followed by privileged process creation within a short time window.
How to Mitigate CVE-2025-62221
Immediate Actions Required
- Deploy the December 2025 Microsoft security update for CVE-2025-62221 to all affected Windows builds without delay.
- Treat the vulnerability as actively exploited and prioritize remediation in line with CISA KEV due-date guidance.
- Audit endpoints for prior exploitation indicators before considering systems clean.
Patch Information
Microsoft has released cumulative security updates that remediate the use-after-free in cldflt.sys. Refer to the Microsoft CVE-2025-62221 Advisory for the specific KB articles mapped to each affected Windows release. Confirm KEV listing requirements via the CISA Known Exploited Vulnerabilities Catalog.
Workarounds
- No vendor-supplied workaround removes the vulnerability; patching is the only complete remediation.
- Where patching must be staged, restrict interactive logon and remove unnecessary local accounts to reduce the population of users who can trigger the flaw.
- Disable the Cloud Files API surface on systems that do not require OneDrive or third-party cloud sync, recognizing that this will break placeholder file functionality.
# Verify the cldflt.sys driver version after patching
Get-Item C:\Windows\System32\drivers\cldflt.sys | Select-Object VersionInfo
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
