CVE-2026-50401: Windows Cloud Files Driver Vulnerability

CVE-2026-50401 Overview

CVE-2026-50401 is an out-of-bounds read vulnerability [CWE-125] in the Windows Cloud Files Mini Filter Driver (cldflt.sys). An authorized local attacker can exploit the flaw to read memory outside of intended buffer boundaries, resulting in local information disclosure. Microsoft published the advisory on July 14, 2026.

The vulnerability requires local access and low-level privileges. It does not permit code execution or data modification, but it can expose sensitive kernel or process memory contents to a logged-on user. Successful exploitation compromises confidentiality without affecting integrity or availability.

Critical Impact
An authorized local user can trigger an out-of-bounds read in the Cloud Files Mini Filter Driver to disclose sensitive in-memory data on affected Windows systems.

Affected Products

Microsoft Windows (Cloud Files Mini Filter Driver component)
Refer to the Microsoft Security Update CVE-2026-50401 advisory for the definitive list of affected builds
Systems with the Cloud Files feature enabled (used by OneDrive and other cloud sync providers)

Discovery Timeline

2026-07-14 - CVE-2026-50401 published to NVD
2026-07-15 - Last updated in NVD database

Technical Details for CVE-2026-50401

Vulnerability Analysis

The Cloud Files Mini Filter Driver (cldflt.sys) is a kernel-mode file system filter that supports the Windows Cloud Files API. This API underpins on-demand file synchronization providers such as OneDrive Files On-Demand. The driver mediates I/O between user-mode sync engines and the file system, handling reparse points, placeholder files, and hydration requests.

The vulnerability is an out-of-bounds read [CWE-125]. The driver reads data past the end of an allocated buffer when processing a specific input or IOCTL. Because the read occurs in kernel context, the returned bytes can include adjacent kernel memory contents that should not be exposed to user mode.

Exploitation requires an authenticated local user session. The attacker interacts with the driver through user-mode APIs or direct device I/O to trigger the boundary condition. The impact is limited to information disclosure, which can include memory addresses useful for bypassing Kernel Address Space Layout Randomization (KASLR) or fragments of sensitive data.

Root Cause

The root cause is missing or insufficient bounds checking on a length or offset field that governs a memory read within cldflt.sys. When the driver trusts an attacker-influenced size parameter without validating it against the actual buffer allocation, the read crosses the buffer boundary and returns adjacent memory to the caller.

Attack Vector

The attack vector is local. An authenticated user with standard privileges opens a handle to the Cloud Files driver interface and submits a crafted request that causes the vulnerable read path to execute. No user interaction from another account is required. The disclosed memory is returned to the attacker's process, where it can be inspected offline.

No public proof-of-concept code or exploit is available at time of publication. Technical specifics are limited to the information in the Microsoft Security Update CVE-2026-50401 advisory.

Detection Methods for CVE-2026-50401

Indicators of Compromise

No file-based indicators of compromise are published for CVE-2026-50401 because the flaw produces no persistent artifacts on disk.
Unusual user-mode processes opening handles to the Cloud Files filter driver device object outside of legitimate sync clients (OneDrive.exe, approved cloud storage providers) warrant investigation.
Repeated or anomalous FSCTL calls targeting Cloud Files reparse points from non-standard binaries may indicate probing activity.

Detection Strategies

Monitor process creation and image load events for unsigned or unexpected binaries interacting with cldflt.sys or the Cloud Files API surface.
Correlate local user sessions with unusual kernel information disclosure primitives such as repeated small reads that fail with STATUS_INVALID_PARAMETER followed by successful reads.
Baseline legitimate cloud sync client behavior and alert on deviations, including new processes issuing Cloud Files IOCTLs.

Monitoring Recommendations

Enable Windows audit policies for kernel object access and forward Sysmon Event ID 1 (process creation) and Event ID 7 (image load) to a centralized log store.
Track patch status for cldflt.sys across the fleet and flag hosts running pre-July 2026 file versions.
Review endpoint telemetry for local privilege escalation chains that pair information disclosure with subsequent exploitation attempts.

How to Mitigate CVE-2026-50401

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft Security Update CVE-2026-50401 advisory to all Windows systems where the Cloud Files feature is present.
Prioritize patching on multi-user systems, terminal servers, and workstations that host sensitive data or credentials.
Verify that endpoint protection and EDR agents are current and forwarding kernel telemetry.

Patch Information

Microsoft addressed CVE-2026-50401 in the July 2026 security update cycle. The fix is delivered through the standard Windows Update channel. Consult the Microsoft Security Update CVE-2026-50401 advisory for the specific KB article and updated cldflt.sys file version corresponding to each supported Windows release.

Workarounds

No official workaround is published by Microsoft. Applying the security update is the supported remediation.
On systems that do not require cloud file sync, disabling or removing cloud storage clients that depend on the Cloud Files API reduces the attack surface.
Restrict interactive logon and local account access on high-value systems to limit which users can reach the vulnerable code path.

bash

# Verify the installed cldflt.sys version on a Windows host
Get-Item C:\Windows\System32\drivers\cldflt.sys | Select-Object VersionInfo

# Confirm the July 2026 cumulative update is installed
Get-HotFix | Sort-Object -Property InstalledOn -Descending | Select-Object -First 10