CVE-2026-69279: Windows Cloud Files Use After Free Vulnerability

CVE-2026-69279 Overview

CVE-2026-69279 is a use-after-free vulnerability [CWE-416] in the Windows Cloud Files Mini Filter Driver (cldflt.sys). An authorized local attacker can trigger the flaw to elevate privileges on an affected system. Microsoft published the advisory on September 8, 2026, and updated it the following day.

The issue affects the kernel-mode filter driver that backs Windows Cloud Files functionality, including OneDrive placeholder file handling. Successful exploitation yields high impact to confidentiality, integrity, and availability on the local host.

Critical Impact
A low-privileged local user who wins a race in the Cloud Files Mini Filter Driver can execute code in kernel context, achieving full SYSTEM-level privilege escalation on the affected Windows host.

Affected Products

Microsoft Windows (Cloud Files Mini Filter Driver, cldflt.sys)
Windows client and server editions that ship the Cloud Files Mini Filter component
Refer to the Microsoft Security Update Guide for the authoritative list of affected builds

Discovery Timeline

2026-09-08 - CVE-2026-69279 published to NVD
2026-09-09 - Last updated in NVD database

Technical Details for CVE-2026-69279

Vulnerability Analysis

The vulnerability resides in the Windows Cloud Files Mini Filter Driver, the kernel component that mediates I/O between user-mode clients and cloud-backed placeholder files. A use-after-free condition allows a local attacker to reference kernel memory after it has been released, then reclaim that allocation with attacker-controlled data.

Exploitation requires an authenticated local account and involves a high-complexity condition, likely a race between concurrent I/O requests handled by the filter driver. Once the freed object is reused, subsequent kernel operations dereference attacker-influenced state, producing arbitrary kernel read, write, or control-flow hijack. The attacker gains SYSTEM privileges without user interaction.

See the Microsoft Security Update Guide for the vendor's technical description.

Root Cause

The root cause is improper lifetime management of a kernel object owned by cldflt.sys. One code path frees the object while another retains and later dereferences a stale pointer. This matches the [CWE-416] use-after-free pattern common to filter drivers that manage asynchronous callback and completion contexts.

Attack Vector

The attack vector is local. An attacker with a standard user account issues crafted I/O or IOCTL requests against the Cloud Files filter driver to trigger the free-and-reuse race. No network access and no user interaction are required. The high attack complexity reflects the timing precision needed to hit the race window and reclaim the freed allocation reliably.

No public proof-of-concept, exploit code, or in-the-wild exploitation has been reported at the time of publication. The CVE is not listed in the CISA Known Exploited Vulnerabilities catalog.

Detection Methods for CVE-2026-69279

Indicators of Compromise

Unexpected process token elevation on endpoints where a low-privileged user session spawns SYSTEM-context children.
Bugchecks or driver faults referencing cldflt.sys, which may indicate failed exploitation attempts.
Anomalous handle or IOCTL activity targeting the Cloud Files filter from non-OneDrive processes.

Detection Strategies

Monitor for parent-child process anomalies where user-context processes create services, scheduled tasks, or processes running as NT AUTHORITY\SYSTEM.
Alert on kernel crash dumps that name cldflt.sys in the faulting module, which can indicate exploitation attempts against the race window.
Correlate local privilege escalation behaviors with recent execution of unsigned or newly written binaries on the host.

Monitoring Recommendations

Ingest Windows kernel and WER (Windows Error Reporting) telemetry into a central analytics platform to surface repeated cldflt.sys faults.
Track Sysmon Event ID 1 and 10 for suspicious token duplication and process access patterns following user logon.
Review endpoint patch state continuously and flag hosts missing the September 2026 Microsoft security update.

How to Mitigate CVE-2026-69279

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft Security Update Guide to all affected Windows systems.
Prioritize patching multi-user hosts, jump servers, and VDI infrastructure where local accounts are commonly authenticated.
Audit local account inventories and remove unnecessary interactive logon rights to reduce the attacker population.

Patch Information

Microsoft has released a security update addressing CVE-2026-69279. Consult the Microsoft Security Update Guide for the KB article, build numbers, and download links applicable to each supported Windows version. Apply the update through Windows Update, WSUS, Microsoft Intune, or your standard patch management workflow.

Workarounds

No official workaround has been published by the vendor; patching is the required remediation.
Where patching must be deferred, restrict interactive and remote local logons to trusted administrators only.
Enforce application allowlisting to limit the ability of low-privileged users to execute arbitrary binaries capable of triggering the race.

bash

# Verify the Cloud Files Mini Filter driver version on a Windows host
Get-ItemProperty -Path "$env:SystemRoot\System32\drivers\cldflt.sys" |
  Select-Object VersionInfo

# List installed updates to confirm the relevant KB is present
Get-HotFix | Sort-Object -Property InstalledOn -Descending