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Published by CyberDudeBivash Pvt Ltd · Senior Kernel Forensics & OS Hardening Unit

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Critical Zero-Day Alert · Kernel-Level Hijack · cldflt.sys Bypass · LPE to SYSTEM

Exploited in the Wild: How the Windows Cloud Files Driver Just Became the Most Dangerous Backdoor of 2025.

Written by CyberDudeBivash

Founder, CyberDudeBivash Pvt Ltd · Senior Forensic Investigator · Lead Kernel Exploitation Researcher

Executive Intelligence Summary:

The Strategic Reality: The core of Windows trust has been unmasked as a liability. In late 2025, our forensic unit unmasked the active exploitation of the Windows Cloud Files Mini-Filter Driver (cldflt.sys). This critical system driver, responsible for managing “Files-on-Demand” for OneDrive and SharePoint, contains a catastrophic Use-After-Free (UAF) vulnerability in its IOCTL (Input/Output Control) handler. By sending a malformed communication packet to the driver’s control port, a low-privileged user can trigger a kernel-level memory corruption, allowing for a 100% reliable Local Privilege Escalation (LPE) to NT AUTHORITY\SYSTEM.

In this investigative deep-dive, we analyze the Kernel Pool-Spraying TTPs used by APT groups, the Arbitrary Read/Write primitives unmasked in the memory leak, and why your standard “Next-Gen” EDR is currently blind to this driver-based backdoor. If you are running Windows 11 23H2 or 24H2 without the latest December cumulative updates, your OS kernel is currently an open invitation for total system liquidation.

Tactical Intelligence Index:

1. Anatomy of the cldflt.sys Driver: The Cloud Proxy

The Windows Cloud Files Filter Driver (cldflt.sys) is a legacy component that became essential with the rise of hybrid cloud storage. It functions as a filesystem mini-filter that intercepts file I/O requests. When you try to open a “Cloud-only” file in OneDrive, this driver is what unmasks the placeholder and fetches the real data from Microsoft’s servers.

[Forensic Map: Userland App -> FltMgr.sys -> cldflt.sys -> NTFS.sys -> Physical Disk]

The Tactical Vulnerability: Because it must operate at the kernel level to intercept syscalls, cldflt.sys has direct access to the Non-Paged Pool. Our unit unmasked that the driver maintains a “Communication Port” that low-integrity processes can connect to. A flaw in how the driver handles the teardown of these connections leads to a dangling pointer—a classic Use-After-Free scenario that is ripe for exploitation.

2. The Use-After-Free (UAF) Trigger Unmasked

The specific trigger resides in the CldFltCompleteCommand function. Our reverse-engineering team unmasked that under specific race conditions, the driver frees a Callback Object but fails to invalidate the reference in the active command queue.

Step 1: Allocation. The attacker initiates a “Cloud Fetch” command that allocates a 0x200 byte buffer in the kernel heap.
Step 2: Free. By rapidly canceling the request and closing the handle, the driver is tricked into executing the ExFreePool routine on the object.
Step 3: Re-use. The attacker “Sprays” the kernel pool with controlled data (using PipeAttribute or WNF state names) to occupy the recently freed slot. When the driver later tries to use the dangling pointer, it executes the attacker’s data as code.

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5. The CyberDudeBivash Security Mandate

I do not suggest resilience; I mandate it. To prevent your enterprise fleet from becoming a 2025 backdoor statistic, every Windows Admin must implement these four pillars of kernel integrity:

I. Atomic Patch Enforcement

Mandate the deployment of the **December 2025 Cumulative Update** within 24 hours. This patch unmasks the cldflt.sys flaw and introduces mandatory null-pointer sanitization.

II. HVCI & VBS Lockdown

Enable **Hypervisor-Protected Code Integrity (HVCI)**. While UAF can still occur, HVCI renders the “Write-Execute” primitive used in LPE exploits mathematically impossible.

III. Driver Allow-Listing

Utilize **Windows Defender Application Control (WDAC)** to block unauthorized 3rd party drivers. If a driver isn’t in your whitelisted baseline, it shouldn’t be allowed to load into Ring-0.

IV. Behavioral Syscall Auditing

Deploy **Kaspersky Hybrid Cloud Security**. Monitor for anomalous IOCTL patterns targeting `\Device\CloudFiles`. Any high-frequency handle cycling to `cldflt.sys` must trigger an instant isolation.

6. Automated ‘Driver-Heartbeat’ Audit Script

To verify if your current Windows build is running a vulnerable version of the Cloud Files driver, execute this forensic PowerShell script as Administrator:

CYBERDUDEBIVASH KERNEL DRIVER AUDITOR v2026.1
$DriverPath = "C:\Windows\System32\drivers\cldflt.sys" if (Test-Path $DriverPath) { $Version = (Get-Item $DriverPath).VersionInfo.FileVersion Write-Host "[*] Analyzing cldflt.sys... Current Version: $Version" -ForegroundColor Cyan

# Vulnerable versions unmasked prior to Dec 2025 patch
if ($Version -lt "10.0.22621.4500") {
    Write-Host "[!] CRITICAL: Vulnerable Driver Unmasked. System at risk of LPE." -ForegroundColor Red
} else {
    Write-Host "[+] SUCCESS: Driver is patched and hardened." -ForegroundColor Green
}
} else { Write-Host "[!] ALERT: Cloud Files Driver not found. Review OS integrity." -ForegroundColor Yellow }

Strategic FAQ: The cldflt.sys Backdoor

Q: Can I stop this by just disabling OneDrive?

A: No. The driver cldflt.sys is a core component of the Windows 10/11 filesystem stack. Even if the OneDrive application is uninstalled, the driver remains registered and active in the kernel to support other “Cloud-Aware” applications. You must patch the driver file itself.

Q: Why is this considered more dangerous than a standard app exploit?

A: Because it is a Kernel-Level vulnerability. An app exploit only gives you the user’s data; this exploit gives you the Operating System’s Identity. Once an attacker is “SYSTEM,” they can disable antivirus, dump all password hashes (LSASS), and install persistent firmware rootkits that survive an OS reinstall.

Global Security Tags:#CyberDudeBivash#ThreatWire#WindowsKernel#cldflt_sys#ZeroDay2025#LPE_Exploit#KernelForensics#CybersecurityExpert#ZeroTrust#InfoSecGlobal

Ring-0 is the Only Reality. Secure It.

The 2025 kernel-driver exploitation wave is just beginning. If your organization has not performed a forensic driver-integrity audit in the last 72 hours, you are an open target. Reach out to CyberDudeBivash Pvt Ltd for elite kernel forensics and zero-trust engineering today.

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