CVE-2026-20857: Windows Cloud Files Privilege Escalation

CVE-2026-20857 Overview

CVE-2026-20857 is an untrusted pointer dereference vulnerability in the Windows Cloud Files Mini Filter Driver that enables an authorized local attacker to elevate privileges on the affected system. This kernel-mode driver vulnerability allows attackers with local access and low-level privileges to gain complete control over the system by exploiting improper pointer validation in the filter driver component.

Critical Impact

Successful exploitation allows a local attacker with low privileges to escalate to SYSTEM-level access, potentially compromising the entire Windows host including access to sensitive data, credentials, and the ability to execute arbitrary code with elevated permissions.

Affected Products

• Windows Cloud Files Mini Filter Driver (cldflt.sys)
• Windows operating systems with Cloud Files API support
• Systems utilizing Windows cloud provider synchronization features

Discovery Timeline

• 2026-01-13 - CVE-2026-20857 published to NVD
• 2026-01-13 - Last updated in NVD database

Technical Details for CVE-2026-20857

Vulnerability Analysis

This vulnerability stems from CWE-822 (Untrusted Pointer Dereference), a dangerous memory corruption issue where the Windows Cloud Files Mini Filter Driver (cldflt.sys) improperly validates pointer values from user-controlled input before dereferencing them. The driver operates in kernel mode, meaning any successful exploitation immediately grants the attacker kernel-level code execution capabilities.

The attack requires local access with low privileges, meaning an attacker must first obtain initial access to the target system through other means such as phishing, malware, or compromised credentials. Once on the system, the attacker can craft malicious requests to the Cloud Files Mini Filter Driver that contain specially crafted pointer values. When the driver processes these requests without proper validation, it dereferences the untrusted pointer, allowing the attacker to read or write arbitrary kernel memory.

Root Cause

The root cause is insufficient validation of pointer values received from user-mode applications before the kernel-mode driver dereferences them. The Windows Cloud Files Mini Filter Driver fails to properly verify that pointer values passed through I/O control requests or filter communication ports originate from trusted kernel memory regions. This allows an attacker to supply arbitrary pointer values that the driver then blindly trusts and dereferences.

Attack Vector

The attack vector is local, requiring the attacker to have authenticated access to the target system with at least low-level privileges. The exploitation process typically involves:

1. The attacker creates a malicious user-mode application that interacts with the Cloud Files Mini Filter Driver
2. The application sends specially crafted I/O requests containing controlled pointer values to the driver
3. The driver processes these requests and dereferences the untrusted pointer without adequate validation
4. The attacker leverages the arbitrary memory access to corrupt kernel structures or execute code in kernel context
5. Privilege escalation to SYSTEM is achieved, granting full control over the compromised host

Since no user interaction is required and the attack complexity is low, this vulnerability presents a significant risk for systems where attackers have gained initial local access.

Detection Methods for CVE-2026-20857

Indicators of Compromise

• Suspicious process activity interacting with the Cloud Files Mini Filter Driver (cldflt.sys) from non-standard applications
• Unexpected system crashes or blue screens (BSOD) related to cldflt.sys that may indicate exploitation attempts
• Processes running with elevated SYSTEM privileges that originated from low-privileged user contexts
• Anomalous DeviceIoControl calls targeting the Cloud Files filter driver from suspicious executables

Detection Strategies

• Monitor for unusual process privilege changes, particularly processes escalating from standard user to SYSTEM
• Implement kernel-mode driver integrity monitoring to detect tampering or exploitation of filter drivers
• Deploy endpoint detection solutions capable of identifying kernel exploitation techniques and token manipulation
• Audit access patterns to the Cloud Files Mini Filter Driver for anomalous behavior from non-OneDrive processes

Monitoring Recommendations

• Enable Windows Event Logging for process creation (Event ID 4688) with command-line auditing to track suspicious activity
• Configure Sysmon or equivalent to monitor filter driver communications and detect exploitation attempts
• Implement real-time kernel memory integrity monitoring where available
• Alert on any unexpected privilege escalation patterns or token impersonation events

How to Mitigate CVE-2026-20857

Immediate Actions Required

• Apply the latest Microsoft security updates addressing CVE-2026-20857 immediately
• Restrict local access to systems to only authorized personnel and limit low-privileged account usage
• Review and harden endpoint protection configurations to detect privilege escalation attempts
• Consider disabling non-essential Cloud Files functionality on critical systems until patched

Patch Information

Microsoft has released a security update to address this vulnerability. System administrators should consult the Microsoft Security Update Guide for detailed patch information, affected product versions, and download links. Apply the appropriate cumulative update or standalone security patch through Windows Update, WSUS, or manual deployment.

Workarounds

• Limit local access to affected systems and enforce the principle of least privilege for all user accounts
• Implement application control policies to prevent unauthorized executables from interacting with kernel-mode drivers
• Consider temporarily disabling the Cloud Files Mini Filter Driver on critical systems if the feature is not required, though this may impact OneDrive and cloud provider functionality
• Deploy additional endpoint monitoring to detect exploitation attempts until the patch can be applied

# Check Cloud Files Mini Filter Driver status
fltmc filters | findstr cldflt

# Verify installed Windows security updates
wmic qfe list brief /format:table | findstr /i "KB"

# Review driver load status
sc query cldflt