CVE-2026-27926: Windows Cloud Files Privilege Escalation

CVE-2026-27926 Overview

CVE-2026-27926 is a race condition vulnerability in the Windows Cloud Files Mini Filter Driver that enables an authorized local attacker to elevate privileges on affected systems. The vulnerability stems from concurrent execution using shared resources with improper synchronization (CWE-362), a classic timing-based flaw that can be exploited to bypass security controls and gain elevated system access.

Critical Impact

Local privilege escalation allowing authenticated attackers to gain elevated system privileges through a race condition in the Windows Cloud Files Mini Filter Driver.

Affected Products

• Windows Cloud Files Mini Filter Driver
• Windows operating systems with Cloud Files Mini Filter Driver component

Discovery Timeline

• 2026-04-14 - CVE-2026-27926 published to NVD
• 2026-04-14 - Last updated in NVD database

Technical Details for CVE-2026-27926

Vulnerability Analysis

This vulnerability exists within the Windows Cloud Files Mini Filter Driver (cldflt.sys), a kernel-mode driver responsible for handling cloud file synchronization operations. The flaw manifests as an improper synchronization issue when multiple threads concurrently access shared resources within the driver.

The race condition occurs during file operations where the driver fails to properly serialize access to shared data structures. When an attacker can manipulate the timing of concurrent operations, they can exploit this window of vulnerability to modify critical data or gain unauthorized access to privileged operations.

The local attack vector requires the attacker to have authenticated access to the target system, but the high complexity rating indicates that successful exploitation requires precise timing to win the race condition. However, once exploited, the impact is significant—allowing complete compromise of confidentiality, integrity, and availability at the local system level.

Root Cause

The root cause is improper synchronization primitives in the Windows Cloud Files Mini Filter Driver when handling concurrent operations on shared resources. The driver fails to implement proper locking mechanisms or atomic operations when accessing shared data structures, creating a Time-of-Check to Time-of-Use (TOCTOU) window that attackers can exploit.

This type of vulnerability typically occurs when:

• Multiple threads access shared memory without proper mutex or spinlock protection
• The driver assumes single-threaded execution in code paths that can be reached concurrently
• Improper use of synchronization APIs in kernel-mode driver code

Attack Vector

The attack requires local access with low privileges. An attacker must execute malicious code on the target system and precisely time operations to exploit the race condition window. The attack flow involves:

1. The attacker initiates multiple concurrent operations that trigger the vulnerable code path in cldflt.sys
2. By carefully timing these operations, the attacker manipulates the shared resource during the unprotected window
3. The race condition allows the attacker to corrupt memory or hijack execution flow
4. Successful exploitation results in code execution with SYSTEM privileges

The vulnerability is exploitable through the Windows Cloud Files Mini Filter Driver interface, which is accessible to authenticated local users. While the complexity is high due to the timing requirements, determined attackers with local access can achieve reliable exploitation through techniques like heap spraying or by repeatedly attempting the race condition.

Detection Methods for CVE-2026-27926

Indicators of Compromise

• Unusual kernel-mode crash dumps or blue screens related to cldflt.sys
• Suspicious process behavior showing unexpected privilege escalation from low-privileged accounts
• Anomalous activity patterns involving rapid, repeated cloud file operations

Detection Strategies

• Monitor for kernel driver crashes or stability issues associated with the Cloud Files Mini Filter Driver
• Implement endpoint detection rules for privilege escalation attempts from standard user contexts
• Deploy behavioral analysis to detect abnormal file operation patterns targeting cloud file synchronization

Monitoring Recommendations

• Enable Windows Event Log monitoring for kernel driver errors (Event ID 41, 1001)
• Configure security information and event management (SIEM) alerts for unexpected privilege transitions
• Monitor process creation events for SYSTEM-level processes spawned by low-privilege users

How to Mitigate CVE-2026-27926

Immediate Actions Required

• Apply the latest Windows security updates from Microsoft immediately
• Review and restrict local access to systems where cloud file synchronization is enabled
• Implement application control policies to limit execution of untrusted code on affected systems

Patch Information

Microsoft has released a security update to address this vulnerability. Refer to the Microsoft Security Update Guide for detailed patch information and affected system versions. Organizations should prioritize deployment of this update on all affected Windows systems, particularly those with the Cloud Files Mini Filter Driver component enabled.

Workarounds

• If patching is not immediately possible, consider disabling the Cloud Files Mini Filter Driver on critical systems where it is not required
• Implement strict access controls to limit which users can log in locally to affected systems
• Enable Windows Defender Credential Guard and other exploit protection features to reduce the attack surface

# Check if Cloud Files Mini Filter Driver is loaded
fltMC.exe

# If not required, the driver can be unloaded temporarily (requires administrator privileges)
fltMC.exe unload cldflt

# Note: This may impact cloud file synchronization functionality
# Consult Microsoft documentation before disabling in production environments