CVE-2026-27927: Windows Projected File System RCE Flaw

CVE-2026-27927 Overview

CVE-2026-27927 is a race condition vulnerability (CWE-362) affecting the Windows Projected File System (ProjFS) that allows an authorized local attacker to elevate privileges on affected systems. The vulnerability stems from concurrent execution using a shared resource with improper synchronization, enabling attackers with local access to exploit timing windows and gain elevated privileges.

Critical Impact

Local privilege escalation vulnerability in Windows Projected File System could allow authenticated attackers to gain SYSTEM-level privileges through race condition exploitation.

Affected Products

• Windows Projected File System (ProjFS)
• Windows operating systems with ProjFS enabled

Discovery Timeline

• April 14, 2026 - CVE-2026-27927 published to NVD
• April 14, 2026 - Last updated in NVD database

Technical Details for CVE-2026-27927

Vulnerability Analysis

This vulnerability exists in the Windows Projected File System, a Windows component that enables file system providers to project hierarchical data from a backing data store into the file system. The race condition occurs due to improper synchronization when multiple threads or processes access shared resources concurrently within the ProjFS subsystem.

The vulnerability is classified under CWE-362 (Concurrent Execution Using Shared Resource with Improper Synchronization), commonly known as a race condition. This type of flaw occurs when the correctness of a computation depends on the relative timing of multiple concurrent threads or processes accessing shared resources, and proper synchronization mechanisms are not implemented.

Root Cause

The root cause is improper synchronization of shared resources within the Windows Projected File System component. When multiple operations access the same resource simultaneously, the lack of proper locking mechanisms creates a window of opportunity where an attacker can manipulate the state between the time a condition is checked and when the result is used, a classic Time-of-Check Time-of-Use (TOCTOU) scenario.

Attack Vector

The attack requires local access to the target system with low-privilege authentication. An attacker must be able to execute code locally and time their operations to exploit the synchronization gap in ProjFS. The attack does not require user interaction, making it potentially automatable once an attacker has established initial access to the system.

The exploitation involves triggering concurrent operations against the Projected File System that race against each other. By carefully timing malicious operations to coincide with legitimate ProjFS operations, an attacker can manipulate shared state to achieve privilege escalation from a standard user context to elevated privileges.

Detection Methods for CVE-2026-27927

Indicators of Compromise

• Unusual process activity involving ProjFS-related operations from low-privileged accounts
• Unexpected privilege escalation events in Windows Security Event logs
• Abnormal timing patterns in file system operations targeting virtualized directories
• Processes spawning with elevated privileges from unexpected parent processes

Detection Strategies

• Monitor for suspicious sequences of ProjFS API calls, particularly PrjStartVirtualizing and related callback operations
• Implement behavioral detection rules for rapid, repeated file system operations that could indicate race condition exploitation attempts
• Deploy endpoint detection and response (EDR) solutions capable of identifying privilege escalation patterns
• Enable detailed auditing of ProjFS operations and correlate with process creation events

Monitoring Recommendations

• Enable Windows Security Event logging for process creation (Event ID 4688) with command line auditing
• Monitor for privilege escalation indicators such as token manipulation or impersonation
• Implement file integrity monitoring on sensitive system directories that may be targeted via ProjFS
• Deploy SentinelOne Singularity for real-time behavioral analysis and automated threat response

How to Mitigate CVE-2026-27927

Immediate Actions Required

• Apply the latest Windows security updates from Microsoft as soon as available
• Review and disable Windows Projected File System if not required for business operations
• Implement principle of least privilege to minimize the impact of potential exploitation
• Enable enhanced monitoring on systems where ProjFS cannot be disabled

Patch Information

Microsoft has released a security update addressing this vulnerability. Refer to the Microsoft Security Advisory for specific patch information and affected Windows versions. Organizations should prioritize patch deployment through standard Windows Update channels or WSUS/SCCM for managed environments.

Workarounds

• Disable the Windows Projected File System feature if it is not required for application functionality
• Restrict local logon rights to trusted users only to reduce the attack surface
• Implement application control policies to prevent unauthorized code execution
• Use network segmentation to limit lateral movement if a system is compromised

To disable Windows Projected File System (if not required), administrators can use the following approach. Note that disabling ProjFS may affect applications that depend on it, such as certain virtualization features and development tools.

# Check if Windows Projected File System is enabled
Get-WindowsOptionalFeature -Online -FeatureName Client-ProjFS

# Disable Windows Projected File System (requires restart)
Disable-WindowsOptionalFeature -Online -FeatureName Client-ProjFS -NoRestart

# Verify the feature is disabled
Get-WindowsOptionalFeature -Online -FeatureName Client-ProjFS | Select-Object State