CVE-2021-31969: Windows 10 Privilege Escalation Flaw

CVE-2021-31969 Overview

CVE-2021-31969 is an elevation of privilege vulnerability affecting the Windows Cloud Files Mini Filter Driver (cldflt.sys). This kernel-mode driver is responsible for managing cloud file operations and synchronization in Windows, particularly for OneDrive and other cloud storage providers. The vulnerability allows a local attacker with low-level privileges to escalate their access to SYSTEM-level privileges on affected systems.

Critical Impact

Successful exploitation enables complete system compromise, allowing attackers to execute arbitrary code with SYSTEM privileges, install persistent malware, access sensitive data, and potentially move laterally across the network.

Affected Products

• Microsoft Windows 10 (versions 1909, 2004, 20H2, 21H1)
• Microsoft Windows Server 2016 (versions 2004, 20H2)
• Microsoft Windows Server 2019

Discovery Timeline

• 2021-06-08 - CVE-2021-31969 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-31969

Vulnerability Analysis

The Windows Cloud Files Mini Filter Driver vulnerability is classified under CWE-269 (Improper Privilege Management). The cldflt.sys driver operates at the kernel level to intercept and manage file system operations for cloud-synchronized files. Due to improper privilege management within the driver, a local attacker can manipulate driver operations to gain elevated privileges.

The attack requires local access to the system and low-level user privileges, but no user interaction is needed for exploitation. Once exploited, the vulnerability provides the attacker with complete control over the confidentiality, integrity, and availability of the affected system.

Root Cause

The root cause of this vulnerability lies in improper privilege management within the Cloud Files Mini Filter Driver. The driver fails to properly validate or restrict certain operations, allowing unprivileged users to perform actions that should be reserved for higher-privileged accounts. This improper access control at the kernel level creates an exploitable path for privilege escalation.

Attack Vector

The attack is executed locally on the target system. An attacker would need to first gain initial access to the system with a low-privileged account. From there, they can interact with the vulnerable cldflt.sys driver to trigger the improper privilege management condition, ultimately elevating their privileges to SYSTEM level.

The vulnerability mechanism involves the following exploitation flow:

1. Attacker gains initial access with low-privilege user account
2. Attacker identifies and interacts with the vulnerable Cloud Files Mini Filter Driver
3. Specially crafted operations trigger the improper privilege management flaw
4. Driver executes operations with elevated privileges
5. Attacker achieves SYSTEM-level access

For detailed technical information, refer to the Zero Day Initiative Advisory ZDI-21-797.

Detection Methods for CVE-2021-31969

Indicators of Compromise

• Unusual process activity spawning from cloud file synchronization services with SYSTEM privileges
• Unexpected modifications to cldflt.sys driver or related registry entries
• Suspicious process creation chains involving low-privileged users executing high-privileged operations
• Anomalous kernel-mode driver loading or unloading events

Detection Strategies

• Monitor for privilege escalation attempts using endpoint detection and response (EDR) solutions
• Implement Windows event log monitoring for security events related to privilege changes (Event IDs 4672, 4673)
• Deploy behavioral analysis to detect unusual process elevation patterns
• Enable kernel-mode driver auditing to track interactions with cldflt.sys

Monitoring Recommendations

• Configure SentinelOne Singularity platform to monitor for kernel-mode exploitation attempts
• Enable detailed logging for Windows file system filter drivers
• Implement real-time alerting for SYSTEM-level process creation from non-standard parent processes
• Regularly audit systems for missing security patches related to this vulnerability

How to Mitigate CVE-2021-31969

Immediate Actions Required

• Apply the Microsoft security update for CVE-2021-31969 immediately on all affected systems
• Prioritize patching systems with cloud file synchronization features enabled
• Restrict local access to systems where possible to reduce the attack surface
• Review and audit user accounts with local system access

Patch Information

Microsoft has released security updates to address this vulnerability as part of their June 2021 security updates. Administrators should apply the appropriate patch for their Windows version. Detailed patch information and download links are available in the Microsoft Security Advisory CVE-2021-31969.

Workarounds

• Limit local user access to critical systems until patches can be applied
• Implement application whitelisting to prevent unauthorized code execution
• Consider temporarily disabling cloud file synchronization features on high-value targets if patching is delayed
• Deploy SentinelOne Singularity for real-time protection against exploitation attempts

# Verify patch installation status
wmic qfe list brief | findstr /i "KB5003637"

# Check Cloud Files Mini Filter Driver version
driverquery /v | findstr /i "cldflt"