CVE-2021-31969 Overview
CVE-2021-31969 is an elevation of privilege vulnerability in the Windows Cloud Files Mini Filter Driver (cldflt.sys). The flaw allows a locally authenticated attacker to escalate privileges on affected Windows 10 and Windows Server systems. Microsoft addressed the issue in the June 2021 Patch Tuesday release. The vulnerability maps to [CWE-269: Improper Privilege Management] and carries a CVSS 3.1 base score of 7.8. Successful exploitation grants the attacker the ability to execute code with elevated kernel-level rights, fully compromising confidentiality, integrity, and availability of the host.
Critical Impact
A local attacker with low-privileged access can exploit the Cloud Files Mini Filter Driver to obtain SYSTEM-level privileges on affected Windows endpoints and servers.
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 and Microsoft releases security patch
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2021-31969
Vulnerability Analysis
The vulnerability resides in the Windows Cloud Files Mini Filter Driver, cldflt.sys. This kernel-mode component supports the Cloud Files API used by OneDrive and other cloud storage providers to present remote files as locally available placeholders. The driver runs in the kernel and processes requests originating from user-mode callers. Improper privilege management within the driver allows a low-privileged local user to manipulate driver operations and gain code execution in a higher-privileged context. The flaw is classified under [CWE-269] and falls into the category of Driver Vulnerability and Privilege Escalation.
Root Cause
The root cause is improper validation of operations or trust boundaries within cldflt.sys. The driver fails to correctly enforce privilege separation when handling requests from user-mode processes. This permits an authenticated local attacker to influence kernel-mode behavior in ways the driver did not intend. Because the driver executes with kernel privileges, any exploitable flaw can be leveraged to break out of standard user-mode restrictions.
Attack Vector
Exploitation requires local access and low-privileged authentication on the target system. The attack does not require user interaction. An attacker first obtains code execution as a standard user, typically through phishing, a foothold from another compromise, or malicious software. The attacker then issues crafted requests to the Cloud Files Mini Filter Driver to trigger the privilege management flaw. Successful exploitation yields SYSTEM-level execution, allowing the attacker to install programs, modify data, or create new privileged accounts. Refer to the Zero Day Initiative Advisory ZDI-21-797 for additional technical context.
Detection Methods for CVE-2021-31969
Indicators of Compromise
- Unexpected loading or interaction with cldflt.sys by non-system processes attempting to issue driver control codes.
- Creation of new privileged accounts or services shortly after a standard user logon session.
- Anomalous child processes spawned by user-mode applications running as NT AUTHORITY\SYSTEM.
- Crash dumps or event log entries referencing cldflt.sys faults outside normal OneDrive activity.
Detection Strategies
- Monitor for processes opening handles to the Cloud Files filter driver device object from non-cloud-sync contexts.
- Detect token manipulation and privilege escalation patterns where a user-mode process suddenly executes with SYSTEM integrity.
- Apply behavioral analytics to flag local privilege escalation sequences such as user-context process spawning SYSTEM children.
Monitoring Recommendations
- Enable Windows Event Log auditing for privilege use (Event IDs 4672, 4673) and process creation (Event ID 4688) with command line logging.
- Track installation status of the June 2021 security updates across the fleet to identify unpatched endpoints.
- Forward driver-related telemetry and Sysmon events to a centralized analytics platform for correlation.
How to Mitigate CVE-2021-31969
Immediate Actions Required
- Apply the June 2021 Microsoft security update for all affected Windows 10 and Windows Server systems without delay.
- Inventory endpoints running Windows 10 versions 1909, 2004, 20H2, 21H1 and Windows Server 2016/2019 to confirm patch coverage.
- Restrict local logon rights and remove unnecessary administrative privileges on workstations.
- Investigate hosts showing anomalous interactions with cldflt.sys for evidence of prior exploitation.
Patch Information
Microsoft released the official fix on June 8, 2021 as part of the monthly security update cycle. Patch details and update package links are available at the Microsoft Security Advisory for CVE-2021-31969. Administrators should deploy the cumulative update corresponding to each affected Windows build.
Workarounds
- No officially supported workaround replaces patching; deploy the Microsoft update as the primary remediation.
- Where patching is delayed, limit local access to trusted users only and enforce application allowlisting to block untrusted binaries from executing.
- Disable or restrict cloud storage sync clients that rely on the Cloud Files API on systems that do not require the feature.
# Verify patch installation status on Windows
wmic qfe list brief | findstr /i "KB5003637 KB5003646 KB5003635 KB5003638"
# Confirm cldflt.sys file version after patching
Get-Item C:\Windows\System32\drivers\cldflt.sys | Select-Object VersionInfo
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
