CVE-2021-43207: Windows 10 Privilege Escalation Flaw

CVE-2021-43207 Overview

CVE-2021-43207 is an elevation of privilege vulnerability in the Windows Common Log File System (CLFS) driver. The flaw allows an authenticated local attacker to execute code with elevated privileges on affected Windows endpoints and servers. Microsoft addressed the issue in the December 2021 Patch Tuesday release. The vulnerability impacts the full range of supported Windows client and server platforms, including Windows 7 through Windows 11, and Windows Server 2008 through Windows Server 2022.

Critical Impact
A local attacker with low privileges can escalate to SYSTEM by abusing the CLFS kernel driver (clfs.sys), enabling full compromise of the affected host.

Affected Products

Microsoft Windows 10 (multiple builds including 1607, 1809, 1909, 2004, 20H2, 21H1)
Microsoft Windows 11 and Windows 7 SP1, Windows 8.1, Windows RT 8.1
Microsoft Windows Server 2008, 2012, 2016, 2019, 2022, and Server 20H2

Discovery Timeline

2021-12-15 - CVE-2021-43207 published to NVD
2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-43207

Vulnerability Analysis

The vulnerability resides in the Common Log File System driver (clfs.sys), a kernel-mode component that provides high-performance, general-purpose logging services to user-mode and kernel-mode clients. CLFS is widely used by Windows subsystems including the Kernel Transaction Manager and TxF (Transactional NTFS).

Because CLFS executes in kernel context, any memory safety or validation flaw in its parsing logic can be leveraged to corrupt kernel data structures. An authenticated local user can craft a malicious log file (.blf or container files) and trigger the vulnerable code path to gain SYSTEM-level execution. The issue requires local access and low privileges, but no user interaction.

Root Cause

Microsoft has not published detailed root-cause information for CVE-2021-43207, and the NVD entry lists the CWE as NVD-CWE-noinfo. CLFS elevation-of-privilege bugs disclosed in the same period have generally involved improper validation of attacker-controlled fields in log file metadata, leading to memory corruption inside the driver.

Attack Vector

Exploitation is local. An attacker must already be able to execute code on the target as a standard user, for example through a phishing payload, a malicious installer, or a compromised service account. The attacker then interacts with the CLFS driver through the documented user-mode API or by directly opening crafted log containers. Successful exploitation yields code execution at kernel privilege, allowing the attacker to disable security tooling, install drivers, and establish persistence.

No public proof-of-concept exploit is listed in the enriched data, and the vulnerability is not present on the CISA Known Exploited Vulnerabilities catalog. The EPSS probability is 0.319%.

Detection Methods for CVE-2021-43207

Indicators of Compromise

Creation of unusual .blf, .bin, or container log files in user-writable directories outside standard CLFS usage patterns.
Unexpected processes spawning child processes as NT AUTHORITY\SYSTEM shortly after interacting with CLFS APIs.
Kernel crashes or bugchecks referencing clfs.sys in the faulting module field of WER or minidump reports.

Detection Strategies

Hunt for non-system processes calling CreateLogFile, AddLogContainer, or related CLFS APIs followed by privilege transitions.
Correlate Windows Security event ID 4688 (process creation) with parent processes running at medium integrity that produce SYSTEM-integrity children.
Monitor Sysmon event ID 11 (file create) for .blf and CLFS container files written by non-administrative users.

Monitoring Recommendations

Centralize Windows kernel telemetry, Sysmon, and EDR process-lineage events into a SIEM for cross-host correlation.
Alert on integrity-level escalations where a child process runs at a higher integrity level than its parent without an explicit elevation prompt.
Track patch state of clfs.sys across the fleet and flag endpoints running pre-December 2021 versions.

How to Mitigate CVE-2021-43207

Immediate Actions Required

Apply the December 2021 Microsoft security updates that address CVE-2021-43207 across all affected Windows client and server SKUs.
Prioritize patching multi-user systems, terminal servers, and developer workstations where local code execution by standard users is more likely.
Audit local administrator group membership and remove unnecessary standing privileges to reduce the impact of any residual elevation flaws.

Patch Information

Microsoft released fixed binaries for clfs.sys through the December 2021 Patch Tuesday cycle. Refer to the Microsoft Security Advisory CVE-2021-43207 for the per-platform KB numbers and updated file versions.

Workarounds

No vendor-supplied workaround replaces the patch. Apply the security update as the primary remediation.
Restrict interactive and remote-desktop logon rights to trusted administrative users on servers until patching is complete.
Enforce application allowlisting (for example, Windows Defender Application Control or AppLocker) to limit execution of unsigned binaries that could deliver an exploit.

bash

# Verify the patched clfs.sys version on a Windows host
Get-Item C:\Windows\System32\drivers\clfs.sys | Select-Object VersionInfo

# Query installed updates to confirm December 2021 cumulative update is present
wmic qfe list brief /format:table