CISA Warns of Actively Exploited Windows Privilege Escalation Flaw (CVE-2021-43226)
CISA Adds Windows CLFS Driver Privilege Escalation Vulnerability (CVE-2021-43226) to KEV Catalog
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Executive Summary
On October 6, 2025, the U.S. Cybersecurity and Infrastructure Security Agency (CISA) added CVE-2021-43226 to its Known Exploited Vulnerabilities (KEV) catalog, signaling that the flaw is under active exploitation. This high-severity vulnerability is a privilege escalation bug in the Microsoft Windows Common Log File System (CLFS) Driver. It allows a local attacker who has already gained initial access to a system to elevate their privileges to SYSTEM, the highest level of authority on a Windows machine. The flaw impacts modern Windows operating systems, including Windows 10, Windows 11, and Windows Server 2016/2019/2022. In accordance with Binding Operational Directive (BOD) 22-01, CISA has mandated that all federal civilian agencies apply the necessary patches by October 27, 2025.
Vulnerability Details
CVE-2021-43226 is a local privilege escalation (LPE) vulnerability rooted in the Windows Common Log File System (clfs.sys). The flaw arises from the driver's failure to properly validate user-supplied metadata within a CLFS log file. An authenticated attacker can craft a malicious log file that, when processed, triggers a buffer overflow, leading to arbitrary code execution in the context of the kernel.
- Attack Vector: Local
- Complexity: Low
- Privileges Required: Low (Authenticated User)
- User Interaction: None
Because the attack is local, a threat actor must first gain a foothold on the target system, typically through methods like phishing, malware infection, or exploiting a separate remote code execution vulnerability. Once on the system, this vulnerability provides a reliable pathway to full system control.
Affected Systems
The vulnerability affects a broad range of Microsoft Windows products, including:
- Windows 10
- Windows 11
- Windows Server 2016
- Windows Server 2019
- Windows Server 2022
The availability of public proof-of-concept (PoC) exploit code further increases the risk, as it lowers the barrier for less sophisticated actors to incorporate this exploit into their attacks.
Impact Assessment
Successful exploitation of CVE-2021-43226 grants an attacker SYSTEM privileges. This level of access is catastrophic, as it allows the threat actor to:
- Bypass all security controls on the endpoint (e.g., antivirus, EDR).
- Deploy persistent malware, such as rootkits or backdoors. (
T1547 - Boot or Logon Autostart Execution) - Dump credentials for other users and domain accounts, facilitating lateral movement across the network. (
T1003 - OS Credential Dumping) - Install ransomware and destroy backups.
- Exfiltrate any data on the system.
This type of vulnerability is a crucial component in the attack chain for ransomware groups and APTs, who use it to escalate privileges after gaining an initial foothold.
Cyber Observables for Detection
Detecting exploitation of this vulnerability involves monitoring for suspicious interactions with the CLFS driver and subsequent privilege escalation activity.
| Type | Value | Description |
|---|---|---|
file_name |
*.blf |
Monitor for the creation or modification of unusual Base Log File (.blf) files, especially by low-privileged user processes. |
process_name |
svchost.exe |
Look for svchost.exe or other high-privilege processes spawning from unexpected parent processes like explorer.exe or cmd.exe. |
event_id |
4688 |
In Windows Security Logs, hunt for process creation events where a low-privilege user process spawns a child process running with SYSTEM integrity. |
log_source |
Microsoft-Windows-Kernel-General |
Kernel-level errors or unexpected reboots could be an indicator of a failed exploit attempt. |
Detection & Response
Security teams should focus on endpoint monitoring to detect exploitation of this LPE flaw.
Detection Methods
- EDR Rules: Configure EDR solutions to alert on processes that exhibit suspicious behavior related to CLFS, such as a non-system process writing to
.blffiles in system directories. Use D3FEND'sProcess Analysisto baseline normal process behavior. - SIEM Correlation: Create correlation rules in your SIEM to detect a chain of events, such as a user downloading a file from the internet, executing it, and that process subsequently spawning a new process with
SYSTEMprivileges. - File Integrity Monitoring (FIM): Monitor for the creation of suspicious
.blffiles in temp directories or user profiles, as these may be used to stage the exploit.
Remediation Steps
Immediate patching is the primary remediation for this vulnerability.
- Apply Patches: Deploy the security updates released by Microsoft that address
CVE-2021-43226. Prioritize patching for critical servers and systems accessible by a large number of users. This is a direct application of D3FEND'sSoftware Update. - Vulnerability Scanning: Run authenticated vulnerability scans to verify that the patch has been applied correctly and to identify any missed assets.
- Principle of Least Privilege: As a general best practice, enforce the principle of least privilege for all user accounts. While this does not prevent the vulnerability's exploitation, it limits the opportunities for attackers to gain the initial foothold needed to run the local exploit. This relates to D3FEND's
User Account Permissions.
Timeline of Events
MITRE ATT&CK Mitigations
Update Software
The primary mitigation is to apply the security updates provided by Microsoft to patch the vulnerability.
Mapped D3FEND Techniques:
Antivirus/Antimalware
Use EDR and antivirus solutions to detect and block the initial malware droppers that are required to execute this local exploit.
D3FEND Defensive Countermeasures
The definitive countermeasure for CVE-2021-43226 is to deploy the corresponding security update from Microsoft. Given its KEV status, this patch should be treated as high priority. Organizations should use their patch management systems (e.g., WSUS, SCCM, Intune) to push the update to all affected endpoints, including workstations (Windows 10, 11) and servers (Server 2016-2022). Prioritize patching of multi-user systems like terminal servers and critical infrastructure servers, as these are high-value targets for privilege escalation. After deployment, run authenticated vulnerability scans to validate that the patch has been successfully applied and that no systems were missed. This action directly remediates the root cause of the vulnerability, preventing exploitation.
To detect the exploitation of CVE-2021-43226, security teams must leverage process analysis capabilities, typically found in EDR solutions. Since this is a local privilege escalation, the attack signature is a low-privilege process spawning a child process with SYSTEM integrity. Configure detection rules to look for this specific parent-child relationship anomaly. For example, an alert should be triggered if outlook.exe or chrome.exe spawns a cmd.exe process which in turn spawns a process running as NT AUTHORITY\SYSTEM. Baseline normal system behavior to reduce false positives. Additionally, monitor for processes interacting with the CLFS driver (clfs.sys) or creating .blf files in unexpected locations (e.g., C:\Users\<user>\AppData\Local\Temp). This provides a critical detection layer for post-compromise activity.
As a foundational hardening measure, enforce the principle of least privilege across the enterprise. While this does not patch the vulnerability itself, it reduces the attack surface and opportunities for an attacker to gain the initial foothold required to exploit CVE-2021-43226. Ensure standard users do not have local administrator rights. Implement application control or allowlisting (using tools like AppLocker or WDAC) to prevent users from running unauthorized executables downloaded from the internet or received via phishing emails. By limiting the attacker's ability to execute their initial payload, you prevent them from ever reaching the stage where they can attempt to exploit this local privilege escalation vulnerability. This is a strategic control that raises the difficulty for attackers across the board.
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Article Author
Jason Gomes
• Cybersecurity PractitionerCybersecurity professional with over 10 years of specialized experience in security operations, threat intelligence, incident response, and security automation. Expertise spans SOAR/XSOAR orchestration, threat intelligence platforms, SIEM/UEBA analytics, and building cyber fusion centers. Background includes technical enablement, solution architecture for enterprise and government clients, and implementing security automation workflows across IR, TIP, and SOC use cases.
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