CVE-2025-29824: Windows CLFS Zero-Day Exploited by Ransomware

CVE-2025-29824 is a use-after-free vulnerability in the Windows Common Log File System (CLFS) driver, actively exploited by multiple ransomware groups as a privilege escalation step in attacks against US organizations. CVSS 7.8 HIGH. Patched in April 2025 Patch Tuesday.

What Is CVE-2025-29824?

The Windows Common Log File System (CLFS) is a kernel-mode logging infrastructure used by Windows and applications for transaction logging. CVE-2025-29824 is a use-after-free vulnerability in the CLFS driver (clfs.sys) that allows a local attacker to elevate privileges to SYSTEM.

This is the sixth CLFS vulnerability exploited in the wild since 2022 — a pattern that suggests ransomware groups maintain dedicated Windows kernel exploit teams specifically targeting this driver. The CLFS attack surface is productive because it's a complex driver with rich functionality accessible to low-privilege users.

The CLFS Exploitation Pattern in Ransomware

Ransomware operators have refined a highly effective attack chain using CLFS vulnerabilities:

1. Initial Access: Exploit internet-facing vulnerabilities or phishing for initial foothold
2. Limited Privileges: Initial access typically lands with user or service account privileges
3. CLFS LPE: Use CVE-2025-29824 (or previous CLFS CVEs) to escalate to SYSTEM
4. Defense Evasion: With SYSTEM privileges, disable AV/EDR, clear event logs
5. Credential Theft: LSASS dump, DCSync, or pass-the-hash with SYSTEM
6. Ransomware Deployment: Encrypt domain-wide via GPO or PsExec with stolen credentials

Technical Details

The vulnerability involves a use-after-free condition in the CLFS driver's handling of BLF (Base Log File) structures. When certain log file operations are performed with a crafted sequence, the driver can be induced to access memory after it has been freed, allowing an attacker to achieve arbitrary write primitives in kernel memory.

# The exploitation process (simplified):
1. Create a CLFS log file as unprivileged user
2. Trigger the use-after-free via crafted IOCTL sequence
3. Reclaim the freed memory with attacker-controlled data
4. Corrupt EPROCESS token to gain SYSTEM privileges
5. Spawn elevated shell or inject into SYSTEM process

Why CLFS Is A Repeat Target

CLFS is targeted repeatedly because:

• Complex driver code with many state transitions (high vulnerability density)
• Accessible to unprivileged users — no special permissions required to interact with CLFS
• Stable exploitation — kernel address layouts in CLFS structures are predictable
• Functional across all modern Windows versions with minimal modification

Affected Systems

All Windows systems running unpatched versions prior to April 2025 Patch Tuesday:

• Windows 10 (all supported versions)
• Windows 11 (all supported versions)
• Windows Server 2012 R2 through 2025

Detection

Behavioral Indicators

# Detect unusual CLFS operations from user-mode processes
# Using Sysmon Event ID 11 (File Create) for .blf files
Get-WinEvent -FilterHashtable @{LogName='Microsoft-Windows-Sysmon/Operational'; Id=11} |
  Where-Object {$_.Message -like '*.blf*'} |
  Where-Object {$_.Message -notlike '*\System32\*'} |
  Select TimeCreated, Message

# Look for process token privilege elevation
# Sysmon Event 10 (ProcessAccess) targeting lsass from non-system processes
Get-WinEvent -FilterHashtable @{LogName='Microsoft-Windows-Sysmon/Operational'; Id=10} |
  Where-Object {$_.Message -like '*lsass*'}

PipeMagic Backdoor IOCs

• DLL files loaded by dllhost.exe from temp directories
• Outbound HTTPS connections to CDN-fronted C2 infrastructure
• Named pipe creation with randomized names
• Memory-only payloads — no file artifacts after execution

Mitigation

1. Apply April 2025 Patch Tuesday updates: KB5055523 (Windows 10/11) addresses CVE-2025-29824
2. Deploy EDR with behavioral detection: Signature-based AV will not catch zero-day LPE; behavioral EDR is required
3. Enable Windows Defender Credential Guard: Prevents LSASS dumping after privilege escalation
4. Implement application control: Restrict execution of unauthorized binaries that could trigger the exploit
5. Backup and test restores: Assume ransomware deployment is possible; offline backups are non-negotiable
6. Monitor for CLFS file creation: Flag creation of .blf files from unusual locations or processes

KENSAI Detection Capability

• Patch inventory scanning: KENSAI identifies all Windows systems missing the April 2025 cumulative update
• Ransomware attack surface assessment: Maps the full attack chain — from initial access vectors to LPE opportunities
• Exposure prioritization: Ranks unpatched systems by network exposure and data sensitivity
• Historical CLFS CVE coverage: KENSAI tracks all six exploited CLFS CVEs, ensuring comprehensive coverage