CVE-2025-29824

What is the Windows Common Log File System (CLFS) Driver?

The Windows Common Log File System (CLFS) is a kernel-mode logging infrastructure that provides transactional logging services to both kernel-mode and user-mode components. It is used by Windows internals, SQL Server, and other applications requiring crash-consistent log files. The CLFS driver (clfs.sys) processes complex log file structures — .blf (base log files) — in the kernel with full system privileges. Because CLFS parses user-supplied file data in the kernel, vulnerabilities in CLFS provide a path from any user-accessible file operation to SYSTEM-level code execution.

CLFS has been one of the most exploited Windows kernel components in recent years: CVE-2022-37969, CVE-2023-23376, CVE-2023-28252, CVE-2024-20672, CVE-2025-29824, CVE-2025-32701, and CVE-2025-32706 all represent CLFS zero-days actively exploited before Microsoft released patches.

Overview

CVE-2025-29824 is a use-after-free vulnerability (CWE-416) in the Windows CLFS driver that allows a locally authenticated attacker with standard user privileges to escalate to SYSTEM. Microsoft disclosed it as a zero-day on April 8, 2025 — the same day as the patch — and attributed active exploitation to Storm-2460, a financially motivated threat actor that deployed RansomEXX ransomware as the final payload. This is the first CLFS zero-day with confirmed ransomware deployment.

Affected Versions

Note: Windows 10 version 1809 was not initially patched on April 8 — Microsoft released an out-of-band patch subsequently.

Technical Details

The use-after-free (CWE-416) occurs in the CLFS driver's log file processing path. CLFS allocates kernel objects to represent log file metadata during processing; a race condition or ordering error allows these objects to be freed while a reference to them is retained in a related structure. By crafting a specially structured .blf log file and triggering specific CLFS operations, an attacker causes the freed memory to be reused with attacker-controlled content, enabling arbitrary kernel code execution.

The Low attack complexity (AC:L) reflects that the exploit was reliably weaponized — Storm-2460 integrated it into a repeatable attack chain as a privilege escalation step following initial access via other means.

Discovery

Microsoft Threat Intelligence discovered the zero-day exploitation by Storm-2460 prior to patch release.

Exploitation Context

Storm-2460 exploited CVE-2025-29824 in targeted ransomware operations across organizations in the US, Venezuela, Saudi Arabia, and Spain — targeting IT, retail, and financial sectors. The full attack chain:

1. Initial access via an internet-facing system compromise (web server exploitation or credential abuse)
2. CVE-2025-29824 for local privilege escalation from user to SYSTEM
3. PipeMagic malware deployment — a plugin-based backdoor that exfiltrates data and downloads the ransomware
4. RansomEXX (also tracked as Defray/Defray777) ransomware deployment

Storm-2460 is an established ransomware operator. This is the first confirmed case where a CLFS zero-day was used specifically for ransomware delivery, rather than espionage.

Remediation

1. Apply the April 2025 cumulative update immediately. Windows 10 1809 users may need a separate out-of-band patch.
2. Enable Windows Defender Exploit Guard CLFS-related mitigations where available.
3. Restrict local logon and RDP access — Storm-2460's attack chain requires local access as a standard user; limiting who can log on to servers reduces the available attack surface.
4. Monitor for PipeMagic indicators: look for anomalous child processes, network beacon activity, and unusual CLFS log file creation from non-system processes.
5. Apply all CLFS-related patches historically — the repeated exploitation pattern suggests CLFS is a sustained research target; keeping cumulative updates current is essential.