What Happened

Microsoft's April 2025 Patch Tuesday included a fix for CVE-2025-29824, a use-after-free vulnerability in the Windows Common Log File System (CLFS) kernel driver (clfs.sys). The flaw allows a local attacker with standard user privileges to elevate to SYSTEM.

Microsoft's Threat Intelligence team confirmed the vulnerability was exploited in the wild before the patch was released, with attribution to the ransomware-as-a-service groups Storm-0506 and RansomEXX (also tracked as Ransom.Win64.RANSOMEXX).

Attack Chain

Observed intrusions followed this pattern:

  1. Initial access via phishing or exposed RDP
  2. Lateral movement using credential theft tools
  3. CVE-2025-29824 exploitation to achieve SYSTEM privileges
  4. Deployment of PsExec and custom loaders
  5. RansomEXX payload detonation across domain

Targeted sectors include IT, finance, and retail in the US, Venezuela, Spain, and Saudi Arabia.

Recommended Actions

Patch immediately. Apply KB5055523 (Windows 10/11) or KB5055519 (Server 2022).
  • Verify patch deployment: wmic qfe list | findstr KB5055523
  • Enable Credential Guard to limit lateral movement impact
  • Review CLFS-related ETW events for pre-patch exploitation indicators
  • Audit for unusual SYSTEM process lineage in EDR telemetry

CLFS Background

CLFS (clfs.sys) is a kernel-mode logging subsystem present in all modern Windows versions. It has been a repeated target — this is the fifth CLFS privilege escalation CVE exploited in the wild since 2022, including CVE-2022-24521 and CVE-2023-23376.