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

Overview

On April 8, 2025, Microsoft patched CVE-2025-29824 as part of Patch Tuesday — a use-after-free vulnerability in the Windows Common Log File System (CLFS) driver that allows a local attacker to escalate privileges to SYSTEM. The vulnerability was already being actively exploited in the wild at the time of disclosure, making it a true zero-day.

Microsoft Threat Intelligence attributed exploitation to Storm-2460, a threat actor observed deploying PipeMagic malware in post-exploitation phases. Targeted sectors included IT and real estate in the United States, financial institutions in Venezuela, a Spanish software company, and a retail organization in Saudi Arabia — indicating broad, opportunistic targeting rather than a narrowly scoped campaign.

The CLFS Driver: A Recurring Target

The Common Log File System is a kernel-mode driver (clfs.sys) that provides general-purpose logging services to both kernel-mode and user-mode applications. It has become a persistent target for privilege escalation research and exploitation:

• CVE-2022-24521 — CLFS privilege escalation, exploited in the wild
• CVE-2023-23376 — CLFS privilege escalation, exploited in the wild
• CVE-2023-28252 — CLFS privilege escalation, exploited by Nokoyawa ransomware
• CVE-2025-29824 — latest in this pattern

The recurring nature of CLFS vulnerabilities reflects the complexity of the driver's internal state management and the value of a reliable local privilege escalation primitive in ransomware operations.

Technical Details

Use-After-Free in CLFS

A use-after-free (UAF) vulnerability occurs when a program continues to use a pointer after the memory it references has been freed. In kernel drivers, UAFs are particularly dangerous because the freed memory may be reclaimed and written by an attacker-controlled allocation, allowing controlled data to be interpreted as kernel objects.

In CVE-2025-29824, the flaw exists in how CLFS manages log block objects. Under specific conditions, a log block can be freed while a reference to it remains accessible to a user-mode caller. An attacker who can manipulate heap layout and trigger the free at the right moment can reclaim the freed memory with attacker-controlled content.

/* Conceptual representation of the vulnerability class */

// 1. CLFS allocates a log block object
LOG_BLOCK *block = allocate_log_block();

// 2. Under certain conditions, CLFS frees the block
// but does NOT clear all references to it
free_log_block(block);
// block pointer is now dangling

// 3. Attacker reclaims the freed memory with controlled data
// by triggering a kernel allocation of the same size

// 4. Attacker calls a CLFS function that still holds
// the dangling `block` pointer — now pointing to
// attacker-controlled data — leading to controlled
// kernel memory operations

Exploitation Chain

Exploiting this vulnerability to achieve SYSTEM privileges typically follows this pattern:

1. Heap spray: Allocate controlled kernel objects to shape the heap
2. Trigger free: Call the CLFS API path that frees the target object without clearing references
3. Reclaim: Quickly allocate a controlled object of the same size to occupy the freed memory
4. Corrupt: Use the dangling reference to overwrite a kernel structure (e.g., a token pointer in an EPROCESS object)
5. Privilege escalation: Modify the current process token to reflect SYSTEM privileges

Microsoft noted that Windows 11 24H2 was not affected by this specific vulnerability — the internal CLFS structures were refactored in that release.

Observed Exploitation: Storm-2460 and PipeMagic

Initial Access via MSBuild Lure

In the observed campaign, Storm-2460 delivered PipeMagic through a malicious application that displayed a fake DPIA (Data Protection Impact Assessment) error message while executing in the background. The application was crafted to appear legitimate enough to pass initial inspection.

PipeMagic is a plugin-based trojan that has been observed since 2022. It communicates over named pipes (hence the name) and supports a modular payload architecture that allows the operator to deploy additional tools post-compromise.

CVE-2025-29824 for SYSTEM Access

After establishing a foothold as a standard user, the attackers deployed the CLFS exploit to elevate to SYSTEM. With SYSTEM privileges they:

• Dumped credentials from LSASS memory
• Deployed ransomware payloads
• Used certutil.exe (a living-off-the-land binary) to download additional components

Ransomware Deployment

Post-escalation, the campaign deployed ransomware that encrypted files and dropped ransom notes. The specific ransomware family varied across victims, consistent with a broker model where Storm-2460 provides access and exploit capabilities to affiliated ransomware operators.

Detection

Patch Status Verification

# Check if April 2025 Patch Tuesday updates are installed
Get-HotFix | Where-Object {$_.InstalledOn -gt (Get-Date "2025-04-01")} | 
  Select-Object HotFixID, InstalledOn | Sort-Object InstalledOn -Descending

# Check CLFS driver version (patched version will be higher)
Get-Item C:\Windows\System32\drivers\clfs.sys | 
  Select-Object VersionInfo

Behavioral Detection

The exploitation chain produces observable behaviors that EDR and SIEM tools can detect:

• LSASS access: Processes unexpectedly opening a handle to lsass.exe with PROCESS_VM_READ
• Token manipulation: Kernel-level token privilege modifications not initiated by legitimate system processes
• certutil abuse: certutil.exe -urlcache -split -f <url> used to download files
• Named pipe creation: Unusual named pipes created by non-system processes (PipeMagic indicator)

# Hunt for suspicious LSASS access attempts (PowerShell + Sysmon)
Get-WinEvent -LogName "Microsoft-Windows-Sysmon/Operational" |
  Where-Object {$_.Id -eq 10 -and $_.Message -like "*lsass*"} |
  Select-Object TimeCreated, Message | Select-Object -First 20

YARA Rule Concept for PipeMagic

rule PipeMagic_Loader {
    meta:
        description = "Detects PipeMagic loader characteristics"
        reference = "CVE-2025-29824 campaign"
    strings:
        $pipe_pattern = "\\\\.\\pipe\\" wide
        $fake_error = "DPIA" ascii wide
        $certutil = "certutil" ascii nocase
    condition:
        uint16(0) == 0x5A4D and
        2 of ($pipe_pattern, $fake_error, $certutil)
}

Mitigation

Immediate: Apply April 2025 Patches

The fix is included in the April 8, 2025 Patch Tuesday release. Patch immediately — this vulnerability is being actively exploited.

# Force Windows Update check
UsoClient.exe StartScan
# Or via Settings > Windows Update > Check for updates

Upgrade to Windows 11 24H2

Windows 11 24H2 is not affected due to CLFS refactoring. Organizations on older Windows versions should treat migration as a security priority, not just a feature upgrade.

Defense in Depth

Since this is a local privilege escalation, the attacker needs code execution as a standard user first. Reducing the initial access surface reduces exposure:

• Enforce application allowlisting to block unknown executables
• Restrict execution from user-writable directories (%APPDATA%, %TEMP%, Downloads)
• Enable Credential Guard to protect LSASS from memory access
• Deploy EDR with kernel-level visibility to detect token manipulation

Conclusion

CVE-2025-29824 follows a well-established pattern: a CLFS use-after-free provides a reliable SYSTEM escalation primitive, which ransomware operators fold into their deployment chains. The recurring nature of CLFS vulnerabilities suggests organizations should treat this driver as a persistent risk surface and prioritize patching velocity for Windows kernel updates accordingly.

The broader lesson is that ransomware groups are sophisticated enough to develop and deploy kernel-level zero-days. The bar for "commodity" ransomware has risen significantly — defenders cannot assume that only nation-state actors operate at this technical level.