🔬 Technical Deep Dive: CVE-2025-29824

Use-After-Free in CLFS Driver Enabling SYSTEM Privilege Escalation via PipeMagic and RansomEXX Deployment

1. Vulnerability Anatomy: How CVE-2025-29824 Works

This is not your average privilege escalation bug. CVE-2025-29824 abuses a Use-After-Free (UAF) flaw in the Windows Common Log File System (CLFS) driver—clfs.sys—a long-standing kernel-mode logging subsystem.

➤ Key Concepts:

• UAF (CWE-416): A memory safety flaw where a pointer is used after its associated memory is freed.

• clfs.sys: Provides transactional logging to subsystems like the Windows Registry and NTFS. Operates in kernel mode.

• Exploitation Goal: Attacker gains arbitrary write capability in kernel space, used to overwrite process tokens and escalate privileges.

➤ Exploit Flow Breakdown:

1. Memory Leak via NtQuerySystemInformation:
   Leaks kernel memory addresses (bypassing KASLR) by querying sensitive SystemBigPoolInformation class—enabled due to overly permissive access in non-hardened builds.

2. Dangling Pointer Reuse in clfs.sys:
   Exploit triggers an internal CLFS structure deallocation and then forces reuse of the dangling pointer via crafted user-mode interaction—likely by manipulating a log file stream or base log file (.blf).

3. Arbitrary Write Primitive via RtlSetAllBits:
   Once attacker has control of a freed structure, they call RtlSetAllBits() on a manipulated bitmap associated with the freed memory—allowing them to flip every bit in a target object.

4. Token Overwrite:
   The manipulated memory structure overlaps with a security token object, which is overwritten to set its Privileges.Enabled and TokenType fields to all 1s—effectively granting full SYSTEM access to the attacking process.

➤ Technical Notes:

• Uses classic write-what-where methodology common in UAF-to-LPE transitions.

• Exploitation requires fine-grained heap grooming and timing due to race conditions in memory reuse.

• Microsoft’s patch likely involved modifying allocation logic or introducing pool tagging/hardening.

2. Exploit Deployment: From Initial Foothold to SYSTEM Access

The vulnerability is exploited after initial access, making it a post-exploitation weapon—a tool for converting user-mode shells into SYSTEM shells. The entire chain is modular and stealthy, centered around the PipeMagic backdoor, tailored to deliver this and other LPE exploits.

➤ Real-World Exploitation Chain (Storm-2460):

1. Initial Access:

   • Most likely vectors: phishing, compromised VPN credentials, vulnerable public-facing service.

   • Common traits: low-privilege, persistent access without triggering AV/EDR.

2. Payload Delivery:

   • Attacker uses certutil.exe to pull a malicious MSBuild project (.csproj) from a compromised but legitimate site.

   • Inline task in MSBuild decrypts a .NET loader in memory—evading disk detection.

3. PipeMagic Deployment:

   • Acts as a loader and exploit orchestrator.

   • Injects into dllhost.exe to evade parent-child process relationship checks.

   • Maintains modular architecture—payloads loaded via plugins.

4. Privilege Escalation:

   • PipeMagic drops a CLFS base log file: C:\ProgramData\SkyPDF\PDUDrv.blf.

   • Triggers clfs.sys UAF condition and escalates from user to SYSTEM.

5. Credential Dumping:

   • Injects into winlogon.exe to further evade detection.

   • Executes procdump.exe to dump lsass.exe, using:

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     dllhost.exe -accepteula -r -ma lsass.exe C:\ProgramData\[random].dmp

      

6. Ransomware Execution:

   • Drops RansomEXX payload via:

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     dllhost.exe --do C:\foobar

      

   • Appends custom extension and ransom note !_READ_ME_REXX2_!.txt.

7. Cleanup & Evasion:

   • Disables recovery:

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     bcdedit /set {default} recoveryenabled no

      

   • Deletes backup catalogs:

     cpp

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     wbadmin delete catalog -quiet

      

   • Clears event logs:

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     wevtutil cl Application

      

3. Detection Engineering: IoCs, Queries, and Heuristics

➤ Critical File and Process Artifacts:

• .blf File Creation:

  • C:\ProgramData\SkyPDF\PDUDrv.blf

  • Unusual for .blf files to appear outside of \Windows\System32\LogFiles.

• Abnormal dllhost.exe Usage:

  • Legitimate COM host process abused for privilege escalation and payload launching.

• LSASS Dump Behavior:

  • dllhost.exe spawning procdump.exe targeting lsass.exe.

➤ SIEM/Hunting Queries:

Sentinel/Splunk Sample Queries:

➤ EDR/AV Detections:

4. Defensive Strategies: Before and After the Patch

➤ Patch Status Recap (as of April 9, 2025):

➤ Compensating Controls for Windows 10:

• EDR Tuning for clfs.sys:

  • Monitor memory access to clfs.sys.

  • Alert on unexpected .blf file creation in userland directories.

  • Watch dllhost.exe anomalies and token privilege escalations.

• Exploit Tool Hardening:

  • AppLocker/WDAC: Block certutil.exe, procdump.exe, wbadmin.exe, and bcdedit.exe for non-admin users.

  • Log use of MSBuild in non-development contexts.

• Access Control Auditing:

  • Enumerate users/groups with SeDebugPrivilege.

  • Alert on processes acquiring SeDebugPrivilege unexpectedly.

• Fallback Response Planning:

  • Prepare incident playbooks for SYSTEM-level privilege escalation events.

  • Monitor for lateral movement and privilege spread using dumped LSASS credentials.

5. Architectural Weakness: CLFS as a Chronic Target

CLFS is an attractive target for kernel-mode exploitation due to:

• Legacy code complexity and low coverage from fuzzing tools.

• Kernel-level privileges upon compromise.

• Persistent use across various Windows services.

➤ Past CVEs Exploited in CLFS:

• CVE-2022-24521 – UAF in clfs.sys (zero-day exploited)

• CVE-2022-37969 – UAF (reported by multiple threat intel teams)

• CVE-2023-23376 – Privilege escalation (CLFS memory corruption)

• CVE-2023-28252 – Used by Nokoyawa

• CVE-2024-49138 – SYSTEM-level LPE

• CVE-2025-29824 – Current exploit used by RansomEXX

➤ Microsoft’s Response:

• Beginning implementation of HMAC-based integrity validation for CLFS logs.

• Future mitigation may involve signature verification of log file streams.

• At time of writing, no customer-facing mitigation has shipped beyond individual CVE patches.

6. Takeaways for Security Engineers and SOCs

➤ Prioritized Actions:

• Patch Management:

  • Prioritize Windows 11/Server patching.

  • Isolate or compensate for unpatched Windows 10 systems.

• Threat Hunting:

  • Sweep for .blf artifacts.

  • Query for dllhost.exe misuse.

  • Correlate credential dumping with privilege elevation.

• Zero-Day Readiness:

  • Prepare for rapid response when patch delays occur.

  • Use threat intelligence feeds to adjust alerting.

• CLFS Risk Awareness:

  • Flag CLFS driver interaction in telemetry for deeper review.

  • Consider it a high-risk kernel component needing proactive monitoring.

• Cross-Functional Response:

  • Ensure incident response, compliance, and backup teams are aligned on potential impact (especially in healthcare and education sectors).