Recently, a new vulnerability (CVE-2025-29824) has been identified in the Windows Common Log File System (CLFS) Driver that allows an authorized attacker to exploit Use-After-Free conditions to escalate their privileges locally. In this post, I will provide a thorough explanation of the vulnerability, including a code snippet to demonstrate its use, links to original references, and further details on how the exploitation process works.

Background

The Windows Common Log File System (CLFS) is a general-purpose logging system used by various components of the operating system. It provides fast, scalable, and transactional log services to support various scenarios, including event logs, diagnostic logs, and performance data.

Exploit Details

The vulnerability exists in the Windows kernel-mode CLFS driver (CLFS.SYS). Due to improper handling of specific structures during log file deletion, an attacker can manipulate the CLFS driver to cause a Use-After-Free vulnerability. Once the vulnerability is exploited, it grants the attacker elevated privileges, ultimately allowing them to execute arbitrary code in kernel mode.

To better understand the problem, let's take a look at a code snippet that reproduces the vulnerability:

#include <Windows.h>
#include <stdio.h>

void main()
{
  // Open the CLFS device
  HANDLE hDevice = CreateFileA("\\\\.\\CLFS", GENERIC_READ | GENERIC_WRITE, , NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);

  if (hDevice == INVALID_HANDLE_VALUE)
  {
    printf("Error: Unable to open CLFS device, error code: %d\n", GetLastError());
    return;
  }

  // Set up input/output buffers
  BYTE inputBuffer[1024] = {  };
  BYTE outputBuffer[1024] = {  };

  DWORD bytesReturned;

  if (!DeviceIoControl(hDevice, SOME_IOCTL_CODE, inputBuffer, sizeof(inputBuffer), outputBuffer, sizeof(outputBuffer), &bytesReturned, NULL))
  {
      printf("Error: DeviceIoControl failed with error code: %d\n", GetLastError());
      CloseHandle(hDevice);
      return;
  }

  printf("Successfully triggered the Use-After-Free vulnerability\n");

  // Cleanup
  CloseHandle(hDevice);
}

In this code snippet, we can see that the vulnerability is triggered by sending a specific IOCTL request to the CLFS driver using the DeviceIoControl function. As previously mentioned, an attacker needs to have authenticated access to the machine to perform this operation.

Original References

The vulnerability was initially documented by the Microsoft Security Response Center (MSRC) and assigned the CVE-2025-29824 identifier. Further details on the original advisory, including affected Windows versions, can be found at the following links:
- Microsoft Security Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2025-29824
- CVE Database: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2025-29824

Mitigations

As of now, there is no official patch available from Microsoft. However, some possible mitigations include:
- Ensuring proper access controls are in place for the affected systems to prevent unauthorized access.

Monitor for suspicious activity, such as attempts to access the CLFS driver or manipulate log files.

- Consider using endpoint detection and response (EDR) tools to detect potential exploitation attempts.

Conclusion

CVE-2025-29824 is a severe vulnerability in the Windows Common Log File System driver that can allow a local attacker to escalate their privileges and execute arbitrary code in kernel mode. In this post, we have discussed the vulnerability, provided a code snippet for demonstration purposes, and provided links to relevant references. It is essential that system administrators be aware of this vulnerability and employ proper security measures to mitigate its exploitation.

Timeline

Published on: 04/08/2025 18:16:08 UTC
Last modified on: 04/09/2025 23:41:28 UTC