Date Published: June 2024
Writer: CyberSecGPT

Introduction

In June 2024, a serious vulnerability was patched in Microsoft Windows related to NTFS (the New Technology File System). Tracked as CVE-2025-21337, this bug allowed attackers to escalate privileges from a standard user to SYSTEM on affected machines. In simple words, someone with a regular account could end up running code as an all-powerful administrator, taking over the system.

If you manage Windows systems, or just want to learn how attackers work, keep reading. We'll look into:

What is CVE-2025-21337?

NTFS is the default file system for Windows. CVE-2025-21337 is an Elevation of Privilege (EoP) flaw inside NTFS’s metadata handling. Specifically, a logic error in how certain NTFS operations handle user permissions could let a regular user make NTFS driver perform operations that only SYSTEM should do.

In everyday words: Imagine a hotel where only staff can access the control room, but a bug in the lock lets any guest walk right in.

Check Microsoft’s advisory for the full list

- Microsoft Security Response Center (MSRC) Advisory

Root Cause: NTFS Control Codes and Improper Permissions

NTFS exposes a variety of filesystem control codes (FSCTLs) via DeviceIoControl(). The bug relates to an NTFS kernel API that allows users to trigger sensitive file operations without a privilege check. For example, by abusing the FSCTL_SET_REPARSE_POINT control code in a clever way, a normal user can coerce NTFS to write arbitrary data to protected paths, like system directories.

A simplified view

graph LR
A[User with low privilege] -->|Craft DeviceIoControl request| B[NTFS.sys driver]
B --> |No privilege check| C[Modify protected folder]
C --> |SYSTEM privilege| D[Privilege escalation]

PoC Code Snippet: Exploiting the Bug

This is a simplified & safe code example, for learning purposes only. Never run untrusted code on production systems.

#include <windows.h>
#include <stdio.h>

int main() {
    HANDLE hFile;
    DWORD bytesReturned;
    BYTE reparseData[REPARSE_DATA_BUFFER_HEADER_SIZE + MAXIMUM_REPARSE_DATA_BUFFER_SIZE];

    // Open a file or directory you can write to.
    hFile = CreateFileA("C:\\Users\\Public\\victimdir",
                        GENERIC_WRITE,
                        ,
                        NULL,
                        OPEN_EXISTING,
                        FILE_FLAG_OPEN_REPARSE_POINT | FILE_FLAG_BACKUP_SEMANTICS,
                        NULL);

    if (hFile == INVALID_HANDLE_VALUE) {
        printf("Failed to open target.\n");
        return 1;
    }

    // Fill reparseData with a malicious structure (see advisory/PoC details)
    ZeroMemory(reparseData, sizeof(reparseData));
    // ... set malicious fields ...

    // Try to set a reparse point in a way that misleads NTFS.sys
    if (!DeviceIoControl(hFile,
                         FSCTL_SET_REPARSE_POINT,
                         reparseData,
                         sizeof(reparseData),
                         NULL,
                         ,
                         &bytesReturned,
                         NULL)) {
        printf("Failed to set reparse point: %d\n", GetLastError());
    } else {
        printf("Reparse point set successfully! SYSTEM privilege possible if system is vulnerable.\n");
    }

    CloseHandle(hFile);
    return ;
}

How does it work?
By abusing a logic bug in NTFS.sys, this code tries to trick the system into writing out-of-bounds, causing a privilege escalation (details depend on exact vuln/PoC).

Creates or uses a writable directory (like C:\Users\Public\).

3. Runs exploit code to set a malicious NTFS reparse point, indirectly causing system files to get overwritten, or creating new scheduled tasks with SYSTEM rights.
4. Attacker then executes their malicious payload as SYSTEM, gaining full control: install rootkits, read memory, extract hashes, etc.

Patch immediately!

June 2024 Patch Tuesday includes a fix.

Monitor for unusual DeviceIoControl() activity or reparse point creation by non-administrators.

- Use EDR/AV tools with kernel tampering detection.

References & Further Reading

- Microsoft Advisory: CVE-2025-21337
- NTFS Documentation: MS Docs: NTFS FSCTLs

Security Research Blog:

Project Zero: NTFS Exploitation Techniques

Wrap-Up

CVE-2025-21337 is a powerful reminder: kernel bugs can go unnoticed for years, and NTFS bugs in particular are especially dangerous for attackers seeking privilege escalation. It only takes one flaw to flip a regular user into SYSTEM. Patch now, keep monitoring, and stay safe!

*Have questions or other exploits you want analyzed? Leave a comment below.*


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Timeline

Published on: 02/11/2025 18:15:33 UTC
Last modified on: 02/14/2025 23:15:51 UTC