CVE-2025-24985 - Leveraging Integer Overflow in Windows Fast FAT Driver for Local Code Execution

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Introduction

A new vulnerability tracked as CVE-2025-24985 has been discovered in the Windows Fast FAT (File Allocation Table) Driver. This vulnerability involves an integer overflow, also known as "wraparound," which allows local attackers to escalate their privileges and potentially execute arbitrary code on affected Windows systems.

In this long read, we'll break down how the integer overflow occurs, why it's dangerous, provide a code snippet that showcases the underlying issue, and discuss exploit details. We'll also supply authoritative links for those who want to delve deeper. Our explanation is written in straightforward, accessible American English.

What Is the Windows Fast FAT Driver?

The Fast FAT (Fastfat.sys) driver is a core component in Microsoft Windows, responsible for handling FAT12/FAT16/FAT32 filesystems. This driver is present in almost every version of Windows, including the latest ones for compatibility with flash drives, SD cards, and older storage formats.

The Vulnerability: Integer Overflow or Wraparound

CVE-2025-24985 is classified as an "Integer overflow or wraparound" issue. In programming, an integer overflow happens when an arithmetic operation tries to create a numeric value outside the range that can be represented with a given number of bits. For example, a 32-bit unsigned integer can store values from to 4,294,967,295. Adding one to the maximum will "wrap around" to zero, which can lead to dangerous logic errors.

How Does It Happen in Fast FAT?

When the driver handles user-supplied data (like file size or offset), it may calculate buffer lengths or perform pointer arithmetic using these values. If an attacker supplies a huge value, such as xFFFFFFFF, and the driver doesn't properly check for overflow before allocating memory, logic errors and memory corruptions can result.

Code Snippet: A Simplified View

The following is a simplified pseudo-code to illustrate the logic error. This is not the actual driver code but is representative of the vulnerable logic pattern:

// Imagine this function processes a request to read file data
NTSTATUS FastFatReadRoutine(PDEVICE_OBJECT DeviceObject, PIRP Irp) {
    ULONG userOffset = Irp->Parameters.Read.ByteOffset;
    ULONG userLength = Irp->Parameters.Read.Length;

    // Vulnerable: Adding userOffset + userLength without overflow check
    ULONG endOffset = userOffset + userLength;

    // Later, uses the endOffset to allocate buffer or read data
    if (endOffset > fileSize) {
        // error: reading past end of file
        return STATUS_INVALID_PARAMETER;
    }

    UCHAR *buffer = ExAllocatePoolWithTag(NonPagedPool, userLength, 'tfaF');
    if (buffer == NULL)
        return STATUS_INSUFFICIENT_RESOURCES;

    // ...proceeds to fill buffer with file data and return to user
    // ...

    return STATUS_SUCCESS;
}

What’s wrong here?

If an attacker sets userOffset to a very large value (close to the maximum unsigned integer) and a moderate userLength, the sum endOffset will "wrap around" to a smaller number. This tricks the driver into thinking the operation is within safe bounds when it's not.

Local access is required: The attacker must be able to run code on the target system.

- Potential impact: Escalation of privileges, execution of arbitrary code in kernel mode, and system compromise.

Craft a Malicious IOCTL Request:

- The attacker sends an input/output control (IOCTL) request with a userOffset and userLength value that will cause their sum (userOffset + userLength) to overflow.

Cause Controlled Memory Corruption:

- The driver allocates a buffer assuming no overflow occurred, but in reality, the attacker controls how the memory is used, potentially leading to buffer overflows or writing arbitrary data to sensitive locations.

Trigger Code Execution:

- By exploiting the overflow, the attacker may overwrite function pointers or kernel data structures, resulting in the ability to execute arbitrary code with kernel-level privileges.

Proof-of-Concept

Here's a conceptual Python snippet using the Windows DeviceIoControl API to trigger the overflow (for demonstration purposes only):

import os
import ctypes
from ctypes import wintypes

# Open handle to a FAT volume (e.g., \\\\.\\E:)
handle = ctypes.windll.kernel32.CreateFileW(
    u"\\\\.\\E:",
    xC000000,  # GENERIC_READ | GENERIC_WRITE
    ,
    None,
    3,  # OPEN_EXISTING
    ,
    None
)

if handle == -1:
    print("Failed to open FAT device.")
    exit(1)

# Craft malicious buffer
user_offset = xFFFFFFF  # Large value
user_length = x20        # Small value; sum wraps to x10

# Depending on IOCTL requests and driver interface, structure will vary
# Here you would craft the IOCTL/IRP to trigger the bug

# Clean up
ctypes.windll.kernel32.CloseHandle(handle)

> Warning: This is for educational demonstration only — do not attempt unauthorized exploitation on real systems.

Mitigation and Patch

Microsoft has released a patch addressing this vulnerability. If you're responsible for Windows systems:

Apply all security updates immediately, especially for CVE-2025-24985.

- Deploy principle of least privilege: Limit which users have the ability to execute local code on sensitive servers.

References and More Reading

- Microsoft Security Guide for CVE-2025-24985 *(official advisory)*
- Understanding Integer Overflow Vulnerabilities
- NTFS/FAT Windows Driver Programming *(Microsoft driver docs)*

Conclusion

Integer overflows in kernel drivers are dangerous, particularly in widely deployed components like Windows Fast FAT driver. CVE-2025-24985 serves as a reminder that thorough bounds-checking is critical for kernels and drivers. If left unpatched, this issue can give local attackers full control over a vulnerable system.

Timeline

Published on: 03/11/2025 17:16:34 UTC
Last modified on: 03/31/2025 01:40:44 UTC