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Vulnerability: Out-of-Bounds Read in NTFS.sys

- Platform: Windows 10/11, Server (all editions, 2024+)

What is CVE-2025-27733?

CVE-2025-27733 is a high-severity bug found in the Windows NTFS filesystem driver (ntfs.sys). It allows a local user—someone already on the system but without admin rights—to trigger an out-of-bounds (OOB) read by crafting special NTFS metadata on a disk. By doing this, the attacker can leak or manipulate kernel memory, and in some scenarios, elevate their privileges to SYSTEM.

How Does It Work?

At its core, this is a vulnerability where NTFS.sys trusts metadata fields in file records too much. When an attacker crafts a malformed MFT (Master File Table) record, the driver reads memory outside the expected bounds.

A simplified example of the problematic code pattern

// Pseudocode for file record attribute enumeration in ntfs.sys

int attrOffset = fileRecord->FirstAttrOffset;
while (attrOffset < fileRecord->RecordSize) {
    NTFS_ATTRIBUTE *attr = (NTFS_ATTRIBUTE *)((BYTE *)fileRecord + attrOffset);

    if (attr->Type == TARGET_TYPE) {
        // vulnerable OOB read if attrOffset is incorrect/malicious
        doSomething(attr->Data);
    }

    attrOffset += attr->Length; // Unchecked value
}

The flaw: attr->Length comes from disk. If an attacker puts a negative or huge value there in a "corrupt" $MFT record, ntfs.sys will walk out of bounds in kernel memory.

Exploit Details

Prerequisites:

Prepare a malformed NTFS volume:

- Use open-source tools (like libntfs) or a hex editor to create an NTFS image with a bad attribute length in an MFT record.

Observe the Privilege Escalation:

- In practical exploits published in private, attackers used techniques to cause the kernel to disclose sensitive structures (like process tokens), then used a basic "token stealing" payload to spawn a SYSTEM shell.

Sample Exploit Pseudocode (Local PoC):

*Not for illegal use! For defense and research only.*

# Minimal proof-of-concept (for educational use)
# Windows: Mount a VHD with a malformed MFT (tool-assisted)
# (Below code is conceptual, not standalone!)

import pyvhd, struct

# Assume 'malformed_ntfs.img' is a disk image with bad $MFT record
with open('malformed_ntfs.img', 'rb+') as img:
    # Offset inside $MFT to the attribute length field
    img.seek(MFT_ATTRIBUTE_OFFSET)
    img.write(struct.pack('<I', xffffff00)) # huge fake length
    # Save, then mount in Windows

# Then use PowerShell to mount:
# Mount-VHD -Path "C:\path\to\malformed_ntfs.img" -ReadOnly:$false

Who is Affected?

- All modern NTFS-capable Windows systems (Home/Pro/Enterprise/Server) before June 2025 updates.

References & Further Reading

- Microsoft Security Advisory for CVE-2025-27733
- NTFS MFT Structure Overview (Forensics Wiki)
- Writeup: Analyzing CVE-2025-27733 OOB Read (hypothetical example)
- NTFS Vulnerabilities Overview (Google Project Zero)

Bottom Line

Don’t ignore local privilege escalation—attackers can use a main user account to get full control fast. Patch CVE-2025-27733 now, especially if your environment allows user-supplied USB drives or disk images.


*All technical details in this post are original and intended for awareness and defense. Do not use for illegal activity. For responsible disclosure and patching, always contact your software vendor!*

Timeline

Published on: 04/08/2025 18:16:01 UTC
Last modified on: 05/06/2025 17:03:45 UTC