CVE-2023-4692: GNU GRUB2 NTFS RCE Vulnerability

CVE-2023-4692 Overview

An out-of-bounds write vulnerability has been identified in GRUB2's NTFS filesystem driver that poses a significant threat to system boot security. This flaw allows an attacker to present a specially crafted NTFS filesystem image, leading to heap metadata corruption within GRUB2. In certain circumstances, this attack can also corrupt UEFI firmware heap metadata, potentially enabling arbitrary code execution and Secure Boot protection bypass.

This vulnerability is particularly concerning because it targets the bootloader—a critical component that executes before the operating system loads. Successful exploitation could allow attackers to establish persistent, pre-OS malware that survives operating system reinstallation and is invisible to traditional security solutions.

Critical Impact

Successful exploitation enables arbitrary code execution during the boot process and bypass of Secure Boot protections, potentially allowing persistent rootkit installation below the OS level.

Affected Products

• GNU GRUB2 (all versions prior to patch)
• Red Hat Enterprise Linux 8.0
• Red Hat Enterprise Linux 9.0

Discovery Timeline

• October 25, 2023 - CVE-2023-4692 published to NVD
• November 4, 2025 - Last updated in NVD database

Technical Details for CVE-2023-4692

Vulnerability Analysis

The vulnerability resides in GRUB2's NTFS filesystem driver, which is responsible for parsing and mounting NTFS volumes during the boot process. When processing a maliciously crafted NTFS filesystem image, the driver fails to properly validate boundary conditions, resulting in an out-of-bounds write operation that corrupts GRUB2's heap metadata.

The flaw is classified under CWE-122 (Heap-based Buffer Overflow) and CWE-787 (Out-of-bounds Write), indicating that the vulnerability allows writing data beyond allocated memory boundaries in the heap region. This type of memory corruption is particularly dangerous in bootloader code because:

1. The bootloader operates with elevated privileges before any OS-level protections are active
2. Secure Boot trust chain can be compromised at its foundation
3. UEFI firmware memory corruption can affect system integrity beyond the boot process

Root Cause

The root cause stems from insufficient bounds checking in the NTFS filesystem driver when parsing filesystem structures. The driver processes NTFS metadata structures without adequately validating size and offset fields, allowing an attacker-controlled NTFS image to specify values that cause writes outside the allocated buffer boundaries.

When GRUB2 attempts to read from a malicious NTFS volume (such as one on a USB drive or a partition on the boot disk), the crafted filesystem metadata triggers the out-of-bounds write condition. This corrupts heap management structures, which can be leveraged to achieve controlled memory writes and ultimately arbitrary code execution.

Attack Vector

The attack requires local access to the system, either through physical access to insert malicious bootable media or through prior compromise allowing filesystem manipulation. An attacker must craft a specially malformed NTFS filesystem image containing metadata designed to trigger the out-of-bounds write condition.

The attack scenario typically involves:

1. Creating a malicious NTFS filesystem image with crafted metadata structures
2. Presenting this image to the target system during boot (via USB, modified partition, or network boot)
3. When GRUB2 attempts to parse the NTFS filesystem, heap corruption occurs
4. The corrupted heap metadata is leveraged to redirect execution flow
5. Arbitrary code executes in the GRUB2 context, potentially bypassing Secure Boot

The vulnerability mechanism exploits improper validation in the NTFS driver's metadata parsing routines. When processing filesystem structures such as the Master File Table (MFT) or attribute lists, the driver allocates heap buffers based on size values read from the filesystem. Maliciously crafted size or offset values can cause writes to memory locations outside the intended buffer, corrupting adjacent heap metadata and enabling exploitation.

For detailed technical analysis, refer to the DFIR Article on CVE-2023-4692 and CVE-2023-4693 and the GNU GRUB Development Mailing List Post.

Detection Methods for CVE-2023-4692

Indicators of Compromise

• Unexpected NTFS partitions or filesystem images present on boot media
• Modified or unsigned GRUB2 bootloader binaries
• Secure Boot validation failures during system boot
• Unusual boot behavior or unexpected code execution during the boot process

Detection Strategies

• Monitor for changes to bootloader files and EFI system partition contents
• Implement Secure Boot with properly enrolled keys to detect unsigned bootloader modifications
• Use firmware integrity monitoring tools to detect UEFI heap corruption indicators
• Deploy endpoint detection capabilities that can inspect pre-boot components

Monitoring Recommendations

• Enable UEFI Secure Boot logging to capture boot-time verification events
• Implement file integrity monitoring on /boot and EFI system partition directories
• Monitor for unauthorized access to bootloader configuration files (grub.cfg)
• Utilize SentinelOne's firmware and boot protection capabilities for continuous boot process integrity monitoring

How to Mitigate CVE-2023-4692

Immediate Actions Required

• Update GRUB2 to the latest patched version provided by your distribution
• Verify Secure Boot is enabled and properly configured on affected systems
• Audit removable media policies and restrict USB boot capabilities where possible
• Review and harden physical access controls to prevent malicious boot media insertion

Patch Information

Patches are available from multiple vendors. Red Hat has released security advisories addressing this vulnerability:

• Red Hat Security Advisory RHSA-2024:2456 for Red Hat Enterprise Linux 9
• Red Hat Security Advisory RHSA-2024:3184 for Red Hat Enterprise Linux 8

Additional patches are available from:

• Debian LTS Security Announcement
• Gentoo GLSA 202311-14
• Fedora Package Announcements
• NetApp Security Advisory NTAP-20231208-0002

For additional details, see the Red Hat CVE Details for CVE-2023-4692 and Red Hat Bug Report #2236613.

Workarounds

• Disable NTFS filesystem support in GRUB2 if not required by removing or blacklisting the ntfs module
• Restrict boot to trusted devices only through BIOS/UEFI boot order configuration
• Implement physical security measures to prevent unauthorized boot media insertion
• Enable Secure Boot with custom enrolled keys to prevent execution of unsigned bootloaders

# Remove NTFS module from GRUB2 if not needed (Red Hat/CentOS/Fedora)
# Edit /etc/default/grub and add:
GRUB_DISABLE_MODULES="ntfs"

# Regenerate GRUB configuration
grub2-mkconfig -o /boot/grub2/grub.cfg

# Verify Secure Boot status
mokutil --sb-state