CVE-2024-56737: GNU GRUB2 Buffer Overflow Vulnerability

CVE-2024-56737 Overview

CVE-2024-56737 is a heap-based buffer overflow vulnerability in GNU GRUB (GRUB2) through version 2.12. The flaw exists in fs/hfs.c, the bootloader's Hierarchical File System (HFS) handler, and is triggered by crafted sblock (superblock) data inside an HFS filesystem. An attacker who supplies a malicious HFS image can corrupt heap memory during the boot process, potentially leading to arbitrary code execution in the pre-boot environment. Because GRUB2 ships as the default bootloader for most Linux distributions and underpins UEFI Secure Boot trust chains, exploitation can undermine boot integrity. The vulnerability is tracked under [CWE-122: Heap-based Buffer Overflow].

Critical Impact

Successful exploitation grants code execution in the bootloader context, which executes before the operating system and Secure Boot policy enforcement completes.

Affected Products

• GNU GRUB2 versions up to and including 2.12
• Linux distributions that bundle vulnerable GRUB2 builds in their boot chain
• UEFI Secure Boot environments relying on signed GRUB2 binaries

Discovery Timeline

• 2024-12-29 - CVE-2024-56737 published to the National Vulnerability Database (NVD)
• 2025-06-24 - Last updated in NVD database

Technical Details for CVE-2024-56737

Vulnerability Analysis

The defect resides in the HFS filesystem driver located at fs/hfs.c within the GRUB2 source tree. When GRUB2 mounts an HFS volume, it parses the volume's superblock (sblock) structure to locate catalog and extent data. The parsing routine fails to correctly validate length or offset fields drawn from the on-disk superblock before copying data into a heap-allocated buffer. A crafted HFS image can specify values that cause the routine to write past the allocated buffer boundary, corrupting adjacent heap metadata and chunks. Because GRUB2 runs prior to kernel initialization, no operating system mitigations such as ASLR, SMEP, or process isolation apply. An attacker who controls heap layout can leverage the overflow to redirect execution within the bootloader.

Root Cause

The root cause is missing bounds validation on attacker-controlled fields parsed from the HFS superblock. GRUB2 trusts size-related metadata read from disk and passes it directly to heap copy operations without sanity checks against the destination buffer size. This pattern is consistent with [CWE-122] heap-based buffer overflow weaknesses in filesystem parsers.

Attack Vector

Exploitation requires the victim system to attempt to boot from or read an attacker-supplied HFS-formatted volume. Realistic attack scenarios include a malicious USB or external disk presented to a target machine, a manipulated virtual machine disk image, or a compromised partition on a multi-boot system. User interaction is required to initiate or permit the boot process, but no authentication to the operating system is needed because the flaw triggers before the OS loads. A successful overflow can be used to bypass Secure Boot guarantees and persist below the kernel as a bootkit.

No public proof-of-concept exploit code is currently available. Technical details are tracked in the GNU Savannah Bug Report.

Detection Methods for CVE-2024-56737

Indicators of Compromise

• Unexpected modifications to /boot, /boot/efi, or the EFI System Partition, including changes to grubx64.efi or core.img outside of authorized package updates.
• Presence of unsigned or unrecognized HFS-formatted partitions or removable media attached during boot.
• Boot-time errors, hangs, or unexplained reboots when removable media is connected.

Detection Strategies

• Inventory installed GRUB2 versions across the fleet and flag any host running grub2 <= 2.12 without the distribution's backported fix.
• Monitor file integrity on bootloader artifacts and the EFI System Partition, alerting on hash changes that do not correlate with patch management activity.
• Audit attached storage devices and disallow boot from untrusted external HFS volumes via firmware policy.

Monitoring Recommendations

• Centralize bootloader and firmware event logs alongside endpoint telemetry to correlate boot-time anomalies with subsequent OS activity.
• Track measured boot values (TPM PCRs 0-9) and alert on unexpected changes to PCRs covering the bootloader.
• Review vendor advisories from each Linux distribution for updated grub2 package versions and track patch deployment status.

How to Mitigate CVE-2024-56737

Immediate Actions Required

• Apply the GRUB2 update provided by your Linux distribution as soon as a patched package is published, prioritizing systems that boot from removable or shared storage.
• Disable booting from USB and other external media in UEFI/BIOS firmware on systems that do not require it, and protect firmware settings with a password.
• Enable and enforce UEFI Secure Boot with a current dbx (forbidden signatures) database to revoke vulnerable GRUB2 binaries once distribution updates ship.

Patch Information

A fixed release of GRUB2 above version 2.12 is required. Track the upstream fix and downstream backports through the GNU Savannah Bug Report and the security advisories of your Linux distribution. After upgrading the grub2 package, regenerate the GRUB configuration and reinstall the bootloader to the target disk or EFI partition so the patched binary is in use at next boot.

Workarounds

• Restrict physical and virtual access to systems so that untrusted HFS volumes cannot be attached during boot.
• Remove or disable the HFS module from GRUB2 builds where HFS support is not required, reducing the bootloader's parser attack surface.
• Use full-disk encryption with TPM-bound keys so that tampering with the boot chain prevents key release and surfaces compromise.

# Example: verify installed GRUB2 version and remove the HFS module from the build
# Check current GRUB2 version
grub-install --version

# After patching, reinstall GRUB2 to the EFI partition (UEFI systems)
sudo grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=GRUB
sudo grub-mkconfig -o /boot/grub/grub.cfg

# Optional: build a GRUB image that excludes the vulnerable HFS module
sudo grub-mkimage -O x86_64-efi -o /boot/efi/EFI/GRUB/grubx64.efi \
    part_gpt fat ext2 normal configfile linux echo