CVE-2025-0689: GNU GRUB2 Buffer Overflow Vulnerability

CVE-2025-0689 Overview

CVE-2025-0689 is a heap-based buffer overflow vulnerability in the GNU GRUB2 bootloader's UDF (Universal Disk Format) filesystem module. When reading data from disk, the UDF filesystem module utilizes user-controlled data length metadata to allocate its internal buffers. In certain scenarios, while iterating through disk sectors, it assumes the read size from the disk is always smaller than the allocated buffer size—a condition that is not guaranteed. A crafted filesystem image may lead to a heap-based buffer overflow resulting in critical data corruption and the risk of arbitrary code execution, potentially bypassing secure boot protections.

Critical Impact

This vulnerability enables attackers to potentially bypass secure boot protections through a crafted UDF filesystem image, leading to arbitrary code execution at the bootloader level—a critical component of the system trust chain.

Affected Products

• GNU GRUB2 (all versions prior to patch)
• Linux distributions using vulnerable GRUB2 versions
• Systems utilizing UDF filesystem support in GRUB2

Discovery Timeline

• 2025-03-03 - CVE-2025-0689 published to NVD
• 2026-01-08 - Last updated in NVD database

Technical Details for CVE-2025-0689

Vulnerability Analysis

This vulnerability resides in GRUB2's UDF filesystem module, specifically in the code responsible for handling disk read operations. The UDF module reads metadata from the disk to determine buffer allocation sizes for internal data structures. The core issue stems from an unsafe assumption in the buffer handling logic: the module assumes that data read from disk sectors will never exceed the size of pre-allocated buffers.

When processing crafted filesystem images, an attacker can manipulate the length metadata fields within the UDF filesystem structures. This manipulation causes the module to allocate insufficiently sized buffers while subsequently writing larger amounts of data during sector iteration, resulting in a classic heap-based buffer overflow condition (CWE-120: Buffer Copy without Checking Size of Input).

The vulnerability is particularly severe because it operates at the bootloader level, which executes before the operating system and its security mechanisms are initialized. Successful exploitation could corrupt critical heap data structures, overwrite function pointers, or manipulate control flow to achieve arbitrary code execution.

Root Cause

The root cause is improper input validation of user-controlled length metadata within the UDF filesystem module. The code fails to verify that the actual data size read from disk sectors does not exceed the allocated buffer size. This boundary check omission allows heap memory corruption when processing maliciously crafted UDF filesystem images with manipulated length fields that exceed expected boundaries.

Attack Vector

The attack requires local access with user interaction—specifically, an attacker must craft a malicious UDF filesystem image and convince a user to boot from or mount a device containing this image. The attack vector involves:

1. Creating a specially crafted UDF filesystem image with manipulated length metadata fields
2. Distributing the malicious image via removable media (USB drives, DVDs) or network boot sources
3. When GRUB2 processes the crafted filesystem during boot, the UDF module allocates undersized buffers
4. Subsequent disk read operations overflow the heap buffer, corrupting adjacent memory
5. Exploitation can lead to arbitrary code execution, potentially bypassing secure boot protections

The vulnerability mechanism involves the UDF module's sector iteration logic. During normal operation, the module iterates through disk sectors and copies data into pre-allocated buffers. When the actual data size exceeds the allocated buffer size due to manipulated metadata, the excess data overwrites adjacent heap memory, enabling potential control flow hijacking.

Detection Methods for CVE-2025-0689

Indicators of Compromise

• Unexpected GRUB2 crashes or hangs during boot from removable media
• System boot failures when processing UDF filesystems on external devices
• Secure boot validation failures or unexpected boot chain modifications
• Anomalous heap corruption signatures in bootloader memory regions

Detection Strategies

• Monitor for boot anomalies when UDF-formatted media is present
• Implement file integrity monitoring on GRUB2 bootloader binaries and modules
• Enable UEFI Secure Boot to help detect unauthorized bootloader modifications
• Audit systems for unexpected UDF filesystem images on boot media

Monitoring Recommendations

• Enable verbose GRUB2 logging where possible to capture filesystem module errors
• Implement boot chain attestation using TPM measurements
• Monitor for unauthorized modifications to GRUB2 configuration and module files
• Review system boot logs for UDF-related error messages or crashes

How to Mitigate CVE-2025-0689

Immediate Actions Required

• Update GRUB2 to the latest patched version provided by your Linux distribution
• Enable UEFI Secure Boot to add an additional layer of protection against bootloader tampering
• Restrict boot from untrusted removable media via BIOS/UEFI settings
• Audit boot media sources and avoid using UDF filesystems from untrusted sources

Patch Information

GNU has released security patches addressing this vulnerability. Organizations should consult the GNU GRUB Development Discussion for official patch details. Red Hat has also published advisory information available at the Red Hat CVE-2025-0689 Advisory. Additional tracking information can be found in Red Hat Bug Report #2346122.

Apply vendor-specific GRUB2 updates as they become available for your distribution. After updating, regenerate the GRUB configuration and update secure boot signatures as required.

Workarounds

• Disable UDF filesystem support in GRUB2 if not required by removing or blacklisting the udf module
• Configure BIOS/UEFI to prevent booting from removable media or network sources
• Implement strict physical access controls to prevent insertion of malicious boot media
• Use measured boot with TPM attestation to detect bootloader modifications

# Configuration example - Disable UDF module loading in GRUB
# Add to /etc/default/grub or grub configuration:
GRUB_PRELOAD_MODULES="part_gpt part_msdos"
# Regenerate GRUB configuration after changes:
# grub-mkconfig -o /boot/grub/grub.cfg