CVE-2025-0684: GNU GRUB2 RCE Vulnerability

CVE-2025-0684 Overview

A heap-based out-of-bounds write vulnerability exists in GNU GRUB2's ReiserFS filesystem module. When performing a symlink lookup from a ReiserFS filesystem, the grub_reiserfs_read_symlink() function uses user-controlled parameters from the filesystem geometry to determine internal buffer sizes. The module improperly checks for integer overflows in these calculations, allowing a maliciously crafted filesystem to cause buffer size calculations to overflow. This results in grub_malloc() allocating a smaller buffer than expected, which is subsequently overwritten when grub_reiserfs_read_real() is called with an overflown length parameter, leading to heap corruption.

Critical Impact

This vulnerability can be leveraged to corrupt GRUB's internal critical data structures, potentially enabling arbitrary code execution and bypassing Secure Boot protections. Systems booting from untrusted or compromised storage media are at risk.

Affected Products

• GNU GRUB2 (all versions with ReiserFS support)
• Linux distributions using GRUB2 bootloader with ReiserFS filesystem support
• Systems configured for Secure Boot with GRUB2

Discovery Timeline

• 2025-03-03 - CVE-2025-0684 published to NVD
• 2025-07-28 - Last updated in NVD database

Technical Details for CVE-2025-0684

Vulnerability Analysis

The vulnerability resides in GRUB2's ReiserFS filesystem driver, specifically in the symlink resolution code path. The flaw stems from improper integer overflow validation when calculating buffer sizes based on filesystem geometry parameters. An attacker who can craft a malicious ReiserFS filesystem image can manipulate the geometry values to trigger an integer overflow during the buffer size calculation.

When the overflow occurs, grub_malloc() allocates a buffer significantly smaller than what the subsequent read operation expects. The grub_reiserfs_read_real() function then writes data beyond the allocated buffer boundaries, corrupting adjacent heap memory. This heap-based out-of-bounds write (CWE-787) can be exploited to overwrite critical GRUB internal data structures, potentially allowing arbitrary code execution during the boot process.

The attack requires local access to modify the boot media or compromise an existing ReiserFS partition. Given GRUB2's role as a bootloader operating before the operating system's security mechanisms are loaded, successful exploitation could bypass Secure Boot protections entirely.

Root Cause

The root cause is an integer overflow vulnerability (CWE-787) in the buffer size calculation within the ReiserFS module. The code fails to properly validate that arithmetic operations on user-controlled filesystem geometry values do not wrap around due to integer overflow. When these values are sourced from a maliciously crafted filesystem, the attacker can engineer specific values that cause the multiplication or addition operations to overflow, resulting in a smaller-than-expected buffer allocation.

Attack Vector

Exploitation requires local access to the system with the ability to either:

1. Modify the boot media (USB drive, hard disk) to contain a malicious ReiserFS filesystem
2. Mount a crafted ReiserFS image that GRUB2 will attempt to read during boot
3. Compromise an existing ReiserFS partition with specially crafted filesystem geometry

The attack vector is local, requiring high privileges to modify boot media or filesystem structures. Once a malicious filesystem is in place, the vulnerability is triggered automatically when GRUB2 attempts to resolve symlinks on the crafted ReiserFS filesystem during the boot process. The attacker does not need any user interaction beyond having the system boot from the compromised media.

The vulnerability mechanism can be described as follows: when GRUB2 encounters a symlink on a ReiserFS filesystem, it reads the filesystem's geometry parameters to calculate the required buffer size for the symlink target. A crafted filesystem can specify geometry values that, when multiplied together, cause an integer overflow. This results in a small heap allocation followed by a large write operation, corrupting heap memory and potentially enabling code execution. For detailed technical analysis, see the Red Hat CVE-2025-0684 Advisory.

Detection Methods for CVE-2025-0684

Indicators of Compromise

• Unexpected boot failures or crashes during GRUB2 initialization
• Modified or suspicious ReiserFS filesystem images on boot media
• Unauthorized changes to boot partition contents
• Secure Boot violations or bypasses being logged by firmware

Detection Strategies

• Implement filesystem integrity monitoring on boot partitions using cryptographic checksums
• Monitor for unauthorized modifications to GRUB2 configuration files and modules
• Enable firmware-level boot logging to detect anomalous bootloader behavior
• Utilize SentinelOne endpoint protection to detect post-exploitation malicious activity

Monitoring Recommendations

• Regularly verify GRUB2 binary and module integrity against known-good hashes
• Monitor system boot logs for ReiserFS-related errors or unusual activity
• Implement physical security controls for removable boot media
• Enable UEFI Secure Boot with properly enrolled keys to limit bootloader modifications

How to Mitigate CVE-2025-0684

Immediate Actions Required

• Apply vendor-provided GRUB2 security patches as soon as they become available
• Disable ReiserFS support in GRUB2 if not required by removing the reiserfs.mod module
• Restrict physical access to systems to prevent boot media tampering
• Verify integrity of existing boot partitions and GRUB2 installations

Patch Information

Consult vendor security advisories for patch availability. Red Hat has acknowledged this vulnerability and is tracking it through Bug Report #2346119. Additional details are available in the Red Hat CVE-2025-0684 Advisory. System administrators should monitor their distribution's security channels for updated GRUB2 packages and follow vendor-specific instructions for regenerating bootloader configurations after patching.

Workarounds

• Remove reiserfs.mod from the GRUB2 modules directory to disable ReiserFS support entirely
• Avoid booting from untrusted or unverified storage media
• Enable UEFI Secure Boot to limit execution of unsigned bootloader code
• Implement full-disk encryption with TPM-backed key storage to protect boot integrity

# Configuration example - Remove ReiserFS module to mitigate vulnerability
# Locate and remove the reiserfs module (requires root privileges)
sudo rm -f /boot/grub/i386-pc/reiserfs.mod
sudo rm -f /boot/grub/x86_64-efi/reiserfs.mod

# Regenerate GRUB configuration without ReiserFS support
sudo grub-mkconfig -o /boot/grub/grub.cfg

# Verify the module has been removed
ls -la /boot/grub/*/reiserfs.mod 2>/dev/null || echo "ReiserFS module successfully removed"