CVE-2020-25632: Gnu Grub2 Use-After-Free Vulnerability

CVE-2020-25632 Overview

A use-after-free vulnerability was discovered in GNU GRUB2 bootloader versions prior to 2.06. The flaw exists in the rmmod implementation, which allows unloading a module used as a dependency without verifying if any other dependent modules are still loaded. This creates a use-after-free condition that could enable arbitrary code execution or bypass of Secure Boot protections, representing a significant threat to system integrity at the pre-boot level.

Critical Impact

This vulnerability allows attackers with local privileged access to execute arbitrary code during the boot process and potentially bypass Secure Boot protections, compromising the entire chain of trust from system startup.

Affected Products

• GNU GRUB2 (versions prior to 2.06)
• Red Hat Enterprise Linux 7.0 and 8.0
• Red Hat Enterprise Linux Server AUS (7.2, 7.3, 7.4, 7.6, 7.7, 8.2)
• Red Hat Enterprise Linux Server EUS (7.6, 7.7, 8.1)
• Red Hat Enterprise Linux Server TUS (7.4, 7.6, 7.7, 8.2)
• Red Hat Enterprise Linux Workstation 7.0
• Fedora 33 and 34
• NetApp ONTAP Select Deploy Administration Utility

Discovery Timeline

• 2021-03-03 - CVE-2020-25632 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-25632

Vulnerability Analysis

This use-after-free vulnerability (CWE-416) resides in the module management functionality of GRUB2. The bootloader's rmmod command is responsible for unloading dynamically loaded modules during the boot process. However, the implementation fails to properly track module dependencies and verify whether other modules still reference a module being unloaded.

When a module is unloaded without checking for active dependencies, any remaining modules that reference the now-freed memory continue to hold dangling pointers. Subsequent access through these pointers triggers a use-after-free condition, potentially allowing an attacker to manipulate execution flow by placing controlled data in the freed memory region.

The exploitation requires local access with high privileges, but the scope extends beyond the vulnerable component itself. A successful attack can compromise the confidentiality and integrity of the entire system by executing code at the bootloader level before the operating system's security controls are initialized.

Root Cause

The root cause is insufficient dependency tracking in the rmmod implementation within GRUB2. When unloading a module, the bootloader does not enumerate and validate that no other loaded modules maintain references to the module being removed. This missing validation check allows the creation of dangling pointers that reference deallocated memory.

The module management subsystem lacks proper reference counting or dependency graph maintenance that would prevent premature deallocation of modules still in use by other components.

Attack Vector

The attack vector is local, requiring the attacker to have high-privilege access to the system. Exploitation involves manipulating the GRUB2 module loading and unloading sequence:

1. Load a base module that provides functionality used by other modules
2. Load dependent modules that reference the base module
3. Use rmmod to unload the base module, bypassing dependency checks
4. The dependent modules now hold dangling pointers to freed memory
5. Trigger access through the dangling pointers to achieve code execution

The vulnerability can be leveraged to bypass Secure Boot protections by executing malicious code during the early boot phase before the operating system's security mechanisms are active. This undermines the chain of trust established by UEFI Secure Boot.

Detection Methods for CVE-2020-25632

Indicators of Compromise

• Unexpected modifications to GRUB2 configuration files in /boot/grub2/ or /boot/efi/EFI/
• Unusual module loading patterns in GRUB2 logs during boot sequence
• Presence of unauthorized or modified GRUB2 modules in the boot partition
• Secure Boot violations or unexpected boot behavior after system updates

Detection Strategies

• Monitor boot partition integrity using file integrity monitoring solutions
• Implement UEFI Secure Boot with custom key enrollment to detect unauthorized bootloader modifications
• Review system boot logs for anomalous module load/unload sequences
• Deploy endpoint detection and response (EDR) solutions capable of monitoring pre-boot environments

Monitoring Recommendations

• Enable TPM-based measured boot to detect bootloader tampering through attestation
• Implement regular integrity verification of boot partition contents
• Configure alerts for any modifications to GRUB2 binaries or configuration files
• Monitor for Secure Boot policy changes or disabled Secure Boot settings

How to Mitigate CVE-2020-25632

Immediate Actions Required

• Update GRUB2 to version 2.06 or later, which includes the fix for this vulnerability
• Apply vendor-specific security patches from Red Hat, Fedora, or your Linux distribution
• Verify Secure Boot is enabled and properly configured with updated revocation databases
• Review and restrict local privileged access to systems running vulnerable GRUB2 versions

Patch Information

GNU GRUB2 version 2.06 addresses this use-after-free vulnerability. Linux distribution vendors have released corresponding security updates:

• Red Hat has issued patches for Enterprise Linux 7.x and 8.x - see the Red Hat Bug Report for details
• Fedora 33 and 34 users should apply updates from the Fedora Package Announcement
• Gentoo users can refer to GLSA 202104-05
• NetApp users should consult the NetApp Security Advisory

After applying GRUB2 updates, the UEFI Secure Boot revocation database (dbx) should be updated to prevent booting with older, vulnerable versions of the bootloader.

Workarounds

• Implement physical security controls to prevent unauthorized local access to affected systems
• Enable UEFI Secure Boot if not already active to add an additional layer of boot integrity protection
• Configure GRUB2 password protection to prevent unauthorized modification of boot parameters
• Restrict boot device options in UEFI/BIOS settings to prevent booting from unauthorized media

# Verify GRUB2 version and Secure Boot status
grub2-install --version
mokutil --sb-state

# Update GRUB2 on Red Hat/CentOS systems
sudo yum update grub2 grub2-efi-x64 grub2-common

# Regenerate GRUB2 configuration after update
sudo grub2-mkconfig -o /boot/grub2/grub.cfg

# Verify Secure Boot database updates
sudo fwupdmgr get-updates
sudo fwupdmgr update