CVE-2020-25647: GNU GRUB2 RCE Vulnerability

CVE-2020-25647 Overview

A critical out-of-bounds write vulnerability was discovered in GNU GRUB2 bootloader versions prior to 2.06. The flaw exists in the USB device initialization process where descriptors are read with insufficient bounds checking. The implementation trusts that USB devices provide sane values without proper validation, allowing a malicious USB device to trigger memory corruption leading to arbitrary code execution and Secure Boot bypass.

Critical Impact

This vulnerability allows physical attackers to execute arbitrary code during the boot process, completely bypassing Secure Boot protections and compromising system integrity before the operating system loads.

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-2020-25647 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-25647

Vulnerability Analysis

This vulnerability represents an Out-of-Bounds Write (CWE-787) condition in the GRUB2 bootloader's USB device handling code. The fundamental issue lies in how GRUB2 processes USB device descriptors during the initialization phase. When a USB device is connected, GRUB2 reads various descriptors (device, configuration, interface, and endpoint descriptors) to understand the device's capabilities and configure it properly.

The vulnerable code path assumes that the USB device provides valid descriptor lengths and counts within expected bounds. However, a malicious USB device can supply crafted descriptor values that exceed buffer boundaries, leading to memory corruption. Since GRUB2 operates in a pre-boot environment with direct hardware access and no memory protection mechanisms, successful exploitation allows an attacker to execute arbitrary code with the highest possible privileges.

The impact is particularly severe because it enables bypassing Secure Boot, a fundamental security mechanism designed to ensure only trusted code runs during the boot process. Once Secure Boot is compromised, an attacker can install persistent bootkits that survive operating system reinstallation.

Root Cause

The root cause is insufficient input validation in GRUB2's USB subsystem. The code responsible for parsing USB descriptors does not adequately verify that field values such as descriptor lengths, counts, and offsets fall within safe boundaries before using them to copy data or access memory. This creates a classic bounds-checking failure where attacker-controlled data from a USB device directly influences memory operations without proper sanitization.

Attack Vector

The attack requires physical access to the target system to connect a malicious USB device. The attacker must craft a USB device (or use a programmable USB device like a USB Rubber Ducky or similar hardware) that presents malformed descriptors to trigger the memory corruption. When GRUB2 enumerates the device during boot, the corrupted memory can be leveraged to redirect execution flow and run attacker-controlled code.

The vulnerability manifests during USB device initialization when GRUB2 processes USB descriptors with insufficient bounds checking. A malicious USB device can provide specially crafted descriptor values that exceed expected buffer sizes, causing out-of-bounds memory writes. This memory corruption can be weaponized to achieve arbitrary code execution in the pre-boot environment, effectively bypassing Secure Boot protections. For detailed technical analysis, refer to the Red Hat Bug Report #1886936.

Detection Methods for CVE-2020-25647

Indicators of Compromise

• Unexpected USB device enumeration events during system boot, particularly from unknown or unrecognized devices
• Modifications to GRUB2 bootloader files or configurations outside of expected update cycles
• System boot failures or unexpected behavior following USB device connection during boot
• Evidence of Secure Boot violations or disabled Secure Boot settings without administrative action

Detection Strategies

• Monitor for unauthorized USB device connections during boot sequences using hardware security modules or USB monitoring tools
• Implement BIOS/UEFI logging and audit USB enumeration events during the pre-boot phase
• Use Secure Boot status monitoring to detect any attempts to disable or bypass boot integrity checks
• Deploy endpoint detection solutions capable of identifying bootloader integrity violations

Monitoring Recommendations

• Enable firmware-level logging capabilities where available to capture boot-time events
• Implement physical security controls to prevent unauthorized USB device connections
• Utilize SentinelOne's Deep Visibility to monitor for post-exploitation behaviors that may indicate successful Secure Boot bypass
• Regularly audit GRUB2 versions across the enterprise to ensure patched versions are deployed

How to Mitigate CVE-2020-25647

Immediate Actions Required

• Update GRUB2 to version 2.06 or later on all affected systems
• Apply vendor-specific patches from Red Hat, Fedora, or your distribution maintainer
• Revoke and update Secure Boot DBX (Forbidden Signatures Database) entries as directed by your OS vendor
• Implement physical security measures to restrict unauthorized USB device connections during boot

Patch Information

GNU GRUB2 version 2.06 addresses this vulnerability with proper bounds checking for USB device descriptors. Enterprise Linux distributions have released patched packages:

• Red Hat Enterprise Linux: Security updates available through Red Hat's standard update channels. See Red Hat Bug Report #1886936 for details.
• Fedora: Updates announced via Fedora Package Announcement
• Gentoo: Security advisory GLSA 2021-04-05
• NetApp: See NetApp Security Advisory ntap-20220325-0001

Workarounds

• Disable USB boot options in BIOS/UEFI settings if USB booting is not required
• Configure BIOS/UEFI passwords to prevent unauthorized boot configuration changes
• Enable BIOS/UEFI USB port controls to restrict which USB ports are active during boot
• Implement chassis intrusion detection to identify unauthorized physical access attempts

# Verify current GRUB2 version on RHEL/CentOS/Fedora
rpm -q grub2

# Update GRUB2 on RHEL/CentOS
sudo yum update grub2

# Update GRUB2 on Fedora
sudo dnf update grub2

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