CVE-2020-14308: GNU GRUB2 Buffer Overflow Vulnerability

CVE-2020-14308 Overview

CVE-2020-14308 is an integer overflow vulnerability affecting the GRUB2 bootloader in versions before 2.06. The GRUB2 memory allocator fails to properly check for arithmetic overflows when processing the requested allocation size. This oversight allows the function to return invalid memory allocations, which can subsequently be exploited to compromise system integrity, confidentiality, and availability during the critical boot process.

Critical Impact

This vulnerability enables attackers with local access and high privileges to potentially bypass Secure Boot protections, compromising the entire system boot chain and allowing for persistent malware installation that survives operating system reinstallation.

Affected Products

• GNU GRUB2 versions before 2.06
• openSUSE Leap 15.1
• openSUSE Leap 15.2

Discovery Timeline

• 2020-07-29 - CVE-2020-14308 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-14308

Vulnerability Analysis

The vulnerability exists within GRUB2's memory allocation subsystem, a critical component responsible for managing memory during the boot process. When allocation requests are processed, the memory allocator performs arithmetic calculations to determine the required memory size. However, the allocator lacks proper bounds checking for integer overflow conditions (CWE-190).

When a specially crafted allocation request triggers an integer overflow, the calculated size wraps around to a smaller value than intended. The allocator then returns a memory buffer that is significantly smaller than what was requested. Subsequent operations that write to this undersized buffer can cause heap corruption, potentially allowing an attacker to overwrite adjacent memory regions.

The local attack vector combined with the requirement for high privileges constrains the exploitability of this vulnerability. However, in scenarios where an attacker has already gained administrative access or can modify boot configuration files, this vulnerability can be leveraged to bypass Secure Boot mechanisms and establish persistent system compromise.

Root Cause

The root cause is a missing integer overflow check in GRUB2's memory allocator function. When calculating the total allocation size, the function multiplies or adds values without validating that the result stays within the bounds of the integer type. This is a classic integer overflow vulnerability (CWE-190) where arithmetic operations exceed the maximum value representable by the data type, causing the value to wrap around.

Attack Vector

Exploitation requires local access to the system with high privileges. An attacker would need to craft malicious GRUB2 configuration files or modules that trigger the memory allocator with specially chosen size parameters designed to cause integer overflow. Once the overflow occurs and an undersized buffer is returned, the attacker can leverage subsequent memory operations to corrupt heap memory.

The attack scenario typically involves:

1. Gaining local administrative access to the target system
2. Modifying GRUB2 configuration or supplying a malicious module
3. Triggering the vulnerable memory allocation path during boot
4. Exploiting the resulting memory corruption to bypass Secure Boot or inject malicious code into the boot process

This vulnerability is particularly concerning in environments relying on Secure Boot to maintain boot chain integrity, as successful exploitation could undermine these protections.

Detection Methods for CVE-2020-14308

Indicators of Compromise

• Unexpected modifications to GRUB2 configuration files (/boot/grub2/grub.cfg, /etc/default/grub)
• Unauthorized GRUB2 modules present in /boot/grub2/ directories
• Integrity check failures on bootloader components
• Secure Boot violations or unexplained boot failures

Detection Strategies

• Monitor file integrity of boot-related files using host-based intrusion detection systems (HIDS)
• Implement and verify Secure Boot with proper key management to detect unauthorized bootloader modifications
• Audit access to boot partitions and GRUB2 configuration files for suspicious activity
• Compare installed GRUB2 package versions against known vulnerable versions

Monitoring Recommendations

• Enable audit logging for file access and modifications within /boot/ directories
• Configure alerts for any changes to GRUB2-related files outside of scheduled maintenance windows
• Regularly verify GRUB2 version against patched releases from your distribution vendor
• Monitor system boot logs for unusual memory allocation errors or boot failures

How to Mitigate CVE-2020-14308

Immediate Actions Required

• Update GRUB2 to version 2.06 or later, or apply distribution-specific security patches
• Verify Secure Boot configuration and ensure UEFI firmware is updated with revoked vulnerable bootloader signatures
• Restrict access to boot partitions and GRUB2 configuration files to authorized administrators only
• Review and audit any recent changes to bootloader configurations

Patch Information

GNU has addressed this vulnerability in GRUB2 version 2.06 and later. Multiple Linux distributions have released security updates to address CVE-2020-14308:

• openSUSE: Security announcements available via openSUSE Security Announcement
• Ubuntu: Patched in Ubuntu Security Notice USN-4432-1
• Gentoo: Addressed in Gentoo GLSA 202104-05
• Red Hat: Details available in Red Hat Bug Report #1852009
• NetApp: Advisory available at NetApp Security Advisory

Workarounds

• Implement strict access controls on boot partitions and GRUB2 configuration files using file system permissions
• Enable and properly configure Secure Boot to prevent unauthorized bootloader modifications
• Use disk encryption with pre-boot authentication to protect against unauthorized boot configuration changes
• Physically secure systems to prevent unauthorized local access

# Verify GRUB2 version on RPM-based systems
rpm -q grub2

# Verify GRUB2 version on Debian-based systems
dpkg -l grub2-common | grep -i version

# Check for available security updates (RHEL/CentOS)
yum check-update grub2

# Check for available security updates (Ubuntu/Debian)
apt list --upgradable | grep grub