CVE-2020-14310: GNU Grub2 Buffer Overflow Vulnerability

CVE-2020-14310 Overview

A heap-based buffer overflow vulnerability exists in GRUB2 (GNU GRand Unified Bootloader) versions prior to 2.06. The vulnerability is located in the read_section_as_string() function, which handles font name parsing. The function expects a font name to be at maximum UINT32_MAX - 1 bytes in length but fails to verify this constraint before proceeding with buffer allocation. An attacker can exploit this by crafting a malicious font file with a name set to UINT32_MAX, causing an arithmetic overflow that results in a zero-sized allocation and subsequent heap-based buffer overflow.

Critical Impact

Attackers with local privileged access can exploit malicious font files to trigger heap corruption in the bootloader, potentially 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 EUS 8.1 and 8.2
• Red Hat Enterprise Linux Server AUS 8.2
• Red Hat Enterprise Linux Server TUS 8.2
• openSUSE Leap 15.1 and 15.2
• Canonical Ubuntu Linux 14.04 ESM, 16.04 ESM, 18.04 LTS, and 20.04 LTS

Discovery Timeline

• July 31, 2020 - CVE-2020-14310 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-14310

Vulnerability Analysis

This vulnerability represents a classic integer overflow condition that leads to memory corruption. The read_section_as_string() function in GRUB2's font parsing code performs arithmetic operations on size values without proper bounds checking. When processing font metadata, the function reads a length value from the font file and uses it to calculate the buffer size needed for the font name string.

The root issue is that when a font name length is set to UINT32_MAX (4,294,967,295), adding 1 for the null terminator causes an integer wraparound to zero. This results in a zero-byte allocation being performed, while the subsequent read operation attempts to write the full UINT32_MAX bytes into this undersized buffer, causing massive heap corruption.

The attack requires local privileged access and user interaction is not required. However, successful exploitation could allow an attacker to execute arbitrary code within the GRUB2 bootloader context, potentially bypassing Secure Boot mechanisms and gaining persistent low-level system access.

Root Cause

The vulnerability stems from improper input validation in the font parsing code (CWE-190: Integer Overflow and CWE-122: Heap-based Buffer Overflow). The read_section_as_string() function fails to validate that the font name length value read from the file is within acceptable bounds before performing arithmetic operations. Specifically, the code does not check if adding 1 to the length value for null-terminator space would cause an integer overflow, leading to incorrect memory allocation size calculations.

Attack Vector

Exploitation requires local access with high privileges. An attacker must be able to place a maliciously crafted font file in a location where GRUB2 will process it during boot. The attack flow involves:

1. Crafting a malicious font file with a name section length set to UINT32_MAX
2. Placing the font file where GRUB2 can access it (typically requires privileged access to boot partitions)
3. Triggering GRUB2 to load the malicious font during the boot process
4. The integer overflow causes zero-sized allocation followed by heap buffer overflow
5. Potential for arbitrary code execution within the bootloader context

The vulnerability is particularly concerning in environments where Secure Boot is expected to provide boot integrity guarantees. Successful exploitation could allow attackers to compromise the boot process and establish persistence that survives operating system reinstallation.

Detection Methods for CVE-2020-14310

Indicators of Compromise

• Unexpected or modified font files in GRUB2 configuration directories (/boot/grub/, /boot/grub2/)
• Font files with unusually large metadata sections or suspicious properties
• System boot failures or unexpected behavior during the GRUB2 font loading phase
• Evidence of unauthorized modifications to the EFI System Partition (ESP)

Detection Strategies

• Implement file integrity monitoring on GRUB2 configuration directories and boot partitions
• Monitor for changes to font files in /boot/grub/fonts/ and related directories
• Use Secure Boot with properly enrolled keys to detect unsigned or modified bootloader components
• Audit access to boot partitions and EFI System Partition for unauthorized modifications

Monitoring Recommendations

• Enable audit logging for all file modifications within boot-related directories
• Deploy endpoint detection solutions that can monitor pre-boot and early-boot stages
• Implement TPM-based measurements to detect bootloader tampering
• Regularly verify GRUB2 package integrity using package manager verification tools

How to Mitigate CVE-2020-14310

Immediate Actions Required

• Update GRUB2 to version 2.06 or later immediately on all affected systems
• Apply vendor-specific security patches from Red Hat, Canonical, openSUSE, or Gentoo as applicable
• Revoke and update Secure Boot DBX (forbidden signatures database) to block vulnerable GRUB2 versions
• Audit boot partitions for any suspicious or unexpected font files

Patch Information

Security patches are available from multiple vendors. Red Hat has published detailed information in their Bug Report for CVE-2020-14310. Canonical has released Ubuntu Security Notice USN-4432-1 covering this vulnerability. openSUSE users should refer to their Security Announcements for Leap 15.1 and 15.2. Gentoo has published GLSA 202104-05 with remediation guidance.

The fix ensures proper bounds checking on the font name length value before performing allocation calculations, preventing the integer overflow condition.

Workarounds

• Restrict write access to GRUB2 configuration and font directories to trusted administrators only
• Remove unnecessary custom fonts from GRUB2 configuration if not required
• Enable Secure Boot with up-to-date DBX to reject vulnerable bootloader versions
• Implement mandatory access controls (SELinux, AppArmor) to protect boot-related files

# Configuration example
# Verify current GRUB2 version
grub2-install --version

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

# Apply GRUB2 security updates (Debian/Ubuntu)
sudo apt-get update && sudo apt-get install --only-upgrade grub-efi-amd64-bin grub-pc-bin

# Verify file integrity of GRUB2 fonts directory
find /boot/grub/fonts/ -type f -exec sha256sum {} \;

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