CVE-2022-3775: GNU Grub2 Buffer Overflow Vulnerability

CVE-2022-3775 Overview

CVE-2022-3775 is an out-of-bounds write vulnerability in GRUB2's font rendering code. When rendering certain unicode sequences, GRUB2's font code fails to properly validate whether the informed glyph's width and height is constrained within bitmap size. As a consequence, an attacker can craft malicious input that leads to an out-of-bounds write into GRUB2's heap, resulting in memory corruption and availability issues. Although complex, arbitrary code execution cannot be discarded as a potential outcome.

Critical Impact

This bootloader vulnerability affects the early boot process, potentially allowing attackers to bypass security controls and achieve persistent compromise before the operating system loads.

Affected Products

• GNU GRUB2 (all versions prior to patched releases)
• Red Hat Enterprise Linux 8.0
• Other Linux distributions using vulnerable GRUB2 versions

Discovery Timeline

• December 19, 2022 - CVE-2022-3775 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2022-3775

Vulnerability Analysis

This vulnerability (CWE-787: Out-of-Bounds Write) exists within GRUB2's font rendering subsystem. The core issue stems from insufficient validation of glyph dimensions when processing specially crafted unicode sequences. When GRUB2 renders text using custom fonts, it allocates bitmap buffers based on expected glyph dimensions. However, the font parsing code does not adequately verify that the actual glyph width and height values fit within the allocated bitmap buffer boundaries.

The local attack vector requires the attacker to have local access to the system, which could be achieved through physical access to the boot process or by tampering with boot configuration files. While no authentication is required to trigger the vulnerability once an attacker has positioned malicious font data, the complexity of achieving reliable code execution makes this attack moderately sophisticated.

Root Cause

The root cause is improper input validation in GRUB2's font handling routines. Specifically, the code responsible for rendering glyphs does not enforce boundary checks to ensure that glyph dimensions specified in font data match the actual allocated bitmap size. This allows a malformed font file to specify glyph dimensions that exceed buffer boundaries, triggering heap corruption when the glyph data is written.

Attack Vector

The attack vector is local, requiring the attacker to influence the boot process through:

1. Malicious Font Files: An attacker with access to the boot partition can introduce a crafted font file containing unicode sequences with invalid glyph dimensions
2. Modified GRUB Configuration: Tampering with GRUB configuration to load malicious fonts during the boot process
3. Secure Boot Bypass: In certain configurations, this vulnerability could potentially be leveraged as part of a chain to bypass Secure Boot protections

When GRUB2 processes the malicious font and attempts to render the crafted unicode sequences, the out-of-bounds write corrupts heap memory. This can lead to denial of service through boot failures, or in more sophisticated attacks, potentially achieve code execution during the pre-boot environment.

The vulnerability mechanism involves the font rendering function processing glyph data without proper bounds checking. When a specially crafted font specifies glyph dimensions that exceed the allocated bitmap buffer, subsequent write operations corrupt adjacent heap memory. Technical details and patch information are available in the Red Hat CVE-2022-3775 Advisory.

Detection Methods for CVE-2022-3775

Indicators of Compromise

• Unexpected boot failures or GRUB2 crashes during system startup
• Modified or unexpected font files in the /boot/grub2/fonts/ directory
• Unauthorized changes to GRUB2 configuration files (grub.cfg)
• Evidence of boot partition tampering in system logs

Detection Strategies

• Monitor file integrity of GRUB2 components including fonts, modules, and configuration files
• Implement boot partition integrity monitoring using tools like AIDE or Tripwire
• Enable Secure Boot with proper signature verification to detect unauthorized bootloader modifications
• Review system logs for unexpected GRUB2 errors or boot anomalies

Monitoring Recommendations

• Configure file integrity monitoring (FIM) for /boot and /boot/efi partitions
• Implement alerting on any modifications to GRUB2 font files or configuration
• Deploy endpoint detection capable of monitoring pre-boot and early boot environments
• Conduct regular audits of boot partition contents against known-good baselines

How to Mitigate CVE-2022-3775

Immediate Actions Required

• Update GRUB2 to the latest patched version provided by your Linux distribution
• Verify the integrity of existing GRUB2 installation and boot partition files
• Enable and configure Secure Boot if supported by your hardware
• Restrict physical access to systems and limit write access to boot partitions

Patch Information

Security patches addressing CVE-2022-3775 are available from affected Linux distributions. Administrators should consult their distribution's security advisories for specific patch versions:

• Red Hat Enterprise Linux: Refer to the Red Hat CVE-2022-3775 Advisory for patch details and affected package versions
• Gentoo Linux: Consult GLSA 202311-14 for patching guidance
• Other Distributions: Check your distribution's security tracker for GRUB2 updates

After updating GRUB2, administrators should regenerate GRUB configuration and update the bootloader installation using distribution-specific tools (e.g., grub2-mkconfig, update-grub).

Workarounds

• Implement strict access controls on the boot partition to prevent unauthorized modifications
• Enable Secure Boot with UEFI to validate bootloader integrity before execution
• Remove unnecessary or custom fonts from the GRUB2 fonts directory
• Use read-only boot partitions where operationally feasible

# Verify GRUB2 package version (Red Hat/CentOS)
rpm -qa | grep grub2

# Update GRUB2 packages (Red Hat/CentOS)
sudo dnf update grub2-common grub2-tools grub2-pc

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

# Verify boot partition file integrity
sudo find /boot -type f -exec sha256sum {} \; > /root/boot_checksums.txt