CVE-2021-3696: GNU GRUB2 Buffer Overflow Vulnerability

CVE-2021-3696 Overview

CVE-2021-3696 is a heap out-of-bounds write vulnerability that occurs during the handling of Huffman tables in the PNG reader component of GNU GRUB2 bootloader. This memory corruption flaw can lead to data corruption in the heap space, potentially enabling an attacker to compromise system integrity during the boot process.

Critical Impact

While exploitation is highly complex, successful attacks could potentially lead to arbitrary code execution or secure boot circumvention in the GRUB2 bootloader environment.

Affected Products

• GNU GRUB2 (versions prior to grub-2.12)
• Red Hat Enterprise Linux 8.x and 9.x (multiple variants including EUS, AUS, TUS)
• Red Hat OpenShift Container Platform 4.6, 4.9, 4.10
• Red Hat Developer Tools 1.0
• Red Hat CodeReady Linux Builder
• NetApp ONTAP Select Deploy Administration Utility

Discovery Timeline

• 2022-07-06 - CVE-2021-3696 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-3696

Vulnerability Analysis

This vulnerability resides in the PNG image parsing functionality of GRUB2, specifically within the code responsible for processing Huffman tables. When GRUB2 processes a specially crafted PNG image during boot (such as background images or theme graphics), improper bounds checking during Huffman table decompression can result in writing data beyond allocated heap buffer boundaries.

The flaw is classified as CWE-787 (Out-of-bounds Write), indicating that the software writes data past the end, or before the beginning, of the intended buffer. In the context of GRUB2, this occurs when the PNG reader mishandles certain Huffman encoding structures, causing heap memory corruption.

While the confidentiality, integrity, and availability impacts are considered limited due to the complexity required for an attacker to control the encoding and positioning of corrupted Huffman entries, the bootloader context makes this vulnerability particularly sensitive. Successful exploitation could theoretically allow an attacker to bypass secure boot mechanisms or execute arbitrary code before the operating system loads.

Root Cause

The root cause of CVE-2021-3696 lies in insufficient validation of Huffman table data during PNG decompression within GRUB2's PNG reader module. The code fails to properly verify that decoded Huffman entries remain within the allocated heap buffer boundaries, allowing malformed PNG files to trigger out-of-bounds memory writes.

Attack Vector

The attack vector for this vulnerability is local, requiring an attacker to either:

1. Place a malicious PNG file in a location where GRUB2 will process it during boot (such as the boot partition's theme directory)
2. Have write access to the system's boot configuration to reference a malicious image file
3. Compromise the boot media or EFI system partition to inject crafted PNG files

The exploitation complexity is high because the attacker must carefully craft the PNG file's Huffman table structures to achieve meaningful heap corruption that results in code execution or security bypass, rather than simply causing a crash.

The vulnerability affects systems where GRUB2 is configured to display PNG images during boot, particularly those using custom themes or background images.

Detection Methods for CVE-2021-3696

Indicators of Compromise

• Unexpected or modified PNG files in GRUB2 theme directories (/boot/grub2/themes/ or /boot/efi/EFI/)
• Changes to GRUB2 configuration files referencing new or modified image files
• Unauthorized modifications to the EFI system partition
• System boot failures or unexpected behavior during GRUB2 initialization

Detection Strategies

• Monitor file integrity of GRUB2 configuration and theme directories using tools like AIDE or Tripwire
• Implement Secure Boot with signed bootloader images to prevent loading of modified GRUB2 binaries
• Use endpoint detection solutions to monitor for unauthorized writes to boot partitions
• Audit access to /boot/ and EFI system partition directories

Monitoring Recommendations

• Enable detailed logging for boot partition access attempts
• Configure alerts for any file modifications in GRUB2 directories
• Monitor for UEFI firmware updates or modifications outside of scheduled maintenance windows
• Implement boot integrity measurement using TPM-based attestation where available

How to Mitigate CVE-2021-3696

Immediate Actions Required

• Update GRUB2 to version grub-2.12 or later where the vulnerability has been addressed
• Apply vendor-specific patches from Red Hat, Gentoo, or NetApp as applicable to your environment
• Review and restrict access permissions to boot partitions and EFI system partitions
• Verify the integrity of existing PNG files in GRUB2 theme directories

Patch Information

Organizations should apply patches from their respective vendors:

• Red Hat: Security updates are available for affected Red Hat Enterprise Linux versions. Consult Red Hat Bug Report #1991686 for specific package versions.
• Gentoo: Apply patches referenced in Gentoo GLSA 202209-12.
• NetApp: Review NetApp Security Advisory ntap-20220930-0001 for ONTAP Select Deploy Administration Utility guidance.

The fix involves adding proper bounds checking during Huffman table processing in the PNG reader component to prevent out-of-bounds heap writes.

Workarounds

• Remove or replace PNG images from GRUB2 theme configurations with verified safe alternatives
• Disable custom themes in GRUB2 configuration by setting GRUB_TERMINAL=console in /etc/default/grub
• Restrict write access to boot partitions using filesystem permissions and mount options
• Enable UEFI Secure Boot to prevent execution of unsigned or modified bootloader code

# Disable GRUB2 graphical mode as a workaround
# Edit /etc/default/grub and add:
GRUB_TERMINAL=console

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

# Restrict permissions on boot partition
chmod 700 /boot/grub2/themes/