CVE-2021-3697: GNU GRUB2 JPEG Image RCE Vulnerability

CVE-2021-3697 Overview

CVE-2021-3697 is a heap buffer underflow vulnerability in the GNU GRUB2 bootloader's JPEG image reader. A crafted JPEG image can cause the JPEG reader to underflow its data pointer, allowing user-controlled data to be written to the heap. This vulnerability requires the attacker to perform triage over the heap layout and craft an image with a malicious format and payload to achieve successful exploitation.

Critical Impact

This vulnerability can lead to data corruption, eventual code execution, or secure boot circumvention, potentially allowing attackers to compromise the boot process and bypass security mechanisms designed to ensure system integrity.

Affected Products

• GNU GRUB2 (versions prior to grub-2.12)
• Red Hat Enterprise Linux 8.x and 9.x
• Red Hat Enterprise Linux EUS 8.2, 8.4, 8.6, 9.0
• Red Hat Enterprise Linux for Power Little Endian 8.0 and 9.0
• Red Hat OpenShift Container Platform 4.6, 4.9, 4.10
• Red Hat Developer Tools 1.0
• Red Hat CodeReady Linux Builder

Discovery Timeline

• July 6, 2022 - CVE-2021-3697 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-3697

Vulnerability Analysis

This vulnerability exists in the GRUB2 bootloader's JPEG image parsing functionality. The JPEG reader contains a flaw that allows an attacker to cause a data pointer underflow condition. When processing a specially crafted JPEG image, the parser fails to properly validate pointer boundaries, resulting in the data pointer being decremented beyond its valid range.

The heap buffer underflow condition (CWE-787: Out-of-bounds Write) enables an attacker to write user-controlled data to arbitrary heap locations. This is particularly dangerous in the context of a bootloader, as GRUB2 operates in a privileged environment before the operating system's security mechanisms are active.

Exploitation requires careful heap manipulation and crafting of a malicious JPEG image with both a specific format structure and weaponized payload. While the local attack vector and high complexity requirements provide some mitigation, successful exploitation can completely undermine the system's security posture by bypassing secure boot protections.

Root Cause

The root cause of this vulnerability lies in improper bounds checking within the GRUB2 JPEG reader implementation. The code fails to validate that pointer arithmetic operations remain within the allocated buffer boundaries. When processing certain JPEG image structures, the data pointer can be decremented without verifying it stays within valid memory ranges, leading to an underflow condition.

This type of vulnerability is classified as CWE-787 (Out-of-bounds Write), where the software writes data past the end, or before the beginning, of the intended buffer.

Attack Vector

The attack vector for CVE-2021-3697 requires local access to the target system. An attacker must be able to provide a malicious JPEG image that will be processed by GRUB2 during the boot sequence. This could occur through:

1. Modifying boot theme images on a system with physical access
2. Compromising a system's boot partition to inject malicious JPEG files
3. Leveraging another vulnerability to modify the GRUB2 configuration or theme assets

The vulnerability requires the attacker to carefully analyze and manipulate the heap layout to position their malicious data precisely. The crafted JPEG image must contain both the trigger for the underflow condition and the payload to be written to the heap.

The vulnerability mechanism involves the JPEG decoder's pointer management during image parsing. When specific malformed JPEG structures are encountered, the data pointer arithmetic results in an underflow, allowing writes to memory locations preceding the intended buffer. For detailed technical analysis, refer to the Red Hat Bug Report #1991687.

Detection Methods for CVE-2021-3697

Indicators of Compromise

• Unexpected or modified JPEG image files in GRUB2 theme directories (typically /boot/grub2/themes/)
• Unauthorized modifications to GRUB2 configuration files referencing image assets
• Anomalous boot behavior or secure boot validation failures
• Unexpected changes to files in the /boot/ partition

Detection Strategies

• Monitor file integrity of the boot partition, particularly GRUB2 theme images and configuration files
• Implement secure boot attestation to detect tampering with the boot chain
• Use SentinelOne's file integrity monitoring to track changes to boot-related files
• Audit access logs for any modifications to GRUB2 directories and files

Monitoring Recommendations

• Enable audit logging for all access to /boot/ and /boot/efi/ directories
• Configure alerts for any changes to GRUB2 theme images or configuration files
• Implement continuous integrity verification of boot components using TPM-based attestation
• Deploy endpoint detection capabilities that can identify anomalous boot-time behavior

How to Mitigate CVE-2021-3697

Immediate Actions Required

• Update GRUB2 to version grub-2.12 or later, which contains the fix for this vulnerability
• Apply vendor-provided security patches for your Linux distribution
• Verify secure boot is enabled and properly configured
• Audit boot partition contents for any unexpected or modified JPEG images

Patch Information

GNU has addressed this vulnerability in GRUB2 version grub-2.12 and later. Multiple Linux distribution vendors have released security updates:

• Red Hat: Security patches available for affected Red Hat Enterprise Linux versions. See the Red Hat Bug Report #1991687 for details.
• Gentoo: Security advisory GLSA-202209-12 provides patch information.
• NetApp: Advisory NTAP-20220930-0001 available for affected NetApp products.

Workarounds

• Restrict physical access to systems to prevent unauthorized boot partition modifications
• Remove or disable custom GRUB2 themes that use JPEG images if patching is not immediately possible
• Implement strong access controls on the boot partition to limit write access
• Use disk encryption to protect boot partition integrity from offline attacks

# Verify current GRUB2 version
grub2-install --version

# Check for available GRUB2 updates (RHEL/CentOS)
yum check-update grub2*

# Apply GRUB2 security updates (RHEL/CentOS)
yum update grub2 grub2-common grub2-tools

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

# Verify secure boot status
mokutil --sb-state