CVE-2022-28736: GNU GRUB2 Use-After-Free Vulnerability

CVE-2022-28736 Overview

CVE-2022-28736 is a use-after-free vulnerability in the grub_cmd_chainloader() function of GNU GRUB2 bootloader. The chainloader command is used to boot operating systems that don't support multiboot and lack direct support from GRUB2. When executing the chainloader command more than once, a use-after-free condition is triggered. If an attacker can control GRUB2's memory allocation pattern, sensitive data may be exposed and arbitrary code execution can be achieved at the bootloader level.

Critical Impact

This vulnerability enables local attackers with low privileges to potentially achieve arbitrary code execution during the boot process, compromising system integrity before the operating system's security controls are initialized.

Affected Products

• GNU GRUB2 (all versions prior to patch)
• Systems utilizing GRUB2 bootloader for operating system boot management
• Enterprise and consumer Linux distributions using GRUB2

Discovery Timeline

• 2022-06-07 - Vulnerability disclosed via Openwall OSS-Security Discussion
• 2023-07-20 - CVE-2022-28736 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2022-28736

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption flaw that occurs when a program continues to use memory after it has been freed. In the context of GRUB2's grub_cmd_chainloader() function, the vulnerability manifests when the chainloader command is executed multiple times in succession.

The chainloader command is specifically designed to load and transfer control to another bootloader or operating system that GRUB2 cannot directly boot. This is commonly used for booting Windows or other non-multiboot compliant operating systems from a GRUB2 environment.

The root cause lies in improper memory management within the chainloader implementation. When the command is invoked a second time, previously freed memory regions are accessed, creating a use-after-free condition. An attacker who can influence GRUB2's memory allocation patterns could exploit this to expose sensitive data or achieve code execution at the bootloader level—a particularly dangerous scenario as it occurs before the operating system's security mechanisms are active.

Root Cause

The vulnerability stems from improper memory lifecycle management in the grub_cmd_chainloader() function. When the chainloader command completes its first execution, memory associated with the operation is freed. However, subsequent invocations of the command reference these freed memory locations without proper reallocation or validation, creating the use-after-free condition.

This type of memory corruption vulnerability in bootloader code is particularly concerning because:

• Bootloader code executes with maximum privileges before OS security controls
• Secure Boot mechanisms may not fully protect against exploitation
• Successful exploitation could persist across system reboots

Attack Vector

The attack requires local access to the system with the ability to interact with the GRUB2 bootloader interface. An attacker would need to:

1. Gain access to the GRUB2 command line interface during boot
2. Execute the chainloader command multiple times to trigger the use-after-free condition
3. Manipulate memory allocation patterns to control the freed memory contents
4. Leverage the memory corruption to achieve code execution or information disclosure

The vulnerability mechanism involves the improper handling of memory deallocation in the chainloader command processing. When the command is executed repeatedly, the internal data structures are not properly reset, causing references to previously freed memory. For detailed technical information, refer to the Openwall OSS-Security Discussion which contains the original disclosure details.

Detection Methods for CVE-2022-28736

Indicators of Compromise

• Unexpected GRUB2 bootloader behavior during system startup
• Multiple chainloader command entries in boot logs or GRUB configuration files
• Signs of tampering with GRUB2 configuration files (grub.cfg, grub.conf)
• Modifications to bootloader-related files without authorized changes

Detection Strategies

• Monitor for unauthorized modifications to GRUB2 configuration files and bootloader binaries
• Implement file integrity monitoring on /boot/grub/ directories and related boot partitions
• Review boot logs for anomalous chainloader command usage patterns
• Utilize Secure Boot with proper key management to detect unauthorized bootloader modifications

Monitoring Recommendations

• Enable and monitor UEFI Secure Boot audit logs where available
• Implement endpoint detection and response (EDR) solutions that monitor pre-boot environments
• Regularly verify bootloader integrity using trusted boot measurement mechanisms
• Configure alerts for any changes to boot-critical files and configurations

How to Mitigate CVE-2022-28736

Immediate Actions Required

• Update GRUB2 to the latest patched version provided by your Linux distribution
• Review and restrict physical access to systems that could allow bootloader interaction
• Enable UEFI Secure Boot to help prevent unauthorized bootloader modifications
• Audit GRUB2 configurations for unnecessary chainloader command usage

Patch Information

Organizations should apply security updates from their Linux distribution vendors. The NetApp Security Advisory provides additional guidance for NetApp products affected by this vulnerability. Contact your Linux distribution's security team for distribution-specific patches and update instructions.

For detailed vulnerability information, consult the MITRE CVE Details page.

Workarounds

• Restrict access to the GRUB2 command line interface by setting a bootloader password
• Disable or limit the chainloader command if not required for system operation
• Implement hardware-based security measures such as TPM-backed boot verification
• Consider using alternative bootloaders where the chainloader functionality is not needed

# Set GRUB2 password to restrict bootloader access
# Generate password hash
grub-mkpasswd-pbkdf2

# Add to /etc/grub.d/40_custom:
# set superusers="admin"
# password_pbkdf2 admin <generated-hash>

# Update GRUB configuration
update-grub