CVE-2020-27779: GNU GRUB2 Privilege Escalation Flaw

CVE-2020-27779 Overview

A Secure Boot bypass vulnerability was discovered in GRUB2, the widely-used open-source bootloader, affecting versions prior to 2.06. The flaw exists in the cutmem command, which fails to honor Secure Boot locking mechanisms. This allows a privileged attacker to manipulate memory address ranges, effectively removing critical memory regions and creating an opportunity to circumvent SecureBoot protections after gaining knowledge of GRUB's memory layout.

Critical Impact

This vulnerability enables privileged attackers to bypass Secure Boot protections, potentially compromising data confidentiality, integrity, and system availability by manipulating GRUB2's memory handling during the boot process.

Affected Products

• GNU GRUB2 (versions prior to 2.06)
• Red Hat Enterprise Linux 7.0 and 8.0
• Red Hat Enterprise Linux Server (AUS, EUS, TUS variants)
• Red Hat Enterprise Linux Workstation 7.0
• Fedora 33 and 34
• NetApp ONTAP Select Deploy Administration Utility

Discovery Timeline

• 2021-03-03 - CVE-2020-27779 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-27779

Vulnerability Analysis

This vulnerability represents a Secure Boot Bypass flaw in the GRUB2 bootloader. The core issue lies in the cutmem command's failure to properly respect Secure Boot lockdown states. When Secure Boot is enabled, the bootloader should restrict certain privileged operations that could compromise the boot chain integrity. However, the cutmem command does not implement these security checks, allowing it to execute even when Secure Boot lockdown should prevent such operations.

The flaw requires local access and high privileges to exploit, but once successfully leveraged, an attacker can alter the memory layout during the boot process. By strategically removing memory address ranges, an attacker can manipulate how the system initializes, potentially disabling security mechanisms or loading malicious code that would normally be blocked by Secure Boot verification.

Root Cause

The root cause stems from improper authorization checks (CWE-285) in the GRUB2 codebase. The cutmem command implementation does not verify whether the bootloader is operating in a Secure Boot locked-down state before executing memory manipulation operations. This oversight allows the command to function unrestricted, even in environments where Secure Boot should prevent such low-level memory modifications.

The vulnerability is categorized under CWE-285 (Improper Authorization), indicating that the command lacks the necessary access control checks to determine if the current execution context permits memory manipulation operations.

Attack Vector

The attack requires local access to the system with elevated privileges. An attacker must have the ability to interact with the GRUB2 bootloader during the boot process, typically requiring physical access or pre-existing privileged access to modify boot configurations.

The exploitation scenario involves:

1. Gaining access to the GRUB2 command line interface during boot
2. Executing the cutmem command with carefully crafted address ranges
3. Manipulating memory regions to disable or bypass Secure Boot verification
4. Loading unsigned or malicious boot components that would normally be rejected

Since no verified exploit code is publicly available, the technical mechanism involves using the cutmem command to remove memory regions that contain Secure Boot verification data or critical security structures. For detailed technical analysis, refer to the Red Hat Bug Report.

Detection Methods for CVE-2020-27779

Indicators of Compromise

• Unexpected modifications to GRUB2 configuration files (grub.cfg) containing cutmem commands
• Alterations to boot loader entries or custom boot scripts that include memory manipulation directives
• Evidence of unauthorized access to boot partitions or EFI System Partition (ESP)
• System boot logs showing unusual memory allocation patterns or errors

Detection Strategies

• Monitor for changes to /boot/grub2/grub.cfg and related bootloader configuration files
• Implement file integrity monitoring (FIM) on boot partition contents and GRUB2 binaries
• Enable UEFI Secure Boot violation logging and review for anomalous boot events
• Audit physical access logs and privileged user activities during maintenance windows

Monitoring Recommendations

• Deploy endpoint detection and response (EDR) solutions capable of monitoring boot-time activities
• Configure alerts for any modifications to bootloader configurations or boot partition files
• Regularly verify Secure Boot status and attestation records on critical systems
• Implement centralized logging for boot events across enterprise infrastructure

How to Mitigate CVE-2020-27779

Immediate Actions Required

• Update GRUB2 to version 2.06 or later on all affected systems
• Apply vendor-specific patches from Red Hat, Fedora, or your distribution vendor
• Review and restrict physical access to systems requiring Secure Boot protection
• Audit existing GRUB2 configurations for suspicious cutmem command usage

Patch Information

Security patches are available from multiple vendors. Red Hat has issued advisories for Enterprise Linux 7 and 8 systems, and Fedora has released updated packages for Fedora 33 and 34. For detailed patch information, consult the following resources:

• Red Hat Bug Report - Primary vulnerability tracking
• Fedora Package Announcement - Fedora security update
• Gentoo GLSA 202104-05 - Gentoo Linux advisory
• NetApp Security Advisory - NetApp ONTAP advisory

Workarounds

• Enable UEFI password protection to prevent unauthorized access to the bootloader interface
• Restrict physical access to systems where Secure Boot integrity is critical
• Consider using UEFI Secure Boot with custom key enrollment to limit bootloader modifications
• Monitor boot configurations through centralized management platforms

# Verify current GRUB2 version and Secure Boot status
grub2-install --version
mokutil --sb-state

# Check for unauthorized cutmem commands in configuration
grep -r "cutmem" /boot/grub2/

# Update GRUB2 on RHEL/CentOS systems
sudo yum update grub2

# Update GRUB2 on Fedora systems
sudo dnf update grub2