CVE-2020-15705: GNU GRUB2 Auth Bypass Vulnerability

CVE-2020-15705 Overview

CVE-2020-15705 is a Secure Boot Bypass vulnerability in GNU GRUB2 bootloader that allows attackers to bypass secure boot protections when the system boots directly without the shim bootloader. This vulnerability specifically affects systems where the kernel signing certificate has been imported directly into the secure boot database, enabling unsigned or maliciously modified kernels to execute during the boot process.

Critical Impact

Attackers with local access and high privileges can bypass secure boot protections, potentially loading unsigned or tampered kernel images. This undermines the entire secure boot chain of trust, enabling persistent rootkits and boot-level malware that survive system reboots.

Affected Products

• GNU GRUB2 version 2.04 and prior versions
• Red Hat Enterprise Linux 7.0 and 8.0
• Canonical Ubuntu Linux 14.04 ESM, 16.04 LTS, 18.04 LTS, 20.04 LTS
• Debian Linux 10.0
• openSUSE Leap 15.1 and 15.2
• SUSE Linux Enterprise Server 11, 12, 15
• Microsoft Windows 10 (multiple versions), Windows 8.1, Windows RT 8.1
• Microsoft Windows Server 2012, 2016, 2019
• Red Hat OpenShift Container Platform 4.0
• Red Hat Enterprise Linux Atomic Host

Discovery Timeline

• July 29, 2020 - CVE-2020-15705 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-15705

Vulnerability Analysis

This vulnerability resides in the GRUB2 bootloader's signature verification logic. Under normal secure boot configurations, the shim bootloader acts as an intermediate trust layer that validates the GRUB2 bootloader and subsequent kernel images. However, when GRUB2 is configured to boot directly without shim—with the kernel signing certificate imported directly into the UEFI Secure Boot database—the bootloader fails to properly validate kernel signatures before execution.

The flaw stems from improper verification of cryptographic signatures (CWE-347) in the kernel loading path. When booting without shim, GRUB2 does not enforce signature checks on the kernel image, even though the system is configured for secure boot. This creates a gap in the chain of trust that secure boot is designed to maintain.

Root Cause

The root cause is a failure in GRUB2's kernel signature validation logic when operating without the shim bootloader. GRUB2 incorrectly assumes that signature verification is handled externally or skips the verification step entirely in this specific boot configuration. The code path for direct booting lacks the necessary cryptographic validation checks that would normally be performed by shim, leaving the kernel loading process unprotected.

Attack Vector

Exploitation requires local access with high privileges to the target system. An attacker must be able to modify the GRUB2 configuration or replace the kernel image on the boot partition. The attack scenario involves:

1. Gaining local access to a system configured to boot GRUB2 directly without shim
2. Replacing the legitimate signed kernel with an unsigned or maliciously modified kernel
3. Rebooting the system, causing GRUB2 to load the tampered kernel without signature validation
4. Achieving persistent boot-level code execution that survives system reinstallation

This attack is particularly dangerous because it operates below the operating system level, making detection extremely difficult and enabling rootkits that persist across OS reinstallations.

Detection Methods for CVE-2020-15705

Indicators of Compromise

• Unexpected modifications to GRUB2 configuration files in /boot/grub2/ or /boot/grub/
• Changes to kernel images in the boot partition that do not correspond to legitimate updates
• Boot logs showing kernel images being loaded without signature verification warnings
• Presence of unsigned kernel modules or boot components on systems configured for secure boot

Detection Strategies

• Monitor boot partition integrity using file integrity monitoring solutions to detect unauthorized modifications to bootloader and kernel files
• Implement UEFI Secure Boot audit logging to capture signature verification events during the boot process
• Use endpoint detection and response (EDR) solutions capable of monitoring pre-boot and boot-time activities
• Regularly verify that shim bootloader is in use rather than direct GRUB2 booting on secure boot-enabled systems

Monitoring Recommendations

• Deploy SentinelOne agents with firmware and bootloader monitoring capabilities to detect boot-level tampering
• Configure system logging to capture UEFI and GRUB2 boot events for forensic analysis
• Implement regular automated verification of secure boot configuration and certificate chain integrity
• Monitor for changes to UEFI Secure Boot database entries that could indicate attempted exploitation

How to Mitigate CVE-2020-15705

Immediate Actions Required

• Verify that all systems boot using the shim bootloader rather than directly booting GRUB2
• Update GRUB2 to patched versions provided by your Linux distribution vendor
• Review and audit UEFI Secure Boot database entries for proper certificate chain configuration
• Apply vendor-specific security updates from Red Hat, Canonical, Debian, SUSE, Microsoft, and other affected vendors

Patch Information

Multiple vendors have released security updates to address this vulnerability. Key vendor resources include:

• GNU GRUB Development Mailing List Announcement
• Microsoft Security Advisory ADV200011
• Red Hat GRUB2 Vulnerability Information
• Ubuntu Security Notice USN-4432-1
• Debian GRUB UEFI Secure Boot Advisory
• SUSE GRUB2 Secure Boot Issue

Organizations should prioritize updating GRUB2 packages and applying UEFI Secure Boot database updates (DBX revocations) as provided by their respective vendors.

Workarounds

• Ensure all systems use the shim bootloader in the secure boot chain rather than booting GRUB2 directly
• If direct GRUB2 booting is required, consider temporarily disabling secure boot until patches are applied (note: this reduces security)
• Implement physical security controls to prevent unauthorized access to boot partitions
• Use full disk encryption with TPM-sealed keys to detect unauthorized boot chain modifications

# Verify shim bootloader is in use (Linux)
efibootmgr -v | grep -i shim

# Check GRUB2 version for patched release
grub2-install --version

# Verify secure boot status
mokutil --sb-state

# List UEFI secure boot database entries
mokutil --list-enrolled