CVE-2020-15707: Gnu Grub2 RCE Vulnerability

CVE-2020-15707 Overview

CVE-2020-15707 is an integer overflow vulnerability discovered in the grub_cmd_initrd and grub_initrd_init functions within the efilinux component of GRUB2. This vulnerability affects distribution-specific implementations shipped in Debian, Red Hat, and Ubuntu, while the upstream GRUB2 codebase is not affected. The integer overflow condition leads to a heap-based buffer overflow, which can be exploited to execute arbitrary code and bypass UEFI Secure Boot restrictions.

The vulnerability can be triggered through two attack vectors: providing an extremely large number of arguments to the initrd command on 32-bit architectures, or utilizing a crafted filesystem containing very large files on any architecture. Successful exploitation allows an attacker with local access to compromise the boot process integrity and potentially install persistent bootkit malware.

Critical Impact

Attackers can bypass UEFI Secure Boot protections to execute arbitrary code during the boot process, potentially leading to persistent firmware-level compromise that survives OS reinstallation.

Affected Products

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

Discovery Timeline

• 2020-07-29 - CVE-2020-15707 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-15707

Vulnerability Analysis

The vulnerability resides in the initrd handling code within GRUB2's efilinux component, specifically affecting the grub_cmd_initrd and grub_initrd_init functions. These functions are responsible for loading initial ramdisk images during the boot process under UEFI environments.

The integer overflow occurs during size calculations when processing initrd arguments. When an attacker provides an extremely large number of arguments on 32-bit systems, or crafts a filesystem with very large files, the arithmetic operations used to calculate buffer sizes can wrap around, resulting in the allocation of an undersized heap buffer. Subsequent operations then write beyond the allocated buffer boundaries, creating a classic heap-based buffer overflow condition.

This vulnerability is particularly significant because it affects the Secure Boot chain of trust. GRUB2 executes with full system privileges during the boot process, and compromising it allows attackers to load malicious code before the operating system's security controls are initialized.

Root Cause

The root cause of CVE-2020-15707 is improper integer arithmetic handling in the initrd processing functions. The code fails to validate that size calculations do not exceed the maximum representable integer value before performing memory allocation. When the calculation overflows, the resulting value wraps around to a small number, causing malloc to allocate an insufficient buffer size. This is classified as CWE-190 (Integer Overflow or Wraparound).

Additionally, CWE-362 (Race Condition) has been associated with this vulnerability, indicating potential concurrent access issues during the exploitation chain.

Attack Vector

Exploitation of CVE-2020-15707 requires local access to the target system. An attacker can trigger the vulnerability through two primary methods:

1. Argument Overflow Attack: On 32-bit architectures, the attacker can invoke the initrd command with an extremely large number of arguments, causing the size calculation to overflow when summing argument sizes.

2. Crafted Filesystem Attack: On any architecture, the attacker can prepare a malicious filesystem containing specially crafted files with very large sizes that cause integer overflow during size calculations when GRUB2 attempts to load them.

Both attack vectors ultimately lead to heap corruption, which can be leveraged for arbitrary code execution. Since the exploit runs within the Secure Boot context, successful exploitation bypasses the cryptographic verification chain, allowing unsigned code execution at the firmware level.

The local attack vector combined with the requirement for high privileges reduces the immediate risk compared to remote exploits, but the ability to bypass Secure Boot makes this a high-impact vulnerability for systems relying on firmware-level security guarantees.

Detection Methods for CVE-2020-15707

Indicators of Compromise

• Unexpected modifications to GRUB2 configuration files (/boot/grub/grub.cfg or /boot/grub2/grub.cfg)
• Presence of unusually large initrd images or suspicious files in /boot directory
• Modified or unsigned GRUB2 binaries when Secure Boot should be enforced
• Anomalous boot-time behavior or extended boot times

Detection Strategies

• Implement file integrity monitoring (FIM) on boot-related files and directories
• Monitor for changes to UEFI Secure Boot database entries and DBX revocation lists
• Audit system logs for unexpected GRUB2 errors or boot loader modifications
• Deploy endpoint detection and response (EDR) solutions capable of detecting boot-level compromise

Monitoring Recommendations

• Establish baseline checksums for all bootloader components and monitor for deviations
• Configure alerts for any modifications to /boot/efi/ and GRUB2-related directories
• Monitor for attempts to disable or modify Secure Boot settings
• Review system firmware logs for signs of boot chain manipulation

How to Mitigate CVE-2020-15707

Immediate Actions Required

• Apply vendor-provided GRUB2 security updates immediately for all affected systems
• Update the UEFI Secure Boot DBX (Forbidden Signature Database) to revoke vulnerable GRUB2 binaries
• Verify Secure Boot is enabled and properly configured after applying patches
• Audit systems for signs of previous exploitation before remediation

Patch Information

Multiple vendors have released patches addressing CVE-2020-15707. Organizations should consult their respective vendor advisories for specific patch information:

• Red Hat: Security advisory available at Red Hat GRUB2 Vulnerability
• Ubuntu: Ubuntu Security Notice USN-4432-1
• Debian: Debian DSA-4735
• SUSE/openSUSE: SUSE GRUB2 Secure Boot Issue Response
• Microsoft: Microsoft Security Advisory ADV200011
• GNU GRUB: GNU GRUB Development Mailing List

Workarounds

• Enable and enforce UEFI Secure Boot with updated DBX containing revocations for vulnerable GRUB2 versions
• Restrict physical access to systems to prevent boot media manipulation
• Implement boot integrity verification mechanisms where available
• Consider using vendor-specific firmware security features (e.g., Intel Boot Guard, AMD Platform Secure Boot) for additional protection

# Verify current GRUB2 version on Debian/Ubuntu
dpkg -l | grep grub

# Verify current GRUB2 version on Red Hat/CentOS
rpm -qa | grep grub2

# Check Secure Boot status
mokutil --sb-state

# Update GRUB2 after applying patches (Debian/Ubuntu)
sudo update-grub

# Update GRUB2 after applying patches (Red Hat/CentOS)
sudo grub2-mkconfig -o /boot/grub2/grub.cfg