CVE-2020-27749: GNU GRUB2 Buffer Overflow Vulnerability

CVE-2020-27749 Overview

A stack buffer overflow vulnerability was discovered in GRUB2 bootloader versions prior to 2.06. The flaw exists in how variable names present in command lines are expanded into their corresponding variable contents. GRUB2 uses a 1kB stack buffer for temporary storage during this expansion process, but fails to implement sufficient bounds checking. When the function is called with a command line referencing a variable with a sufficiently large payload, an attacker can overflow the stack buffer, corrupt the stack frame, and potentially gain control of execution flow. This vulnerability is particularly concerning as successful exploitation could circumvent Secure Boot protections.

Critical Impact

Successful exploitation enables stack buffer overflow leading to arbitrary code execution during boot, potentially bypassing Secure Boot protections and compromising system integrity before the operating system loads.

Affected Products

• GNU GRUB2 (versions prior to 2.06)
• Red Hat Enterprise Linux 7.0 and 8.0
• Red Hat Enterprise Linux Server AUS 7.2, 7.3, 7.4, 7.6, 7.7, 8.2
• Red Hat Enterprise Linux Server EUS 7.6, 7.7, 8.1
• Red Hat Enterprise Linux Server TUS 7.4, 7.6, 7.7, 8.2
• Red Hat Enterprise Linux Workstation 7.0
• Fedora 33 and 34
• NetApp ONTAP Select Deploy Administration Utility

Discovery Timeline

• 2021-03-03 - CVE-2020-27749 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-27749

Vulnerability Analysis

This vulnerability (CWE-121: Stack-based Buffer Overflow, CWE-787: Out-of-bounds Write) resides in the command line parsing functionality of GRUB2. The bootloader expands environment variables referenced in command lines by copying their contents into a fixed 1KB stack buffer. The lack of proper boundary validation means that a variable containing more than 1024 bytes of data will overflow the stack buffer when expanded.

The implications of this vulnerability are severe in the bootloader context. GRUB2 executes in a privileged environment before the operating system loads, meaning exploitation occurs outside the protection of standard OS security mechanisms. An attacker who successfully exploits this flaw could execute arbitrary code with full system privileges, potentially installing persistent malware that survives operating system reinstallation.

The local attack vector requires elevated privileges, as modifying GRUB2 configuration or environment variables typically requires administrative access. However, in scenarios where an attacker has already gained local access with high privileges, this vulnerability provides a mechanism to establish persistence below the operating system level and potentially bypass Secure Boot protections entirely.

Root Cause

The root cause is insufficient bounds checking in the variable expansion routine. When GRUB2 processes command lines containing variable references (e.g., ${variable_name}), it allocates a static 1KB buffer on the stack to hold the expanded content. The code does not validate that the variable contents fit within this buffer before performing the copy operation, resulting in a classic stack-based buffer overflow condition.

Attack Vector

Exploitation requires local access with high privileges to manipulate GRUB2's command line or environment variables. An attacker would craft a malicious GRUB2 environment variable with a payload exceeding 1KB that, when expanded during command line processing, overflows the stack buffer. By carefully constructing the overflow data, the attacker can overwrite the return address on the stack and redirect execution to attacker-controlled code. This technique could be used to:

• Execute arbitrary code during the boot process
• Bypass Secure Boot signature verification
• Install boot-level rootkits or implants
• Persist across operating system reinstallations

The vulnerability can be triggered through malicious GRUB configuration files or by manipulating GRUB environment variables stored on disk. Since the exploitation occurs during the boot phase, traditional operating system security controls are ineffective at preventing or detecting the attack.

Detection Methods for CVE-2020-27749

Indicators of Compromise

• Unexpected modifications to GRUB2 configuration files (/etc/default/grub, /boot/grub2/grub.cfg)
• Unusually large environment variables defined in GRUB configuration
• Changes to the GRUB environment block file (grubenv) with oversized variable values
• Modifications to boot partition files outside of normal system update processes

Detection Strategies

• Monitor file integrity of GRUB2 configuration files and boot partition contents using tools like AIDE or Tripwire
• Implement Secure Boot with custom signing keys to detect unauthorized bootloader modifications
• Audit changes to /boot partition and GRUB-related files for unauthorized modifications
• Review system logs for unexpected boot behavior or failures that may indicate exploitation attempts

Monitoring Recommendations

• Enable TPM-based measured boot to detect bootloader integrity violations
• Configure UEFI Secure Boot and monitor for enrollment of unauthorized signing keys
• Implement boot-time attestation solutions to verify bootloader integrity
• Regularly audit GRUB2 configuration for unusually long variable definitions or suspicious command line parameters

How to Mitigate CVE-2020-27749

Immediate Actions Required

• Update GRUB2 to version 2.06 or later which includes the fix for this vulnerability
• Apply vendor-specific patches from Red Hat, Fedora, or your distribution's security updates
• Revoke vulnerable GRUB2 bootloader signatures in UEFI Secure Boot dbx database
• Audit GRUB2 configuration files for any signs of tampering or suspicious variable definitions

Patch Information

The vulnerability is fixed in GRUB2 version 2.06 and later. Multiple Linux distributions have released patched versions:

• Red Hat: Security advisories available via Red Hat Bug Report
• Fedora: Updates available via Fedora Package Announcement
• Gentoo: GLSA 2021-05 provides patched packages
• NetApp: Consult the NetApp Security Advisory for ONTAP Select Deploy updates

Organizations should update their UEFI Secure Boot revocation lists (dbx) to include the signatures of vulnerable GRUB2 versions to prevent rollback attacks.

Workarounds

• If immediate patching is not possible, restrict physical access to systems to prevent boot-time manipulation
• Enable UEFI password protection to prevent unauthorized access to boot settings
• Use full disk encryption with TPM binding to detect bootloader modifications
• Monitor boot partition integrity and alert on any unauthorized changes

# Verify current GRUB2 version
grub2-install --version

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

# Apply GRUB2 updates
yum update grub2 grub2-tools grub2-efi-x64

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

# Verify Secure Boot status
mokutil --sb-state