CVE-2020-14309: Gnu Grub2 Buffer Overflow Vulnerability

CVE-2020-14309 Overview

CVE-2020-14309 is an integer overflow vulnerability in GRUB2 (GNU GRand Unified Bootloader) affecting all versions prior to 2.06. The vulnerability exists in the squashfs filesystem handler when processing symbolic links with specially crafted name lengths. When a squashfs filesystem contains a symbolic link with a name length of UINT32 bytes, the size value triggers an arithmetic overflow, resulting in a zero-size memory allocation. This subsequently leads to a heap-based buffer overflow condition where attacker-controlled data can overwrite adjacent heap memory.

Critical Impact

Successful exploitation allows local attackers with high privileges to achieve arbitrary code execution at the bootloader level, potentially bypassing Secure Boot protections and compromising system integrity before the operating system loads.

Affected Products

• GNU GRUB2 (all versions before 2.06)
• openSUSE Leap 15.1
• openSUSE Leap 15.2

Discovery Timeline

• 2020-07-30 - CVE-2020-14309 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-14309

Vulnerability Analysis

This vulnerability is classified as CWE-190 (Integer Overflow or Wraparound). The flaw resides in GRUB2's squashfs filesystem parsing code, which fails to properly validate symbolic link name lengths before performing arithmetic operations and memory allocations.

When GRUB2 parses a squashfs filesystem, it reads metadata about symbolic links including their name lengths. The vulnerability occurs when the name length field is set to UINT32_MAX (4,294,967,295 bytes). When this value is used in subsequent size calculations, an integer overflow occurs, wrapping the computed allocation size to zero or a very small value.

The memory allocator then allocates an undersized buffer, but the subsequent copy operation attempts to write the full attacker-controlled symbolic link name data into this buffer. This mismatch between the allocated buffer size and the data being written results in a classic heap-based buffer overflow condition.

Since GRUB2 operates at the pre-boot stage with full system privileges and before operating system security controls are active, successful exploitation could allow attackers to execute arbitrary code in the bootloader context, potentially bypassing Secure Boot mechanisms and establishing persistent boot-level malware.

Root Cause

The root cause is improper validation of integer values before arithmetic operations in the squashfs symbolic link handling code. The code fails to check if the symbolic link name length would cause an integer overflow when used in size calculations, and does not implement safe arithmetic operations that would detect and prevent overflow conditions. This allows a crafted squashfs image to trigger the overflow-to-zero allocation pattern, a well-known exploitation primitive in memory corruption vulnerabilities.

Attack Vector

The attack requires local access and high privileges to mount or boot from a maliciously crafted squashfs filesystem image. An attacker would need to:

1. Craft a squashfs filesystem containing a symbolic link with a name length field set to UINT32_MAX
2. Deliver this malicious filesystem to the target system (via removable media, network boot, or by modifying existing boot partitions)
3. Trigger GRUB2 to parse the malicious filesystem during boot or through manual filesystem browsing

The vulnerability is exploited when GRUB2's squashfs handler reads the symbolic link metadata, performs the overflowing size calculation, allocates the undersized buffer, and then copies attacker-controlled data beyond the buffer boundaries.

Detection Methods for CVE-2020-14309

Indicators of Compromise

• Presence of suspicious or unexpected squashfs filesystem images in boot partitions or removable media
• GRUB2 crashes or unexpected behavior during boot with error messages related to squashfs parsing
• Modified boot configuration pointing to untrusted squashfs images
• Evidence of Secure Boot bypass attempts or unsigned bootloader components

Detection Strategies

• Monitor boot partition integrity using file integrity monitoring solutions to detect unauthorized modifications to GRUB2 binaries or configuration
• Implement boot-time attestation to verify the integrity of bootloader components before system startup
• Scan removable media and network boot images for malformed squashfs filesystems with anomalous symbolic link metadata
• Deploy endpoint detection solutions capable of monitoring pre-boot environment changes

Monitoring Recommendations

• Regularly audit GRUB2 version information across the infrastructure to identify systems running vulnerable versions
• Monitor for changes to /boot partition contents, particularly GRUB2 modules and configuration files
• Review boot logs and UEFI event logs for anomalies indicating exploitation attempts
• Track security advisory notifications from distribution vendors for related updates

How to Mitigate CVE-2020-14309

Immediate Actions Required

• Update GRUB2 to version 2.06 or later across all affected systems
• Apply vendor-specific security patches from Red Hat, openSUSE, Ubuntu, Gentoo, or NetApp as applicable
• Verify Secure Boot database updates have been applied to revoke vulnerable GRUB2 binaries
• Restrict physical access and removable media boot options to prevent delivery of malicious squashfs images

Patch Information

Security patches addressing CVE-2020-14309 are available from multiple vendors. The fix involves adding proper bounds checking and safe arithmetic operations to prevent integer overflow conditions in the squashfs symbolic link parsing code. Refer to the following vendor advisories for distribution-specific patching guidance:

• openSUSE Security Announcements for Leap 15.1 and 15.2
• Red Hat Bug Report for RHEL-based systems
• Ubuntu Security Notice USN-4432-1 for Ubuntu systems
• Gentoo GLSA 202104-05 for Gentoo Linux
• NetApp Security Advisory for NetApp products

Workarounds

• Disable GRUB2 squashfs module loading if squashfs boot is not required (may impact functionality)
• Configure UEFI/BIOS settings to prevent booting from untrusted media sources
• Implement strict boot partition access controls to prevent unauthorized modification of boot components
• Enable UEFI Secure Boot with updated revocation lists to block known vulnerable GRUB2 binaries

# Verify current GRUB2 version (example for Debian/Ubuntu)
grub-install --version

# Check for available security updates
apt update && apt list --upgradable | grep grub

# Apply GRUB2 updates
apt upgrade grub-efi-amd64-signed grub2-common

# Regenerate GRUB configuration after patching
update-grub