CVE-2024-56738: GNU GRUB2 Information Disclosure Vulnerability

CVE-2024-56738 Overview

CVE-2024-56738 affects GNU GRUB (also known as GRUB2) through version 2.12. The grub_crypto_memcmp function does not use a constant-time algorithm when comparing cryptographic values. This implementation flaw enables side-channel attacks against sensitive comparisons performed by the bootloader.

An attacker who can measure timing differences during memory comparison operations may recover secrets such as password hashes used to protect GRUB menu entries. The weakness maps to [CWE-208] Observable Timing Discrepancy and [CWE-203] Observable Discrepancy.

Critical Impact
Timing analysis of grub_crypto_memcmp can leak byte-by-byte information about secret values compared inside GRUB, weakening bootloader authentication controls.

Affected Products

GNU GRUB2 versions through 2.12
Linux distributions shipping vulnerable GRUB2 builds
Systems relying on GRUB2 password protection for boot entries

Discovery Timeline

2024-12-29 - CVE-2024-56738 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2024-56738

Vulnerability Analysis

GRUB2 provides grub_crypto_memcmp as a helper for comparing cryptographic material such as password digests. A constant-time comparison must process every byte regardless of where the first mismatch occurs. The affected implementation returns early on mismatch, so the total execution time correlates with the length of the shared prefix between the two inputs.

An attacker who can submit candidate values and observe timing responses can iterate byte positions until the comparison lengthens. Repeating the process across all positions reconstructs the target secret without ever guessing the full value at once. The issue is a classic timing side channel in a security-sensitive primitive.

The attack does not require memory corruption or code execution. It exploits observable behavior of a correctly reachable code path, which is why it falls under [CWE-208] and [CWE-203].

Root Cause

The root cause is the use of a data-dependent comparison in grub_crypto_memcmp. Because the function exits as soon as two bytes differ, execution time varies with input similarity. Cryptographic comparisons must instead accumulate differences across the full buffer and only branch on the final result.

Attack Vector

Exploitation requires the ability to submit comparison inputs and observe timing feedback. Realistic vectors include local attackers with console access measuring password validation delays. Network-facing attack surfaces exist where GRUB2 code is reused in environments that expose the comparison remotely, which is reflected in the network attack vector rating.

The vulnerability yields confidentiality impact only. It does not directly enable code execution or persistence, but recovered secrets can subsequently unlock bootloader protections. See the GNU Savannah Bug Report #66603 for upstream discussion.

Detection Methods for CVE-2024-56738

Indicators of Compromise

Repeated GRUB password authentication attempts against a single boot entry
Console or serial-line activity showing scripted, rapidly repeated boot menu interactions
Unexplained delays followed by successful bootloader authentication on hardened hosts

Detection Strategies

Inventory installed GRUB2 packages and flag versions at or below 2.12 that lack distribution backports
Compare grub-install --version output across fleet baselines to identify unpatched hosts
Review distribution security trackers for the specific GRUB2 build hash deployed on each system

Monitoring Recommendations

Log physical and serial console access to systems with GRUB password protection enabled
Alert on repeated failed GRUB authentication events captured by out-of-band management controllers
Track patch state of grub2 and grub2-common packages through configuration management

How to Mitigate CVE-2024-56738

Immediate Actions Required

Apply vendor-supplied GRUB2 updates as soon as distributions publish fixed packages
Restrict physical and remote console access to systems that rely on GRUB2 password protection
Rotate any GRUB2 passwords that may have been exposed to untrusted operators

Patch Information

Upstream tracking is available in GNU Savannah Bug Report #66603. Fixes replace the non-constant-time comparison in grub_crypto_memcmp with an implementation that processes the entire buffer before returning. Consult your Linux distribution's security advisories for backported package versions.

Workarounds

Enforce full-disk encryption so bootloader password recovery alone does not grant data access
Enable UEFI Secure Boot and firmware passwords to reduce reliance on GRUB2 password protection
Limit console exposure by disabling serial redirection and locking down BMC interfaces

bash

# Verify installed GRUB2 version and check for available updates
grub-install --version
# Debian/Ubuntu
apt list --upgradable | grep -i grub
# RHEL/Fedora
dnf check-update grub2 grub2-common grub2-tools