CVE-2025-61663: GRUB2 Bootloader Use-after-Free DoS Vulnerability

CVE-2025-61663 Overview

CVE-2025-61663 is a use-after-free vulnerability in the GRUB2 bootloader's normal command. The flaw occurs because the normal command is not properly unregistered when its module is unloaded. An attacker with local access to the GRUB2 command interface can trigger access to freed memory, causing system instability or a complete crash. The issue is tracked under CWE-825: Expired Pointer Dereference and primarily affects availability, though impacts on confidentiality and integrity are not excluded.

Critical Impact
Local attackers with access to the GRUB2 shell can halt system boot or crash the loader, denying system availability during the pre-boot phase.

Affected Products

GRUB2 bootloader (normal command module)
Linux distributions shipping affected GRUB2 builds (see Red Hat CVE-2025-61663 Advisory)
Downstream vendor packages tracked in Red Hat Bug Report #2414684

Discovery Timeline

2025-11-18 - CVE-2025-61663 published to the National Vulnerability Database
2026-10-18 - Last updated in NVD database

Technical Details for CVE-2025-61663

Vulnerability Analysis

The vulnerability resides in GRUB2's module management logic for the normal command. GRUB2 supports loading and unloading modules at runtime from its command interface. When the module providing the normal command is unloaded, the command handler is not correctly removed from GRUB2's command registry. The dangling command entry continues to reference memory that has been freed.

When the normal command is subsequently invoked, GRUB2 dereferences the stale pointer and executes code tied to memory that is no longer valid. This condition matches [CWE-825: Expired Pointer Dereference], a subclass of use-after-free. The outcome is undefined behavior inside the bootloader, typically producing a hang or crash of the pre-boot environment.

Root Cause

The root cause is missing cleanup during module unload. GRUB2 should call the command unregister routine as part of the normal module teardown, releasing the command structure before freeing the module memory. Because unregistration does not occur, the command table retains a pointer into memory that the module teardown has released back to the allocator.

Attack Vector

Exploitation requires local access to the GRUB2 command shell or an equivalent configuration path that can issue module load/unload sequences followed by invocation of the normal command. The attack is constrained to the pre-boot environment and requires the attacker to reach the GRUB2 prompt, which is typically protected on hardened systems. No authentication within the running operating system is required, but physical or console access to the bootloader is needed. See the Red Hat CVE-2025-61663 Advisory for vendor-specific exposure details.

Detection Methods for CVE-2025-61663

Indicators of Compromise

Unexpected system hangs or kernel-less crashes during the GRUB2 boot stage
GRUB2 shell access events on systems where the bootloader prompt should be restricted
Boot logs showing repeated reloads or recovery mode entries without administrative action

Detection Strategies

Audit GRUB2 package versions across the fleet and compare against vendor fixed releases listed in the Red Hat CVE-2025-61663 Advisory
Monitor firmware and bootloader integrity using measured boot or TPM event logs for unexpected GRUB2 module activity
Review centralized logging for abnormal reboot patterns, especially systems that fail to progress past the bootloader

Monitoring Recommendations

Collect boot telemetry and uptime metrics in a centralized data store to spot clusters of pre-boot failures
Alert on configuration changes to grub.cfg and GRUB2 password settings across managed endpoints
Track physical and remote console access events on servers that expose BMC or iDRAC interfaces

How to Mitigate CVE-2025-61663

Immediate Actions Required

Apply the GRUB2 updates published by your Linux distribution as soon as they are available
Enforce a GRUB2 superuser password to prevent unauthorized access to the bootloader command shell
Restrict physical and remote console access to systems where bootloader hardening is incomplete

Patch Information

Refer to the Red Hat CVE-2025-61663 Advisory and Red Hat Bug Report #2414684 for fixed package versions. The fix adds proper unregistration of the normal command during module unload, eliminating the dangling pointer in the GRUB2 command registry. Downstream distributions typically ship coordinated updates; verify your vendor's advisory for the exact fixed build.

Workarounds

Configure a GRUB2 password with grub2-setpassword to block interactive access to the bootloader shell
Disable the GRUB2 edit and command-line entries in grub.cfg where interactive boot editing is not required
Enable Secure Boot and measured boot to detect unauthorized changes to bootloader components

bash

# Set a GRUB2 superuser password to restrict access to the command shell
sudo grub2-setpassword

# Rebuild the GRUB configuration after changes
sudo grub2-mkconfig -o /boot/grub2/grub.cfg