CVE-2020-25647: GNU GRUB2 RCE Vulnerability

CVE-2020-25647 Overview

CVE-2020-25647 is an out-of-bounds write vulnerability [CWE-787] in GNU GRUB2 bootloader versions prior to 2.06. The flaw resides in the USB device initialization routine, which reads device descriptors with insufficient bounds checking. GRUB2 trusts the values reported by the connected USB device and copies descriptor data into fixed-size buffers without validation. An attacker with physical access can craft a malicious USB device to trigger memory corruption during boot. Successful exploitation allows arbitrary code execution in the pre-boot environment and bypass of the UEFI Secure Boot mechanism. Affected distributions include Red Hat Enterprise Linux 7 and 8, Fedora 33 and 34, and NetApp ONTAP Select Deploy Administration Utility.

Critical Impact
A malicious USB device presented at boot can corrupt GRUB2 memory, execute attacker-controlled code, and defeat Secure Boot integrity guarantees on affected Linux systems.

Affected Products

GNU GRUB2 versions prior to 2.06
Red Hat Enterprise Linux 7.0, 8.0 (including Server AUS, EUS, TUS, and Workstation variants)
Fedora 33 and 34
NetApp ONTAP Select Deploy Administration Utility

Discovery Timeline

2021-03-03 - CVE-2020-25647 published to NVD
2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-25647

Vulnerability Analysis

The vulnerability exists in the GRUB2 USB stack, which is responsible for enumerating and initializing USB devices during the pre-boot phase. When GRUB2 queries a connected device for its USB descriptors (device, configuration, interface, and endpoint descriptors), it parses fields such as descriptor length and endpoint counts directly from attacker-controlled data. The code assumes these values fall within expected ranges and writes descriptor data into statically sized buffers without verifying length fields against destination buffer sizes.

A crafted USB device advertising oversized or malformed descriptors causes GRUB2 to write past the end of those buffers. This out-of-bounds write corrupts adjacent memory structures within the GRUB2 runtime. Because GRUB2 executes before the operating system kernel and within the Secure Boot trust chain, an attacker who controls execution at this stage can subvert the entire boot integrity model.

Root Cause

The root cause is improper input validation [CWE-787] in the USB descriptor parsing logic. GRUB2 treats descriptor length and count fields from the USB device as trusted inputs rather than untrusted, attacker-controllable values. No bounds checks are performed before copying descriptor payloads into fixed buffers.

Attack Vector

Exploitation requires physical access to the target system. An attacker plugs a malicious USB device into the machine before or during boot, when GRUB2 enumerates USB hardware. The device returns malformed descriptors engineered to overflow GRUB2 buffers, redirect execution, and load unsigned code. Because the attack runs before the kernel loads, Secure Boot signature verification is bypassed and persistent bootkit implants become possible.

No verified public proof-of-concept code is available. See the Red Hat Bug Report #1886936 for additional technical detail.

Detection Methods for CVE-2020-25647

Indicators of Compromise

Unexpected modifications to /boot/grub2/ files, grub.cfg, or shim and GRUB EFI binaries in the EFI System Partition
Boot integrity measurements (TPM PCR values) that diverge from a known-good baseline after physical access events
Presence of unauthorized USB device connections logged by firmware or kernel during boot sequences

Detection Strategies

Compare installed grub2 package versions against vendor-fixed releases using package manager queries such as rpm -q grub2 or dnf list installed grub2*
Enroll systems in measured boot with TPM attestation to detect tampering with pre-boot components
Monitor endpoint telemetry for unsigned kernel modules, unexpected EFI variable changes, or bootloader file modification events

Monitoring Recommendations

Collect and centralize boot integrity logs, firmware events, and USB device insertion records into a SIEM for correlation
Alert on physical USB connections to servers, kiosks, or unattended endpoints outside maintenance windows
Track vendor advisories from Red Hat, Fedora, Gentoo, and NetApp for follow-on GRUB2 patches

How to Mitigate CVE-2020-25647

Immediate Actions Required

Apply vendor-supplied GRUB2 updates that raise the package to version 2.06 or the distribution-specific patched release
Update the Secure Boot DBX revocation list (dbxtool or vendor revocation updates) to invalidate vulnerable GRUB2 and shim binaries
Restrict physical access to servers, workstations, and ONTAP Select Deploy hosts through locked cabinets, port blockers, and chassis intrusion detection

Patch Information

Vendors have released fixed packages addressing the USB descriptor bounds checks. Reference the Fedora Package Announcement, Gentoo GLSA 2021-04-05, and NetApp Security Advisory NTAP-20220325-0001 for distribution-specific package versions. After updating, regenerate the GRUB configuration and reinstall the bootloader to ensure the patched binary is in use.

Workarounds

Disable USB boot and external USB ports in firmware (BIOS/UEFI) settings on systems that do not require USB peripherals at boot
Set a firmware administrator password to prevent boot order modification and unauthorized USB device enumeration
Deploy full-disk encryption with TPM-bound keys so that pre-boot tampering invalidates key release

bash

# Verify patched GRUB2 version on Red Hat / Fedora systems
rpm -q grub2 grub2-common grub2-efi-x64

# Apply available updates
sudo dnf update grub2\* shim\*

# Reinstall GRUB to the EFI partition after update
sudo grub2-install --target=x86_64-efi --efi-directory=/boot/efi
sudo grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg

# Apply Secure Boot revocation updates (where provided by vendor)
sudo fwupdmgr refresh
sudo fwupdmgr update