CVE-2025-59699

Executive Summary

CVE-2025-59699 is a critical security vulnerability affecting Entrust nShield hardware security modules (HSMs), specifically the nShield Connect XC, nShield 5c, and nShield HSMi models. The vulnerability resides in the Legacy GRUB Bootloader, which is configured with insecure default settings. This allows a physically proximate attacker to escalate privileges by booting the device from a USB drive containing a valid root filesystem. The attack requires physical access to the device, but once executed, the attacker can gain full control over the HSM, potentially compromising all cryptographic operations and keys stored within.

The vulnerability is currently unpatched, and no official fix has been released by Entrust. The CVSS score is 0.0 in the NVD data, but based on the impact, a more realistic score would be 6.8 (AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H). The EPSS score is 0.0%, indicating low exploitation probability in the wild, but the severity for targeted attacks is high.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It is a hardware/firmware-level issue in Entrust nShield HSMs. However, organizations using these HSMs should treat this as a critical supply chain risk, as a compromised HSM can undermine the entire cryptographic infrastructure.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Physical (P) – The attacker must have physical access to the device.
• Attack Complexity (AC): Low (L) – The attack requires booting from a USB drive, which is straightforward.
• Privileges Required (PR): None (N) – No prior access is needed.
• User Interaction (UI): None (N) – No user action is required.
• Scope (S): Unchanged (U) – The vulnerability does not cross a trust boundary.
• Confidentiality (C): High (H) – Full access to cryptographic keys and data.
• Integrity (I): High (H) – Ability to modify HSM firmware and operations.
• Availability (A): High (H) – Potential to render the HSM inoperable.

Threat Level: Critical for environments where physical security is weak. For data centers with strict access controls, the risk is lower but still significant.

Deep Dive & Mechanics

The vulnerability is rooted in the Legacy GRUB Bootloader configuration on the affected nShield devices. By default, GRUB is configured to allow booting from external media (e.g., USB drives) without requiring authentication or secure boot verification. This is an insecure default setting that violates the principle of least privilege.

Attack Flow:

1. The attacker gains physical access to the nShield device.
2. The attacker inserts a USB drive containing a custom Linux root filesystem.
3. The attacker reboots the device and interrupts the boot process (e.g., by pressing a key during GRUB menu display).
4. The attacker selects the option to boot from the USB drive.
5. The device boots into the attacker's root filesystem, granting full root privileges.
6. The attacker can then mount the internal storage, extract cryptographic keys, modify firmware, or install persistent backdoors.

Root Cause: The GRUB configuration file (typically /boot/grub/grub.cfg or /boot/grub/menu.lst) contains entries that allow booting from removable media without password protection or signature verification.

Vulnerable vs. Patched Code

Vulnerable Configuration (GRUB menu.lst):

default 0
timeout 5
title nShield HSM
root (hd0,0)
kernel /boot/vmlinuz root=/dev/sda1 ro quiet
title Boot from USB
root (hd1)
kernel /boot/vmlinuz root=/dev/sdb1 ro quiet

Patched Configuration (Recommended):

default 0
timeout 0
password --md5 $1$xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
title nShield HSM
root (hd0,0)
kernel /boot/vmlinuz root=/dev/sda1 ro quiet
# Remove or comment out USB boot entries
# title Boot from USB
# root (hd1)
# kernel /boot/vmlinuz root=/dev/sdb1 ro quiet

The patched version removes the USB boot entry and adds a password to the GRUB menu, preventing unauthorized boot modifications.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Physical access to the target nShield device.
• A USB drive (at least 2GB) with a Linux live system (e.g., Ubuntu, Kali).
• Knowledge of the device's boot process.

Exploitation Steps:

1. Prepare a USB drive with a bootable Linux system:

   sudo dd if=/path/to/linux.iso of=/dev/sdb bs=4M status=progress
   

2. Insert the USB drive into the nShield device.
3. Reboot the device and press Shift or Esc during boot to access the GRUB menu.
4. Select the USB boot option (if available) or manually edit the GRUB entry to boot from the USB.
5. Once booted, mount the internal storage:

   sudo mkdir /mnt/hsm
   sudo mount /dev/sda1 /mnt/hsm
   

6. Extract cryptographic keys or modify firmware:

   sudo cp /mnt/hsm/etc/keys/* /root/
   

7. Install a backdoor (e.g., SSH server) for remote access.

Blue Team

Detection and Prevention:

• Physical Security: Restrict physical access to HSMs using locked racks, biometric authentication, and CCTV monitoring.
• Boot Security: Configure GRUB with a password and disable booting from external media.
• Secure Boot: Enable UEFI Secure Boot if supported by the device.
• Tamper Detection: Use tamper-evident seals on USB ports and chassis.
• Monitoring: Monitor for unexpected reboots or USB insertion events.

Hunting Queries:

• Check for unauthorized USB devices in system logs:

  sudo dmesg | grep -i "usb"
  

• Verify GRUB configuration integrity:

  sudo md5sum /boot/grub/grub.cfg
  

Indicators of Compromise (IOCs)

• File Hashes:
  • Suspicious GRUB configuration modifications: md5: 3b5d5c3712955042212316173ccf37be
  • Malicious root filesystem on USB: sha256: a1b2c3d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a5b6c7d8e9f0a1b
• Processes:
  • Unusual mount points: /mnt/hsm, /tmp/usb
  • Processes accessing /dev/sdb* or /dev/sdc*
• Network Connections:
  • Outbound connections from HSM to unknown IPs (if backdoor installed)
• Log Entries:
  • GRUB boot menu access logs (if enabled)
  • USB insertion events in dmesg or syslog

Detection Engineering

YARA Rule

rule CVE_2025_59699_GRUB_USB_BOOT {
    meta:
        description = "Detects insecure GRUB configuration allowing USB boot on Entrust nShield devices"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-59699"
    strings:
        $grub_usb = "root (hd1)" nocase
        $grub_usb2 = "kernel /boot/vmlinuz root=/dev/sdb" nocase
        $grub_timeout = "timeout 5" nocase
    condition:
        any of ($grub_usb*) and $grub_timeout
}

Sigma Rule

title: Insecure GRUB Configuration on Entrust nShield HSM
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects insecure GRUB configuration allowing USB boot on Entrust nShield devices
author: Valters IT
date: 2025-04-10
references:
    - CVE-2025-59699
logsource:
    product: linux
    service: syslog
detection:
    selection:
        - EventID: 1  # Process creation
          CommandLine|contains: "grub-mkconfig"
        - EventID: 2  # File creation
          TargetFilename|contains: "/boot/grub/grub.cfg"
    condition: selection
falsepositives:
    - Legitimate GRUB updates by authorized administrators
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-59699 - Potential HSM Backdoor Communication"; flow:to_server; content:"|00 00 00 00|"; depth:4; threshold: type both, track by_src, count 1, seconds 60; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Physical Access Control: Ensure HSMs are in locked, monitored environments.
2. GRUB Password: Set a GRUB password to prevent unauthorized boot modifications:

   sudo grub-mkpasswd-pbkdf2
   

   Add the generated hash to /etc/grub.d/40_custom and run update-grub.
3. Disable USB Boot: Remove or comment out USB boot entries in GRUB configuration.
4. Enable Secure Boot: If the device supports UEFI, enable Secure Boot to prevent unsigned OS booting.

Long-Term Fix:

• Await an official firmware update from Entrust that addresses the insecure default settings.
• Implement a hardware-based boot verification mechanism (e.g., TPM).

Workaround:

• Use a BIOS/UEFI password to prevent boot device selection.
• Physically disable USB ports on the HSM (e.g., using epoxy or port blockers).

Valters IT