CVE-2021-39301: HP Z1 G5 Privilege Escalation Flaw

CVE-2021-39301 Overview

CVE-2021-39301 is a significant vulnerability affecting UEFI firmware (BIOS) across an extensive range of HP PC products. This firmware-level security flaw enables attackers to escalate privileges and execute arbitrary code on affected systems, potentially compromising the entire system at the lowest hardware level.

UEFI firmware vulnerabilities are particularly concerning because they operate below the operating system level, making traditional security software ineffective at detection and remediation. Successful exploitation could allow attackers to establish persistent access that survives operating system reinstallation and can bypass Secure Boot protections.

Critical Impact

This vulnerability allows privilege escalation and arbitrary code execution at the firmware level, enabling persistent system compromise that can survive OS reinstallation and evade traditional security controls.

Affected Products

• HP EliteBook Series (830, 836, 840, 846, 850, 1050, x360 variants - G4 through G8)
• HP ProBook Series (430, 440, 450, 470, 630, 640, 650, x360 variants - G4 through G8)
• HP ZBook Series (14u, 15, 15u, 17, Create, Firefly, Fury, Power, Studio variants - G5 through G8)
• HP Z Workstation Series (Z1, Z4, Z6, Z8 - G4 through G8)
• HP EliteDesk Series (800, 805, 880 - G4 through G8)
• HP ProDesk Series (400, 405, 480, 600, 680 - G4 through G8)
• HP EliteOne and ProOne All-in-One PCs (400, 440, 600, 800, 1000 series)
• HP Elite Dragonfly, Elite x2, and Zhan Series notebooks
• HP Engage Flex Mini and MP9 G4 Retail Systems

Discovery Timeline

• February 16, 2022 - CVE-2021-39301 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-39301

Vulnerability Analysis

This vulnerability exists within the UEFI firmware (BIOS) implementation across HP's commercial PC product lines. The flaw allows a local attacker with low-level privileges to escalate their access and achieve arbitrary code execution within the firmware environment.

The attack requires local access to the system, meaning an attacker must either have physical access or have already compromised the system at the operating system level. However, once exploited, the attacker gains persistent control at the firmware layer, which executes before and independently of the operating system.

Firmware-level compromise is particularly dangerous because:

• Malicious code persists across OS reinstallations
• Traditional antivirus and EDR solutions cannot detect firmware-level threats
• Secure Boot protections may be bypassed
• The attacker can modify system behavior before any security software loads

Root Cause

The vulnerability stems from insufficient input validation or access control mechanisms within HP's UEFI firmware implementation. While specific technical details have not been publicly disclosed to prevent exploitation, the flaw allows improper privilege escalation within the System Management Mode (SMM) or similar privileged firmware execution contexts.

UEFI firmware operates in highly privileged processor modes that have direct access to all system memory and hardware. Weaknesses in these components can allow attackers to escape normal security boundaries and execute code with the highest possible system privileges.

Attack Vector

The attack vector is local, requiring the attacker to have existing access to the target system. The exploitation flow typically involves:

1. An attacker gains initial access to the system through physical access or prior compromise
2. The attacker leverages the UEFI vulnerability to escalate privileges within the firmware
3. Arbitrary code execution at the firmware level is achieved
4. The attacker can install persistent implants that survive OS reinstallation

This vulnerability does not require user interaction and can be exploited by a user with low-level privileges on the system. The scope is changed, meaning successful exploitation impacts resources beyond the vulnerable component itself.

Detection Methods for CVE-2021-39301

Indicators of Compromise

• Unexpected BIOS/UEFI firmware version changes or modifications
• System Management Mode (SMM) anomalies detected by firmware integrity monitoring tools
• Secure Boot validation failures or unexpected secure boot configuration changes
• Unusual pre-boot activity or extended POST times

Detection Strategies

• Deploy firmware integrity monitoring solutions that can detect unauthorized BIOS modifications
• Utilize HP Sure Start or similar hardware-based firmware verification technologies on supported systems
• Monitor for UEFI firmware update attempts outside of authorized change windows
• Implement attestation services to verify firmware integrity at boot time

Monitoring Recommendations

• Enable UEFI secure boot and monitor for configuration changes
• Configure centralized logging for firmware update events via management tools like HP Client Management Solutions
• Establish baseline firmware versions across the fleet and alert on deviations
• Integrate firmware health monitoring with SentinelOne endpoint protection for comprehensive visibility

How to Mitigate CVE-2021-39301

Immediate Actions Required

• Identify all affected HP systems in your environment using the comprehensive product list
• Download and apply the latest BIOS/UEFI firmware updates from HP Support
• Enable HP Sure Start (where available) to provide hardware-enforced firmware recovery capabilities
• Restrict local administrative access to limit the attack surface for privilege escalation

Patch Information

HP has released security updates to address this vulnerability. Administrators should consult the HP Security Advisory to identify the specific firmware version required for each affected product model.

The advisory provides model-specific softpaq downloads and installation instructions. Organizations should prioritize patching based on system criticality and exposure level, giving precedence to systems with sensitive data or elevated network access.

Firmware updates typically require a system reboot and should be scheduled during maintenance windows. Consider using HP Image Assistant or similar tools to streamline firmware deployment across large fleets.

Workarounds

• Enable UEFI Secure Boot and configure BIOS administrator passwords to prevent unauthorized firmware modifications
• Implement physical security controls to restrict access to affected systems
• Use SentinelOne's endpoint protection with firmware monitoring capabilities for detection of anomalous behavior
• Restrict local administrator privileges using principle of least privilege

# Check current BIOS version on Windows PowerShell
Get-WmiObject -Class Win32_BIOS | Select-Object SMBIOSBIOSVersion, ReleaseDate

# For Linux systems
sudo dmidecode -s bios-version
sudo dmidecode -s bios-release-date