CVE-2021-39298: HP Z1 Entry Tower G5 Privilege Escalation

CVE-2021-39298 Overview

A potential vulnerability exists in the AMD System Management Mode (SMM) interrupt handler that may allow an attacker with high privileges to access the SMM, resulting in arbitrary code execution. This vulnerability could be exploited by malicious actors to bypass security mechanisms provided in the UEFI firmware, potentially compromising the integrity of the entire system at a level below the operating system.

Critical Impact

Successful exploitation enables attackers with elevated privileges to execute arbitrary code within SMM, bypassing UEFI security mechanisms and potentially establishing persistent, undetectable malware at the firmware level.

Affected Products

• HP EliteBook Series (830, 840, 850, 1050 across G5-G8 generations)
• HP ProBook Series (430, 440, 450, 470, 630, 640, 650 across G4-G8 generations)
• HP ZBook Series (14u, 15, 15u, 17, Create, Firefly, Fury, Power, Studio across G5-G8 generations)
• HP EliteDesk Series (800, 805, 880 across G4-G8 generations)
• HP ProDesk Series (400, 405, 480, 600, 680 across G4-G8 generations)
• HP Z Workstation Series (Z1, Z4, Z6, Z8 G4-G8)
• HP Elite Dragonfly, Elite x2, EliteOne, and ProOne All-in-One systems

Discovery Timeline

• February 16, 2022 - CVE-2021-39298 published to NVD
• February 24, 2026 - Last updated in NVD database

Technical Details for CVE-2021-39298

Vulnerability Analysis

This vulnerability resides in the AMD System Management Mode (SMM) interrupt handler implementation within affected HP systems utilizing AMD processors. SMM is a highly privileged execution mode in x86 processors that operates transparently to the operating system and all software running above it. When triggered via a System Management Interrupt (SMI), the processor enters SMM and executes code from a protected memory region called SMRAM.

The vulnerability allows an attacker who has already obtained high privileges on the system to improperly access SMM execution context. Once access is achieved, the attacker can execute arbitrary code at the SMM privilege level, which operates at Ring -2, below even the hypervisor level (Ring -1). This represents one of the most privileged execution contexts available on modern x86 systems.

Root Cause

The root cause stems from improper handling within the SMM interrupt handler that fails to adequately validate or restrict access to SMM resources. The insufficient access controls in the SMI handler code allow a privileged attacker to leverage the interrupt mechanism to gain unauthorized execution within the SMM context. This type of vulnerability typically arises from inadequate input validation, improper memory access controls, or flaws in the SMM entry/exit logic within the UEFI firmware implementation.

Attack Vector

The attack requires local access with high privileges on the target system. An attacker must first compromise the system at an administrative or kernel level before attempting to exploit this vulnerability. Once sufficient privileges are obtained, the attacker can craft malicious inputs or trigger specific conditions that cause the SMM interrupt handler to execute attacker-controlled code.

The exploitation flow typically involves:

1. Gaining administrative or kernel-level access to the target HP system
2. Identifying the vulnerable SMM interrupt handler entry point
3. Crafting malicious data or triggering specific SMI conditions
4. Achieving code execution within the SMM context
5. Leveraging SMM access to bypass Secure Boot, modify firmware, or install persistent implants

This attack bypasses traditional security boundaries including operating system protections, hypervisor isolation, and UEFI Secure Boot mechanisms. Code executing in SMM has unrestricted access to all system memory, can modify firmware stored in SPI flash, and remains invisible to security software running in the operating system.

Detection Methods for CVE-2021-39298

Indicators of Compromise

• Unexpected modifications to UEFI firmware images or SPI flash contents
• Anomalous SMI activity patterns detected through hardware-based monitoring
• Unauthorized changes to Secure Boot configuration or trust databases
• Persistence of malicious code across operating system reinstallation

Detection Strategies

• Deploy firmware integrity monitoring solutions that can detect unauthorized changes to UEFI components
• Implement hardware-based security monitoring using platform security features like Intel Boot Guard or AMD Platform Secure Boot where available
• Utilize endpoint detection and response (EDR) solutions with firmware-level visibility capabilities
• Conduct regular firmware hash verification against known-good baselines

Monitoring Recommendations

• Enable and monitor UEFI Secure Boot logs for unauthorized signing certificate additions or policy modifications
• Implement continuous firmware integrity monitoring using Trusted Platform Module (TPM) measured boot attestation
• Monitor for unusual privilege escalation attempts that could precede SMM exploitation
• Track BIOS/UEFI update events and validate authenticity of all firmware updates

How to Mitigate CVE-2021-39298

Immediate Actions Required

• Identify all affected HP systems in your environment using the comprehensive affected products list
• Apply the latest BIOS/UEFI firmware updates from HP for all affected systems immediately
• Ensure Secure Boot is enabled and properly configured on all affected devices
• Restrict administrative access to critical systems and implement least-privilege principles

Patch Information

HP has released updated BIOS/UEFI firmware versions to address this vulnerability. Organizations should consult the AMD Security Bulletin #1027 and AMD Security Bulletin #1032 for detailed technical information and mitigation guidance. HP provides model-specific firmware updates through their support portal, and administrators should verify they are installing the correct firmware version for their specific hardware model.

Firmware updates should be applied during planned maintenance windows, as BIOS updates require system reboots. Organizations should maintain backup configurations and test updates in non-production environments where possible before widespread deployment.

Workarounds

• Implement strict administrative access controls to limit the attack surface for privilege escalation
• Enable and configure hardware-based security features including Secure Boot, TPM, and UEFI password protection
• Deploy application whitelisting and kernel integrity verification to detect unauthorized privilege escalation attempts
• Consider implementing network segmentation to isolate systems pending firmware updates from potential lateral movement

# Verify current BIOS version on HP systems (Windows PowerShell)
Get-WmiObject -Class Win32_BIOS | Select-Object SMBIOSBIOSVersion, ReleaseDate

# Check Secure Boot status
Confirm-SecureBootUEFI

# Monitor for firmware integrity (requires appropriate tooling)
# Consult HP security advisories for model-specific updated firmware versions