CVE-2021-33123: Intel Xeon Bronze Privilege Escalation

CVE-2021-33123 Overview

CVE-2021-33123 is a privilege escalation vulnerability affecting the BIOS authenticated code module (ACM) in numerous Intel processors. The vulnerability stems from improper access control within the BIOS ACM, which could allow an attacker with existing privileged local access to escalate their privileges further on the affected system.

This firmware-level vulnerability is particularly concerning because it affects a foundational security component that executes during the system boot process. The BIOS authenticated code module is responsible for establishing a hardware root of trust, and compromising this component could undermine the entire security chain of the system.

Critical Impact

A privileged local attacker can exploit improper access controls in the BIOS authenticated code module to achieve further privilege escalation, potentially gaining persistent low-level access to the system that survives operating system reinstallation.

Affected Products

• Intel Xeon Scalable Processors (Bronze, Silver, Gold, Platinum series)
• Intel Core i3, i5, i7, i9 Processors (7th through 10th Generation)
• Intel Xeon D, E, and W Series Processors
• Intel Xeon E3, E5, and E7 v4-v6 Series Processors

Discovery Timeline

• 2022-05-12 - CVE CVE-2021-33123 published to NVD
• 2025-05-05 - Last updated in NVD database

Technical Details for CVE-2021-33123

Vulnerability Analysis

This vulnerability resides in the BIOS authenticated code module, a critical firmware component that executes during the early stages of system boot before the operating system loads. The ACM is part of Intel's Trusted Execution Technology (TXT) and plays a crucial role in establishing a measured launch environment.

The improper access control flaw means that security boundaries within the ACM are not properly enforced, allowing an attacker who already possesses local privileged access (such as administrator or root) to bypass intended restrictions and gain elevated capabilities. Because this occurs at the firmware level, successful exploitation could provide an attacker with persistent, deeply embedded access that is difficult to detect and remediate.

The attack requires local access and some level of existing privilege, which limits the attack surface but makes this vulnerability particularly dangerous in scenarios involving insider threats, compromised privileged accounts, or post-exploitation privilege escalation chains.

Root Cause

The root cause of CVE-2021-33123 is improper access control implementation within the BIOS authenticated code module. Specifically, the firmware code fails to properly validate or restrict access to sensitive operations or memory regions that should only be accessible under more restrictive conditions. This allows a privileged user to perform operations beyond their intended scope within the firmware execution context.

Attack Vector

Exploitation of this vulnerability requires local access to the target system with an already-privileged account. The attacker must be able to interact with low-level system components, typically through operating system interfaces that communicate with firmware. The attack flow involves:

1. Gaining initial privileged access to the target system (administrator, root, or equivalent)
2. Leveraging the improper access controls in the BIOS ACM to execute operations with elevated firmware-level privileges
3. Potentially modifying firmware state or gaining persistent access that survives system reboots

Due to the sensitive nature of firmware exploitation and the lack of verified public exploit code, specific technical exploitation details should be obtained from the Intel Security Advisory SA-00601.

Detection Methods for CVE-2021-33123

Indicators of Compromise

• Unexpected changes to system firmware or BIOS settings without authorized administrator action
• Anomalous behavior during system boot sequence or extended boot times
• Unauthorized firmware update activity or tampering with flash memory regions
• System event logs showing unusual privileged operations targeting firmware interfaces

Detection Strategies

• Implement firmware integrity monitoring to detect unauthorized modifications to BIOS/UEFI components
• Enable Intel TXT measured launch logging and monitor for unexpected measurement changes
• Deploy endpoint detection solutions capable of monitoring firmware-level activity
• Audit privileged account usage and correlate with firmware-related system calls

Monitoring Recommendations

• Configure SIEM rules to alert on firmware update operations outside of maintenance windows
• Monitor for attempts to access System Management Mode (SMM) or firmware flash regions
• Implement hardware-based attestation to verify firmware integrity at boot time
• Review system logs for signs of privilege escalation attempts followed by firmware access

How to Mitigate CVE-2021-33123

Immediate Actions Required

• Inventory all systems with affected Intel processors to determine exposure scope
• Apply BIOS/UEFI firmware updates from your system OEM that address this vulnerability
• Restrict physical and remote access to privileged accounts on affected systems
• Enable BIOS write protection features where available

Patch Information

Intel has released firmware updates to address this vulnerability as documented in Intel Security Advisory SA-00601. Organizations should obtain updated BIOS/UEFI firmware from their system or motherboard manufacturer (OEM), as Intel provides microcode and reference implementations that OEMs then integrate into their specific firmware packages.

NetApp customers should also consult the NetApp Security Advisory NTAP-20220818-0003 for product-specific guidance.

Workarounds

• Implement strict privileged access management to limit accounts with local administrator access
• Enable Secure Boot and Intel TXT features to establish additional verification of boot integrity
• Deploy endpoint protection solutions with firmware monitoring capabilities
• Segment critical systems with affected processors from less trusted network segments

# Verify current BIOS version on Linux systems
dmidecode -s bios-version

# Check Intel processor information to identify affected systems
cat /proc/cpuinfo | grep -E "model name|microcode"

# On Windows, check BIOS version via PowerShell
Get-WmiObject -Class Win32_BIOS | Select-Object SMBIOSBIOSVersion, ReleaseDate