CVE-2021-26381: Trusted OS Privilege Escalation Flaw

CVE-2021-26381 Overview

CVE-2021-26381 is a high-severity vulnerability affecting AMD's Trusted OS component. The vulnerability stems from improper system call parameter validation that allows a malicious driver to perform mapping or unmapping operations on a large number of pages, potentially resulting in kernel memory corruption.

This vulnerability exists within the secure execution environment of AMD processors, specifically in the Trusted OS layer that manages secure operations. A privileged attacker with the ability to load malicious drivers could exploit this flaw to corrupt kernel memory, potentially leading to system instability, privilege escalation, or further security compromises.

Critical Impact

A malicious driver can exploit improper parameter validation in AMD Trusted OS system calls to corrupt kernel memory, potentially enabling privilege escalation or system compromise.

Affected Products

• AMD Trusted OS (specific versions detailed in AMD Security Bulletins)
• AMD processors utilizing Trusted Execution Environment
• Systems running affected AMD Platform Security Processor (PSP) firmware

Discovery Timeline

• 2026-02-10 - CVE CVE-2021-26381 published to NVD
• 2026-02-10 - Last updated in NVD database

Technical Details for CVE-2021-26381

Vulnerability Analysis

The vulnerability resides in the parameter validation logic of system calls within AMD's Trusted OS. When a driver makes system calls to perform memory mapping or unmapping operations, the Trusted OS fails to properly validate the parameters specifying the number of pages to be operated upon.

This improper validation (classified as CWE-770: Allocation of Resources Without Limits or Throttling) allows a malicious driver to request operations on an excessively large number of memory pages. Without proper bounds checking, the Trusted OS processes these requests, leading to potential kernel memory corruption.

The attack requires local access and high privileges to execute, as the attacker must be able to load a malicious driver onto the target system. However, once a malicious driver is loaded, it can leverage this vulnerability to perform unauthorized memory operations that could compromise system integrity.

Root Cause

The root cause is improper input validation in the Trusted OS system call interface, specifically the failure to enforce appropriate limits on the number of pages that can be mapped or unmapped in a single operation. This allows resource allocation without proper throttling or boundary checks, enabling memory corruption through excessive page operations.

Attack Vector

The attack requires local access to the system with high privileges. An attacker would need to:

1. Gain sufficient privileges to load a kernel-mode driver on the target system
2. Develop or deploy a malicious driver that makes crafted system calls to the Trusted OS
3. Pass malicious parameters specifying an excessively large number of pages for mapping/unmapping operations
4. Trigger the improper memory operations to corrupt kernel memory

The vulnerability exploitation requires a sophisticated attacker with local system access and the ability to execute privileged code. The attack complexity is considered high due to the need for precise crafting of malicious parameters.

For detailed technical information about this vulnerability, refer to the AMD Security Bulletin SB-4013 and AMD Security Bulletin SB-6024.

Detection Methods for CVE-2021-26381

Indicators of Compromise

• Unexpected kernel crashes or blue screens related to memory corruption
• Unusual driver loading activity, especially from untrusted sources
• Anomalous system call patterns targeting the Trusted OS interface
• Memory integrity violations detected by security monitoring tools

Detection Strategies

• Monitor for unsigned or suspicious driver installations on AMD-based systems
• Implement driver signing enforcement policies to prevent malicious driver loading
• Deploy endpoint detection and response (EDR) solutions capable of detecting kernel-level anomalies
• Use behavioral analysis to identify unusual memory mapping operations

Monitoring Recommendations

• Enable Windows Driver Signature Enforcement and Secure Boot
• Monitor system event logs for driver installation events (Event ID 7045)
• Implement kernel integrity monitoring to detect unauthorized memory modifications
• Deploy SentinelOne agents for comprehensive endpoint visibility and threat detection

How to Mitigate CVE-2021-26381

Immediate Actions Required

• Apply the latest AMD firmware and BIOS updates from your system manufacturer
• Review AMD Security Bulletins SB-4013 and SB-6024 for specific patch information
• Enforce driver signing requirements to prevent unauthorized driver loading
• Limit administrative privileges to reduce the attack surface

Patch Information

AMD has released security updates addressing this vulnerability. System administrators should consult the AMD Security Bulletin SB-4013 and AMD Security Bulletin SB-6024 for detailed patch information and affected product lists.

Firmware updates are typically distributed through OEM vendors (Dell, HP, Lenovo, etc.) as BIOS updates. Contact your system manufacturer for the latest security updates applicable to your specific hardware.

Workarounds

• Enforce strict driver signing policies using Windows Device Guard or similar technologies
• Implement application whitelisting to control which drivers can be loaded
• Restrict administrative access to limit who can install kernel-mode drivers
• Enable Secure Boot and verify its configuration is active

# Windows: Verify driver signing enforcement status
bcdedit /enum | findstr "testsigning"

# Windows: Enable driver signature enforcement (if disabled)
bcdedit /set testsigning off

# Linux: Check secure boot status
mokutil --sb-state