CVE-2021-26392 is a critical vulnerability found in certain AMD platforms, specifically involving how the system loads Trusted Applications (TA) through a function called LoadModule. Due to missing size verification when loading a TA, an attacker with sufficient privileges can exploit this flaw to write out-of-bounds in memory, leading to arbitrary code execution at the operating system or kernel level. This post breaks down the problem in simple language, walks through the vulnerability with a code snippet, discusses exploitation details, and links to official advisories.

What Systems Are Affected?

This bug mainly impacts certain AMD processors supporting ARM-based Secure Execution environments (like AMD Secure Encrypted Virtualization and AMD Secure Processor, sometimes called Platform Security Processor or PSP). The vulnerable LoadModule function exists in AMD’s firmware for these chips.

What is LoadModule?

LoadModule is a secure firmware function that loads a Trusted Application into the Secure Execution Environment (SEE). TAs are like tiny apps that run with high trust, often handling keys, firmware encryption, or remote attestation.

The Vulnerability

The LoadModule handler reads information about the incoming Trusted Application, such as its size and where to place it in memory. Unfortunately, it does not properly check if the given size parameter is legitimate. If an attacker crafts a specially designed TA whose header claims an extremely large size, the internal memory buffer logic can overflow—causing an out-of-bounds write.

This means that arbitrary data from the TA can overwrite critical kernel or OS memory.

Here’s a simplified (and non-verbatim) code snippet representing the logic

// Simplified vulnerable pseudocode
int LoadModule(void *input, size_t size) {
    ModuleHeader *hdr = (ModuleHeader *)input;
    // ... skips vital validation ...
    void *module_mem = malloc(hdr->module_size); // module_size can be attacker-controlled
    if (!module_mem) return -1;

    memcpy(module_mem, input, hdr->module_size); // OOB if module_size > actual input size!
    // ... further processing ...
    return ;
}

Problem:
hdr->module_size comes from the untrusted TA file. If this value is too big (or negative, cast to a huge unsigned integer), malloc and memcpy will behave unpredictably. It can lead to a memory overwrite *past* the allocated buffer (a classic heap overflow), corrupting memory.

The attacker requires some form of privilege to load a TA, which usually restricts exploitation to

- A user/process with administrative rights (root/Administrator),

An existing malicious component in the VM host or a compromised OS.

However, in environments running semi-trusted code, or where remote attestation services allow user input, this requirement might be achievable.

Exploit scenario (Simplified)

1. Craft a malicious TA binary, setting the module_size field in the header to a value much larger than the actual TA size.
2. Use any interface designed to load TAs (like kernel drivers, firmware update tools, etc.) to load the crafted TA.

On execution, the firmware copies data out-of-bounds, overwriting adjacent critical memory.

4. The attacker’s payload hijacks execution flow, eg. by overwriting a return pointer or function table.
5. Payload runs with high privilege in secure/kernel context.

Detection, Mitigation, and Fix

- Update firmware! AMD addressed this bug in SPI firmware updates. Always check for BIOS/firmware updates from your motherboard or system OEM.
- Monitor for unexpected TA loads: Watch system logs and platform management traffic for suspicious attempts to load TAs.

AMD Official Advisory:

AMD-SB-1037: Insufficient verification of missing size check in LoadModule (CVE-2021-26392)

CVE Report:

NIST NVD: CVE-2021-26392

Additional coverage (external analysis):

Tenable's disclosure writeup
AMD’s whitepapers on Secure Encrypted Virtualization

Conclusion

CVE-2021-26392 is a classic firmware bug: insufficient input validation in a security-critical function enables dangerous memory corruption. If you manage AMD-based servers/workstations or deal with firmware updates, you should ensure your systems are running the latest SPI firmware. Don’t overlook these silent, powerful vulnerabilities—attackers only need a small mistake in trusted code to break the rest wide open. Stay updated, validate your inputs, and always keep privilege boundaries as tight as possible.


*This post used exclusive research and simplified explanations. For any questions or incident reporting, consult the official AMD security bulletin linked above.*

Timeline

Published on: 11/09/2022 21:15:00 UTC
Last modified on: 11/23/2022 14:00:00 UTC