In this post, we’ll dive deep into CVE-2022-26873, a critical vulnerability found in the AMI Aptio 5.x firmware, specifically in the PlatformInitAdvancedPreMem module. We’ll go step by step, explaining what the bug is, why it’s dangerous, and how an attacker can exploit it to break through some of the most important hardware-based security boundaries in modern computers, including those used by cloud providers for confidential computing.

Please note: For learning purposes only. Do not misuse this information.

What is CVE-2022-26873?

This vulnerability is related to the UEFI (Unified Extensible Firmware Interface), which is the first code to run on your PC or server at power-on. Anything that goes wrong here can undermine all software-based security that runs after it, including the operating system, virtualization, or anything in memory.

Problem

The PlatformInitAdvancedPreMem module does not properly protect memory allocations and pointer usages in the PEI phase (Pre-EFI Initialization) of the boot process. Poor validation at this early stage allows arbitrary code injection and execution before operating system security takes control.

Attack Scenario

An attacker with write access to the system’s firmware storage (e.g. BIOS chip or SPI flash, or sometimes with elevated OS privileges) replaces or tampers with the PlatformInitAdvancedPreMem module, or abuses the vulnerability to run arbitrary code during the PEI phase. This allows them to:

Break down VM memory isolation, undermining confidential compute

In cloud environments, this means memory contents from one VM can be stolen by a compromised hypervisor or firmware-level attacker.

How Does the Exploit Work?

The PlatformInitAdvancedPreMem module fails to properly check memory regions and pointers. Let’s highlight a generic example of what would go wrong:

// Simplified code showing weak pointer handling
VOID SomeFunction(IN VOID* Buffer)
{
    // Attacker provides malicious Buffer pointer.
    // No check here to ensure Buffer points to valid memory or is within allowed range
    CopyMem((VOID*)(TARGET_LOCATION), Buffer, BUFFER_SIZE); // Dangerous unchecked copy
}

What attacker does:
They craft a special firmware variable or boot input so that their malicious pointer gets into the Buffer parameter. That lets them overwrite contents at TARGET_LOCATION in early boot – for example, SMRAM or PEI structures.

In a real-world exploit

- They’d produce a crafted firmware image (or inject at runtime) to install a backdoor, leak secrets, or persist malware in SMRAM.
- On reboot, the malware is executed before the OS loads – undetectable by most antivirus or endpoint agents.

1. Gain firmware write access

Attacker gets physical or privileged local access (flashing SPI, running as admin/root, etc).

2. Modify or patch PlatformInitAdvancedPreMem

For example, replace the module in the SPI flash with a trojanized one or overwrite critical pointers.

3. Inject malicious payload

Typically, the payload will be a DMAr table, modified UEFI variable, or a directly embedded shellcode.

Here’s a sketch of how a backdoor gets delivered

// Malicious Buffer contains shellcode
void InjectToSMRAM(void* MaliciousBuffer)
{
    // Assume SMRAM_BASE is not validated
    memcpy((void*)SMRAM_BASE, MaliciousBuffer, PAYLOAD_SIZE);
    // Next, SMRAM will execute payload in SMM context
}

Why is this so serious?

Mitigation Bypass: Secure Boot, VBS, HVCI, VM tenants’ isolation, etc., can be bypassed.

- Physical Memory Disclosure: Secrets from the OS/Hypervisor/VMs – including keys, passwords, cryptographic material – can leak.
- Confidential Computing Breakdown: All cloud confidentiality boundaries are moot. An attacker can see any “protected” VM’s memory.

Detection

- Check UEFI firmware for tampering. Use tools like CHIPSEC or vendor-specific firmware integrity checkers.

References & Further Reading

- National Vulnerability Database Entry for CVE-2022-26873
- AMI Aptio V UEFI BIOS
- Black Hat 2021: How Attackers Abuse UEFI Vulnerabilities
- CHIPSEC Framework
- Intel SMM Security Whitepaper

Conclusion

CVE-2022-26873 is a stark example of how a small bug in early platform firmware can have catastrophic consequences. Attackers able to exploit this in server or cloud environments can bypass almost every modern security technique, leak VM secrets, and build incredibly stealthy malware. Always keep platform firmware updated and audited, and treat your UEFI as a high-value target.

Stay safe out there!  
If you want a deep dive or proof-of-concept discussion, get in touch with your firmware security team or trusted researchers.

Timeline

Published on: 09/20/2022 18:15:00 UTC
Last modified on: 09/26/2022 13:49:00 UTC