CVE-2025-4276: UsbCoreDxe SMM RCE Vulnerability

CVE-2025-4276 Overview

CVE-2025-4276 affects the UsbCoreDxe UEFI driver distributed as part of Insyde H2O firmware. The flaw allows an attacker with local, high-privileged access to write arbitrary data into System Management RAM (SMRAM) and execute arbitrary code at System Management Mode (SMM) level. SMM operates below the operating system and hypervisor, so successful exploitation grants the highest execution privilege available on the platform. The root weakness is improper input validation [CWE-20] within the USB core DXE component. Insyde published advisory SA-2025005 covering this issue.

Critical Impact
Arbitrary code execution at SMM level breaks the firmware trust boundary and can be used to install persistent implants, bypass Secure Boot, and defeat OS-level protections.

Affected Products

Insyde H2O UEFI firmware containing the UsbCoreDxe driver
OEM platform firmware derived from affected Insyde reference code
Refer to Insyde Security Advisory SA-2025005 for specific fixed kernel versions

Discovery Timeline

2025-08-13 - CVE-2025-4276 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-4276

Vulnerability Analysis

The UsbCoreDxe driver participates in USB stack initialization during the DXE phase and registers Software SMI (System Management Interrupt) handlers that continue to run after boot. These handlers accept input from the operating system through the SMM communication buffer. The vulnerability arises because the driver does not fully validate pointers and length fields supplied by the caller before using them as write targets inside SMRAM.

Weakness [CWE-20] applies: input crossing the OS-to-SMM trust boundary is treated as trusted. An attacker able to invoke the SMI handler can direct writes to arbitrary SMRAM addresses, overwriting SMM code or function pointers used by other handlers. Once a controlled pointer is dereferenced during a later SMI, the attacker gains execution in ring -2.

Root Cause

The root cause is missing or insufficient validation of communication buffer parameters passed to the UsbCoreDxe SMI handler. Address and size fields are not checked against SMRAM boundaries, and embedded pointers are not verified to reside outside SMRAM before being written through. This allows the caller to redirect writes into protected memory.

Attack Vector

Exploitation requires local execution with high privileges, typically kernel-level access on the running operating system. The attacker triggers the vulnerable SMI handler through the platform's SMI command port and supplies a crafted communication buffer. No user interaction is required. Because SMM crosses the OS security boundary, the scope is changed and impact extends beyond the OS to firmware persistence and Secure Boot bypass. Detailed exploitation flow is described in the Insyde Security Advisory SA-2025005; no public proof-of-concept has been released.

Detection Methods for CVE-2025-4276

Indicators of Compromise

Unexpected modifications to SPI flash regions or UEFI variables outside of vendor update windows
Kernel drivers or userland tools issuing writes to the SMI command port (0xB2 on most x86 platforms) without a legitimate management context
Firmware measurements in the TPM PCRs diverging from the known-good baseline after reboot

Detection Strategies

Compare firmware images against vendor-signed baselines using tools such as CHIPSEC or the platform vendor's firmware attestation utility
Monitor endpoint telemetry for processes loading kernel drivers that map physical memory or interact with \Device\PhysicalMemory
Correlate boot integrity events with EDR process telemetry to identify pre-OS tampering followed by suspicious post-boot activity

Monitoring Recommendations

Enable Windows Defender System Guard and measured boot logging, and forward the results to your SIEM for baseline deviation analysis
Alert on drivers or utilities that write to model-specific registers, IOMMU tables, or the ACPI communication buffer used by SMM
Track firmware version inventory across the fleet and flag hosts that remain on pre-patch Insyde kernel builds

How to Mitigate CVE-2025-4276

Immediate Actions Required

Identify all systems running Insyde H2O firmware and cross-reference against the fixed versions listed in advisory SA-2025005
Restrict local administrator and kernel-driver loading privileges to reduce the population of accounts able to reach the SMI interface
Enforce Secure Boot, HVCI, and vendor firmware update policies to raise the cost of chaining this flaw with other primitives

Patch Information

Insyde has issued fixed firmware through advisory SA-2025005. OEMs consuming Insyde H2O must integrate the corrected UsbCoreDxe module and ship platform BIOS updates to end customers. Apply the OEM-branded BIOS update for each affected model; generic Insyde patches cannot be flashed directly on most consumer or enterprise hardware.

Workarounds

No supported software workaround eliminates the vulnerability; only the firmware update from the OEM resolves the flaw
Reduce exposure by preventing untrusted code from running at kernel privilege, since exploitation requires local high-privileged access
Enable BIOS administrator passwords and disable unattended firmware modification interfaces to limit tampering after remediation

bash

# Verify installed firmware version on Linux and compare against the OEM's fixed release
sudo dmidecode -s bios-version
sudo dmidecode -s bios-release-date

# On Windows, query firmware information for fleet inventory
wmic bios get smbiosbiosversion,releasedate,manufacturer