CVE-2025-4275: UEFI Secure Boot Bypass Vulnerability

CVE-2025-4275 Overview

CVE-2025-4275 is a Secure Boot bypass vulnerability in the Insyde UEFI firmware digital signature verification process. The flaw originates from improper validation of NVRAM variable attributes during signature checks. An attacker can create a non-authenticated NVRAM variable that the firmware still trusts for signature verification decisions. This allows execution of arbitrary signed UEFI code outside the Secure Boot chain of trust. The issue is tracked in Insyde Security Advisory SA-2025002 and CERT Vulnerability ID 211341.

Critical Impact

Successful exploitation bypasses Secure Boot, enabling pre-OS code execution that can persist below the operating system and evade endpoint security controls.

Affected Products

• Insyde H2O (InsydeH2O) UEFI firmware implementations
• OEM platforms and motherboards shipping Insyde-based firmware
• Refer to Insyde Security Advisory SA-2025002 for the affected firmware kernel versions

Discovery Timeline

• 2025-06-11 - CVE-2025-4275 published to the National Vulnerability Database (NVD)
• 2026-04-15 - Last updated in NVD database

Technical Details for CVE-2025-4275

Vulnerability Analysis

The vulnerability resides in how Insyde firmware handles NVRAM variables consumed by the Secure Boot signature verification routine. Secure Boot relies on authenticated variables, such as db, dbx, KEK, and PK, that must carry the EFI_VARIABLE_AUTHENTICATED_WRITE_ACCESS or EFI_VARIABLE_TIME_BASED_AUTHENTICATED_WRITE_ACCESS attribute. The verification logic reads variable contents but fails to confirm that the variable was created with the expected authentication attributes.

An attacker with local privileges and the ability to write NVRAM variables can create a parallel, non-authenticated variable that the signature verification path subsequently consults. Because the firmware treats the attacker-controlled data as trusted policy input, the verification returns success for code that would otherwise be rejected. The result is execution of arbitrary signed, or improperly validated, UEFI modules during boot, undermining the Secure Boot trust anchor.

Root Cause

The root cause is missing attribute validation on NVRAM variables consumed by the signature verification flow. The firmware does not enforce that policy-bearing variables carry authenticated write attributes before being trusted. This is a configuration and design flaw that aligns with Secure Boot Bypass and Improper Certificate Validation weakness classes.

Attack Vector

Exploitation requires local access and low privileges on the target system, with high attack complexity. An attacker writes a crafted NVRAM variable using standard UEFI runtime services, such as SetVariable(), without the authenticated attribute. On the next boot, the Secure Boot verification path reads the variable, accepts its contents as policy, and validates an attacker-supplied UEFI image. The scope change is significant because pre-OS code can subvert operating system protections, install bootkits, and disable kernel-mode security mechanisms.

No verified public proof-of-concept is available. The vulnerability is described in prose in the referenced Insyde advisory and the CERT/CC note.

Detection Methods for CVE-2025-4275

Indicators of Compromise

• Unexpected NVRAM variables present in the EFI variable store that lack the authenticated attribute but share names or GUIDs with Secure Boot policy variables.
• Boot-time loading of UEFI drivers or applications not previously inventoried by the platform owner.
• Firmware measurements in TPM PCRs 0, 2, 4, and 7 that diverge from a known-good baseline.

Detection Strategies

• Compare firmware versions across the fleet against the fixed builds listed in the Insyde advisory and flag systems running pre-patch firmware.
• Enumerate EFI variables from the operating system using efivar on Linux or Get-SecureBootUEFI on Windows and audit for anomalous entries.
• Validate Secure Boot state and TPM measured boot logs during posture checks and on every reboot.

Monitoring Recommendations

• Ingest UEFI and boot integrity telemetry into a centralized SIEM or data lake for longitudinal analysis.
• Alert on writes to Secure Boot policy variables and on changes to platform firmware versions outside approved maintenance windows.
• Track local administrative actions that touch firmware interfaces, including SetFirmwareEnvironmentVariable calls on Windows and efivarfs writes on Linux.

How to Mitigate CVE-2025-4275

Immediate Actions Required

• Apply the firmware update provided by your OEM that incorporates the Insyde fix referenced in SA-2025002.
• Restrict local administrative access on systems running affected Insyde firmware until patches are deployed.
• Enroll devices in measured boot and remote attestation programs to identify tampered boot chains.

Patch Information

Insyde has released updated firmware kernel versions addressing the variable attribute validation flaw. OEMs must integrate the fix into platform-specific firmware images and distribute them through their standard update channels. Consult the Insyde Security Advisory SA-2025002 and CERT/CC VU#211341 for the authoritative list of fixed versions and OEM coordination status.

Workarounds

• Enforce full disk encryption tied to TPM PCR measurements so unauthorized firmware changes invalidate disk unlock policy.
• Require BIOS administrator passwords and disable unattended firmware update mechanisms on affected platforms.
• Limit local user accounts with privileges sufficient to write NVRAM variables until patched firmware is installed.

# Verify Secure Boot status and firmware version before and after patching
# Linux
mokutil --sb-state
dmidecode -s bios-version
efivar -l | grep -Ei 'PK|KEK|db|dbx'

# Windows (PowerShell, elevated)
Confirm-SecureBootUEFI
Get-WmiObject -Class Win32_BIOS | Select-Object SMBIOSBIOSVersion, Manufacturer
Get-SecureBootUEFI -Name PK