CVE-2026-0390: Windows Boot Loader Auth Bypass Flaw

CVE-2026-0390 Overview

CVE-2026-0390 is a security feature bypass vulnerability in the Windows Boot Loader that stems from reliance on untrusted inputs in a security decision. This flaw allows an authorized attacker with high privileges to bypass security features when exploiting the vulnerability locally. The vulnerability is classified under CWE-807 (Reliance on Untrusted Inputs in a Security Decision), indicating that the boot loader fails to properly validate input data before using it to make critical security decisions during the boot process.

Critical Impact

An attacker with administrative privileges can exploit this vulnerability to bypass Secure Boot protections, potentially allowing unauthorized code execution during the boot process and compromising system integrity before the operating system fully loads.

Affected Products

• Windows Boot Loader (specific versions not disclosed in advisory)

Discovery Timeline

• April 14, 2026 - CVE-2026-0390 published to NVD
• April 14, 2026 - Last updated in NVD database

Technical Details for CVE-2026-0390

Vulnerability Analysis

This vulnerability exists within the Windows Boot Loader component, which is responsible for initiating the Windows operating system startup sequence. The core issue involves improper validation of inputs that are subsequently used to make security-critical decisions during the boot process.

The Windows Boot Loader is a critical component in the chain of trust established by Secure Boot. When the boot loader relies on untrusted inputs without proper validation, an attacker can manipulate these inputs to influence security decisions, effectively bypassing protections designed to ensure only authorized and signed code executes during boot.

The attack requires local access and high privileges, suggesting the attacker must already have administrative control of the system. However, successful exploitation could allow the attacker to subvert Secure Boot protections, potentially enabling persistent rootkits or bootkits that survive system reboots and evade traditional security software.

Root Cause

The root cause is CWE-807: Reliance on Untrusted Inputs in a Security Decision. The Windows Boot Loader makes security-critical decisions based on input data that can be influenced or controlled by an attacker. Instead of using trusted sources or properly validating inputs, the boot loader accepts potentially malicious data and uses it to determine whether to allow certain operations or code execution during the boot process.

Attack Vector

The attack vector for CVE-2026-0390 is local, requiring the attacker to have existing access to the target system with elevated privileges. The exploitation scenario typically involves:

1. An attacker with administrative access to a Windows system identifies the vulnerable boot loader component
2. The attacker crafts malicious input data designed to influence the boot loader's security decisions
3. Upon system reboot, the modified input causes the boot loader to bypass normal security checks
4. The attacker can then load unauthorized code during the boot process, potentially establishing persistence below the operating system level

The vulnerability is particularly concerning for enterprise environments where Secure Boot is relied upon to maintain system integrity and prevent boot-level attacks.

Detection Methods for CVE-2026-0390

Indicators of Compromise

• Unexpected modifications to boot configuration data (BCD) stores
• Anomalous entries in the EFI System Partition related to boot loader files
• Secure Boot violations or disabled Secure Boot without administrator action
• Unexpected boot loader file hashes that don't match known-good Microsoft signatures

Detection Strategies

• Monitor for changes to boot loader configuration files and BCD stores using file integrity monitoring
• Implement Measured Boot with TPM attestation to detect unauthorized boot sequence modifications
• Review Windows Event Logs for boot-related security events and Secure Boot validation failures
• Deploy endpoint detection solutions capable of monitoring pre-boot and early boot activities

Monitoring Recommendations

• Enable Windows Event Log auditing for boot configuration changes (Event IDs related to BCD modifications)
• Implement continuous monitoring of EFI System Partition integrity
• Configure alerts for Secure Boot policy violations or status changes
• Utilize SentinelOne's boot-level protection capabilities to detect and alert on suspicious boot activities

How to Mitigate CVE-2026-0390

Immediate Actions Required

• Apply the latest Windows security updates from Microsoft addressing CVE-2026-0390
• Verify Secure Boot is enabled and properly configured on all affected systems
• Audit administrative access to ensure only authorized personnel have elevated privileges
• Review boot configuration data for any unauthorized modifications

Patch Information

Microsoft has released security updates to address this vulnerability. Organizations should consult the Microsoft CVE-2026-0390 Advisory for specific patch information and update guidance. Apply the appropriate security update for your Windows version through Windows Update or Windows Server Update Services (WSUS).

Workarounds

• Restrict local administrative access to trusted personnel only
• Enable BitLocker with TPM+PIN to add additional boot-time authentication
• Implement Credential Guard and Device Guard to harden the boot process
• Consider deploying hardware-based security solutions with boot integrity monitoring

# Verify Secure Boot status
Confirm-SecureBootUEFI

# Check boot configuration integrity
bcdedit /enum all

# Review EFI partition for unexpected files
mountvol S: /S
dir S:\EFI\Microsoft\Boot\