CVE-2026-21265
📋 TL;DR
This CVE describes a Windows Secure Boot certificate expiration issue where Microsoft's UEFI certificates are expiring in 2026. Devices with affected certificates may lose Secure Boot functionality if firmware defects prevent proper certificate updates, potentially allowing boot-level attacks. All Windows systems using Secure Boot with the expiring certificates are affected.
💻 Affected Systems
- Windows Secure Boot
- UEFI firmware with Microsoft certificates
📦 What is this software?
Windows 10 1607 by Microsoft
Windows 10 1607 by Microsoft
Windows 10 1809 by Microsoft
Windows 10 1809 by Microsoft
Windows 10 21h2 by Microsoft
Windows 10 21h2 by Microsoft
Windows 10 21h2 by Microsoft
Windows 10 22h2 by Microsoft
Windows 10 22h2 by Microsoft
Windows 10 22h2 by Microsoft
Windows 11 23h2 by Microsoft
Windows 11 23h2 by Microsoft
Windows 11 24h2 by Microsoft
Windows 11 24h2 by Microsoft
Windows 11 25h2 by Microsoft
Windows 11 25h2 by Microsoft
⚠️ Risk & Real-World Impact
Worst Case
Complete Secure Boot bypass allowing attackers to load malicious bootloaders or kernel-level malware, leading to persistent system compromise and data theft.
Likely Case
Secure Boot failures causing boot problems, system instability, or inability to apply security updates, requiring manual intervention to restore functionality.
If Mitigated
Temporary boot issues during certificate updates that are resolved through proper firmware and OS updates with minimal disruption.
🎯 Exploit Status
Exploitation requires physical access or administrative privileges to manipulate boot process. The vulnerability is more about certificate expiration causing failures than active exploitation.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Windows updates with new certificate packages and firmware updates from hardware vendors
Vendor Advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21265
Restart Required: Yes
Instructions:
1. Apply latest Windows updates from Microsoft. 2. Check for and apply UEFI/firmware updates from hardware manufacturer. 3. Verify Secure Boot is functioning after updates. 4. Test boot process thoroughly before production deployment.
🔧 Temporary Workarounds
Disable Secure Boot (NOT RECOMMENDED)
windowsTemporarily disable Secure Boot in UEFI settings to avoid boot failures, but this significantly reduces security
🧯 If You Can't Patch
- Maintain physical security controls to prevent unauthorized access to systems
- Implement strict administrative access controls and monitor for unauthorized boot process changes
🔍 How to Verify
Check if Vulnerable:
Check UEFI firmware settings for Secure Boot status and certificate details, or use PowerShell: Confirm-SecureBootUEFI and check certificate expiration datesCheck Version:
PowerShell: Get-SecureBootPolicy, Get-SecureBootUEFI, or msinfo32.exe for system informationVerify Fix Applied:
Verify Secure Boot is enabled and functioning, check that new certificates are present in UEFI KEK/DB, and confirm system boots normally📡 Detection & Monitoring
Log Indicators:
- Windows Event Logs: Secure Boot violations, boot failures, certificate validation errors
- UEFI/BIOS logs: Secure Boot policy changes
Network Indicators:
- Not applicable - this is a local boot process issue
SIEM Query:
EventID=1034 OR EventID=1001 from source 'Microsoft-Windows-Security-SPP' OR 'Secure Boot' in EventData