CVE-2026-21265 addresses a vulnerability in Microsoft Windows 10 Version 1809 related to the expiration and update process of Secure Boot certificates stored in the UEFI KEK (Key Exchange Key) and DB (Signature Database). Secure Boot is a security standard designed to ensure that a device boots using only software trusted by the Original Equipment Manufacturer (OEM). Microsoft’s Secure Boot implementation uses certificates to sign boot loaders, Option ROMs, and the Windows Boot Manager. The certificates in question—Microsoft Corporation KEK CA 2011, Microsoft Corporation UEFI CA 2011, and Microsoft Windows Production PCA 2011—are set to expire between June and October 2026. Failure to update these certificates will cause Secure Boot to lose its ability to verify boot components, potentially allowing unauthorized or malicious code to execute during system startup. The update mechanism for these certificates relies on firmware components that may contain defects, leading to failures or unpredictable behavior when updating the trust chain. This can result in disruption of Secure Boot, causing boot failures or weakening the security posture of the system. The vulnerability has a CVSS score of 6.4 (medium severity), reflecting the requirement for local privileges and high authentication, no user interaction, and the potential for high impact on confidentiality, integrity, and availability. No known exploits are currently reported in the wild. The issue highlights the importance of coordinated updates between Microsoft, hardware vendors, and system administrators to maintain Secure Boot integrity as certificates expire.

For European organizations, the impact of CVE-2026-21265 can be significant, especially for those relying on Windows 10 Version 1809 in critical infrastructure, government, finance, healthcare, and industrial control systems. Failure to update the expiring Secure Boot certificates can lead to Secure Boot malfunction, resulting in boot failures or the inability to verify the integrity of boot components. This could open avenues for rootkits or boot-level malware to persist undetected, compromising system confidentiality, integrity, and availability. Disruption of Secure Boot may also cause operational downtime, impacting business continuity. Organizations with strict compliance requirements around system integrity and secure boot processes may face regulatory and reputational risks if they do not address this vulnerability promptly. The complexity of the update process, involving firmware components, increases the risk of deployment errors or failures, necessitating careful validation and testing. European entities using legacy hardware or firmware with limited update support are particularly vulnerable to prolonged exposure or operational issues.

1. Inventory and identify all systems running Windows 10 Version 1809, especially those with Secure Boot enabled. 2. Coordinate with hardware vendors and firmware providers to obtain and apply firmware updates that support the Secure Boot certificate update process reliably. 3. Apply all available Windows updates and patches that address Secure Boot certificate management and related components. 4. Test the certificate update process in controlled environments before wide deployment to detect and mitigate potential failures or unpredictable behavior. 5. Develop and implement a rollback or recovery plan to restore Secure Boot functionality in case of update failures, including recovery media and firmware re-flashing procedures. 6. Monitor Microsoft advisories and security bulletins for updates or patches related to this CVE and Secure Boot certificate lifecycle management. 7. Consider upgrading affected systems to newer Windows versions with extended support and improved Secure Boot management if feasible. 8. Educate IT and security teams on the importance of Secure Boot certificate updates and the risks of neglecting this maintenance task. 9. Maintain regular backups and incident response readiness to mitigate potential impacts from boot failures or security breaches related to this vulnerability.