Secure Boot bypass in Microsoft-signed UEFI SHIM bootloaders

CVE-2026-8863 is a UEFI Secure Boot security feature bypass affecting multiple Microsoft-signed UEFI SHIM bootloaders, primarily shim version 0.9 and earlier. The vulnerable bootloaders can be abused to subvert Secure Boot validation during the pre-OS boot chain. According to the provided context, an attacker with local administrative privileges, physical access, or the ability to modify the boot process can leverage one of these vulnerable Microsoft-signed shim bootloaders in a BYOVD-style approach to execute arbitrary code during the early boot phase before the operating system initializes. Because the affected shim binaries remain trusted unless explicitly revoked, blocking exploitation requires a specific UEFI DBX update that revokes the vulnerable bootloaders.