Microsoft Secure Boot has been bypassable for a decade, researchers find 11 unrevoked shims

Security firm ESET has uncovered that the Secure Boot standard, designed by Microsoft to protect devices from firmware infections, has been easily bypassable for nearly its entire existence. Researchers identified 11 UEFI shim images, some dating back to 2013, that were known to be vulnerable but remained signed by Microsoft and unrevoked.

Shims were originally created to extend Secure Boot support to Linux devices and utility software. However, using a simple technique, even novice hackers can completely circumvent the protection embedded in the motherboard's UEFI. The flaw stems from Microsoft's failure to revoke these publicly available images after vulnerabilities were discovered.

The threat affects both Windows and Linux users, as the shims can be installed on devices running either operating system. An attacker can use these shims to install malicious firmware that loads early in the boot process and persists even after OS reinstallation or hard drive replacement.

ESET researcher Martin Smolár noted that these old shims are dangerous not because of new vulnerabilities, but because no new exploit is needed to bypass Secure Boot. An attacker only needs a copy of an old, still-trusted but unrevoked shim binary and a basic understanding of how UEFI shims work.

Microsoft finally revoked the 11 shims in its regular June patch release after ESET alerted CERT and Microsoft. The company has not yet explained how the lapse occurred. One possible cause is the complexity of Secure Boot, which involves multiple databases and version-based revocation mechanisms like SBAT and SVN.

Security expert HD Moore of runZero criticized the entire Secure Boot model, stating that Microsoft is the de facto root of trust for the entire UEFI platform, the protection fails to scale properly, and components can boot even after top-level certificates expire. He concluded that the ecosystem is broken and needs a reboot.