Microsoft Secure Boot bypassed for 13 years via unrevoked shims

Microsoft's Secure Boot has been trivial to bypass for 13 of its 14 years of existence. Researchers at ESET discovered 11 firmware images, known as shims and including at least one from 2013, that remained signed by Microsoft despite known defects.

Old shims enable simple UEFI Secure Boot bypasses. The shims were created to extend Secure Boot support to Linux devices and utility software. Attackers can use a basic technique with these still-trusted but unrevoked binaries to circumvent the protection embedded in a device's UEFI firmware. No new vulnerability is required.

ESET researcher Martin Smolár stated that an attacker needs only a copy of an old shim binary and a basic understanding of how UEFI shims work. This is enough to bypass UEFI Secure Boot. The lapse occurred because Microsoft, which oversees shim signing, failed to revoke the publicly available images once vulnerabilities were found.

Threat affects both Windows and Linux users. The vulnerable shims can be installed on devices running either operating system. From there, attackers with brief physical access can subvert the chain of trust to install malicious firmware that loads early in the boot process and persists even after OS reinstallation or hard drive replacement.

Secure Boot was introduced in 2012 to counter bootkits. Without it, such malicious firmware has been used in attacks including LoJax by Russian state hackers in 2018, MosaicRegressor in 2020, CosmicStrand in 2022, and BlackLotus in 2023. Most bootkits require physical access, a threat model Secure Boot is designed to protect against.

Microsoft revokes the shims after ESET disclosure. The 11 shims, mainly version 0.9 and earlier, were added to the DBX revocation list in Microsoft's June 9, 2026 Patch Tuesday update. Windows systems receive the dbx update automatically while Linux distributions get it via LVFS. Some shims were used by distributors including Red Hat, openSUSE, and Oracle as well as third-party software such as PC-Doctor Finland’s Matriculation Examination Board.

Many of the shims were built before protections like SBAT and MOK deny lists existed. Others contain accumulated bugs. CERT/CC issued Vulnerability Note 616257 on the Microsoft-signed UEFI shim bootloaders, acknowledging Martin Smolár of ESET and referencing the full list of affected products and versions.

Complexity of Secure Boot contributed to the oversight. Microsoft has not explained how the failure to revoke the shims occurred over more than a decade in some cases. The highly complex design of Secure Boot, where Microsoft's digitally signed UEFI bootloader serves as the anchor of trust on Windows machines, is cited as a possible factor. Shims function as a secondary trust anchor signed by one of Microsoft's other UEFI certificates.