Forgotten UEFI shims undermining Secure Boot

ESET researchers identified 11 old and forgotten UEFI shim bootloaders at versions 0.9 and below that can be used to bypass UEFI Secure Boot on any UEFI-based machine that trusts Microsoft’s Microsoft Corporation UEFI CA 2011 third-party UEFI certificate authority (CA) certificate, regardless of the installed operating system (OS). Reported shims can be exploited to execute untrusted code during system boot, enabling attackers to deploy malicious UEFI bootkits (such as Bootkitty, HybridPetya, or BlackLotus) even on systems with UEFI Secure Boot enabled. We reported our findings to CERT/CC in February 2026, and the vulnerable UEFI applications were revoked on Microsoft’s June 9th, 2026 Patch Tuesday.

While two CVE IDs were assigned to this case to cover the reported shims, CVE-2026-8863 and CVE-2026-10797, exploitation of each reported shim is not just about a single bug or two that can be found in these old shims directly. In fact, the attack surface is extended by the shims’ trusted, second-stage bootloaders (mostly GRUB 2), which – like the shims themselves – may include outdated versions with known vulnerabilities. The discovered shims come from various tools or software packages, including PC-diagnostics software, Linux distributions, and other UEFI-based utilities. Importantly, exploitation is not limited to systems with the affected software or OS installed, as attackers can bring their own copy of the vulnerable shims to any UEFI system with the Microsoft third-party UEFI certificate enrolled.

The full list of the software products relying on the reported shims along with their affected versions is available in CERT/CC’s Vulnerability Note. In response to ESET researchers’ report, UEFI shim bootloaders with the following PE Authenticode hashes were revoked in the dbx update that was part of Microsoft’s June 9th Patch Tuesday: