Microsoft Left Windows and Linux Secure Boot Bypassable for 13 Years, ESET Finds

A decade-long hole in the one protection that mattered

Secure Boot exists for one reason: stop malware from hijacking a computer before the operating system even loads. Microsoft built it in 2012 and it became an industry standard baked into the UEFI firmware of nearly every PC sold since. For 13 of its 14 years, it turns out, that protection had a hole big enough to drive a truck through.

According to Ars Technica, researchers at the security firm ESET identified 11 firmware images, called shims, that were known to be defective yet remained digitally signed and trusted by Microsoft. At least one of these dates back to 2013.

Shims are small pieces of software Microsoft created to extend Secure Boot coverage to Linux and other non-Windows systems. Because Microsoft controls the signing process, a shim carries Microsoft's stamp of trust. That stamp is supposed to mean the file is safe. In this case, it meant nothing.

No hacking skill required

ESET researcher Martin Smolár did not mince words about how simple this is to exploit. "What makes these old shims dangerous is not a novel vulnerability," Smolár wrote, according to Ars Technica. "It's that no new vulnerability is needed to bypass UEFI Secure Boot. An attacker needs no complicated exploitation primitives, only a copy of an old, still-trusted, but unrevoked shim binary and a basic understanding of how UEFI shims work."

You don't need to be a nation-state hacker. You need an old file that Microsoft should have blacklisted years ago and never did. Ars Technica reported the technique is simple enough for a novice to pull off.

Once an attacker plants one of these old shims, they can subvert the entire chain of trust Secure Boot is supposed to enforce and install a bootkit, malicious firmware that loads before the operating system and survives a full OS reinstall or even a hard drive swap. Secure Boot was designed to prevent exactly this.

Real bootkits, real damage, already in the wild

This isn't a theoretical concern. Ars Technica pointed to a string of bootkits that have already caused damage in the years Secure Boot has existed: LoJax, tied to Russian state hackers in 2018; MosaicRegressor, found in 2020; CosmicStrand in 2022; and BlackLotus in 2023. Others tracked under names like ESpecter, FinSpy, and MoonBounce round out a growing list.

Most of these attacks require an attacker to have brief physical access to a device, even one that's powered off. That's precisely the threat model Secure Boot was built to defend against. A broken Secure Boot doesn't just weaken security on paper, it directly enables the exact attacks that have already happened against real targets.

Who's exposed and why it slipped through

A list of the 11 shims compiled by CERT shows the exposure runs across the industry, not just Microsoft's own software. Linux distributors including Red Hat, OpenSuse, and Oracle had shims on the list. So did third-party software, including one tied to Finland's Matriculation Examination Board through PC-Doctor Finland.

Many of these shims predate security mechanisms that exist today, including SBAT (a revocation mechanism) and MOK deny lists. Others simply accumulated bugs over time, either in their own code or in the second-stage binaries they were designed to authorize. Microsoft's signed UEFI bootloader is the sole anchor of trust for the entire Windows boot chain, meaning any weak link that Microsoft fails to revoke undermines the whole system, not just one product.

How does this even happen

There's no evidence in ESET's findings that Microsoft acted maliciously. The criticism is one of basic operational discipline: Microsoft controls the revocation list for a security mechanism embedded in essentially every PC motherboard sold worldwide, and known-defective signed files sat unrevoked for over a decade without anyone catching it until ESET did.

That's a process failure, not a sophisticated attack. It raises an obvious question for a company that markets itself as a security leader: how many other old, forgotten signed binaries are still sitting out there, trusted by default, waiting to be found by someone with worse intentions than a research team.

Microsoft has not been quoted in available reporting responding directly to ESET's disclosure or laying out a timeline for revoking the affected shims. Until those binaries are formally blacklisted through updated revocation lists, the bypass ESET describes remains available to anyone who can get their hands on one of the 11 old files, whether that's a criminal, a curious teenager, or a state actor with physical access to a target machine.