Microsoft's Secure Boot Could Be Bypassed in a Couple of Clicks for Almost Its Entire Life

Secure Boot is an industry standard that Microsoft developed to protect Windows devices — and later Linux ones as well — from being infected at the firmware level. The idea is simple: the UEFI firmware on the motherboard verifies the digital signatures of the code it loads and refuses to run anything unsigned. In practice, though, as specialists at ESET have discovered, the mechanism remained trivially bypassable for almost its entire history — 13 of its 14 years.

What the problem is

At the heart of the story are so-called shims, intermediate bootloaders. They were devised as a way to extend Secure Boot to Linux devices and utility software: a shim carries Microsoft's signature and then decides for itself what to run next. It is Microsoft that is responsible for signing such images.

The researchers found 11 firmware images, at least one of which dates back to 2013. All of them were known to be defective — their vulnerabilities had been discovered long ago — yet they remained signed by Microsoft and publicly available. The company never revoked the signatures after the problems became known.

As a result, these old shims, forgotten by everyone, can be used to bypass the protection entirely. The technique, according to ESET, is so simple that even a novice attacker could pull it off: a signed image is still treated as trusted, which means arbitrary code can be smuggled through it, circumventing the entire chain of checks.

Why it is hard to fix

Revoking a signature should, in theory, solve the problem, but this is exactly where the main difficulty lies — one that readers pointed out as well. A computer verifies signatures during boot, when there is no question of any internet access. Revoking a compromised image means distributing updated denylists (the very ones stored in UEFI) to devices all over the world — and until that happens, everything that was signed earlier remains trusted, including bootloaders known to be full of holes.

The result is a paradox: a protection conceived as the foundation of trusted boot rested for years on signatures that should long since have been annulled. The blame lies not so much with the Secure Boot architecture itself as with an organizational failure — an unwillingness or an inability to revoke in time what had once been signed.

Source: Schneier on Security