Further complicating the process, even the expiration of the Microsoft certificate that signed the shims, which took place late last month, isn’t enough to revoke the ones ESET identified.
A rogue’s gallery of defective shims
The shims identified by ESET authorize secondary components that are known to be vulnerable to various exploits. The Oracle shim, for instance, signs a binary vulnerable to CVE-2015-5381. Smolár said the skill required to exploit the vulnerability is low. Other vulnerable shims fail to support protections, such as MOK deny-list enforcement and SBAT enforcement, both of which came into effect after the affected shim was released. Still other identified shims contain vulnerabilities in their own code.
In the interest of brevity, many additional details included in Tuesday’s report are omitted from this article.
An unsettling prospect
As noted, these vulnerable shims can be used against Windows and Linux machines alike, although likely not Windows 11 Secured-core PCs in their default state. Any Windows user who has installed Microsoft’s June update batch is no longer vulnerable. Linux users should check the Linux Vendor Firmware Service or consult their distributor. Revocation statuses are available using the uefi-dbx-audit script.
The prospect that attackers have had the means to bypass Secure Boot for more than a decade through what amounts to hack-by-numbers scripts isn’t much of an endorsement of the mechanism proposed by Microsoft in partnership with hardware makers. As mentioned earlier, a key contributor to this debacle is its complexity.
“This is a solid rebuke of the entire secure boot model,” HD Moore, a firmware security expert, CEO and founder of runZero, and a long-time critic of Secure Boot, said in an interview. His complaints include Microsoft being the de facto root of trust for the entire UEFI platform, the inability of the protection to scale sufficiently, and the ability for components to boot even after top-level certificates expire.
“The end result is a huge number of unknown (to everyone but Microsoft) signed things that bypass Secure Boot—some of which can then be used to boot other things—and both have normal security bugs and other mistakes that mean they can be used to boot nearly anything,” Moore added. “The whole ecosystem is somewhat broken and needs a reboot.”
