Linux users who have Secure Boot enabled on their systems knowingly or unknowingly rely on a key from Microsoft that is set to expire in September. After that point, Microsoft will no longer use that key to sign the shim first-stage UEFI bootloader that is used by Linux distributions to boot the kernel with Secure Boot. But the replacement key, which has been available since 2023, may not be installed on many systems; worse yet, it may require the hardware vendor to issue an update for the system firmware, which may or may not happen. It seems that the vast majority of systems will not be lost in the shuffle, but it may require extra work from distributors and users.
The details are complex; it has humorously been called “security by security”.
Hobby Linux users could, as far as I understand , simply disable UEFI secure boot (after weigthing carefully what secure boot provides to them, and what it does not provide). Otherwise, they’ll need a firmware upgrade before any upgrade to a new OS / bootloader chain.
Small companies which use old laptops with Windows might be bitten hard by this because they can become locked out of their hardware with no way to update it, or even make a backup!
And by the way, Intel motherboards which are running your Linux system may contain a copy of Minix - yes, the Minix from the historic Tanenbaum vs. Torvalds debate - which runs below the OS in the system management mode engine and is controlled by the vendor, which can e.g. update firmware via the network. SMM is normally not visible by the user but it can cause problems e.g. for real-time applications because it has higher privileges than the kernel and can interrupt all of the kernel at any time.