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UEFI Secure Boot signing keys can be updated at runtime, and old keys can be explicitly marked as untrusted. Validation of key updates is performed by the firmware, not the OS, so this can't be trivially subverted.

Right, agree. What I'm asking is: why can't the OEM over-the-air-update Secure Boot on all the online devices? Is there no channel to re-key Secure Boot from an online source?

I agree that anyone who wants to muck about their device on purpose could just not connect to the Internet, but I'm focused on protecting the users who don't want to be victims of hacks.

(My only mental model for this is the bugs in the early protection on the original XBox and how Microsoft fixed it with OTA updates and specific games they released that patched the bug).

They key is burned into the device.

> Is there no channel to re-key Secure Boot from an online source?

There is no such method. Any such method would be potentially vulnerable if the device was jailbroken (from a purely software hack) - in which case, you would need some sort of permanent key to verify the re-keying which defeats the point. As a result, the Secure Boot key is burned into the chip and unchangeable.

> Got the Apple signing key? Revoke any app you like and have it instantly lock out from millions of iPhones. Break every app for everybody on iPhone and give Apple employees and users a miserable week and cause a political panic. Sky's the limit.

How are you getting to the iPhones? You need to be able to intercept any internet connection too.

nb I'm not sure there is such a single signing key.

> Send out fake Windows updates, approved by Microsoft, to do whatever you want.

Send them... How? You're planning on setting up a MITM attack on everyone's internet access?

At a nation state level attack that is completely possible.

If a state wants something from you, they'll send armed agents at your door.

If 'your' state that is. There are any number of states unfriendly to each other.