Boot making itself immutable?

[BootSec]

My understanding is that secure boot calls an interface to make portion flash that contains a bootloader immutable. Since this is called after the bootloader start to run, does it mean there is no immutability before this instruction?

Is that not a security issue because the bootloader can be replaced before immutable settings are called with a version that doesn't include instruction for such setting?

[Simon]

The immutable bootloader will lock itself in the start 

i_https://github.com/nrfconnect/sdk-nrf/blob/799d96f2802b648d9c0d62e8b62bf4703354cafe/samples/bootloader/src/main.c#L72.

Since this is done immediately, there is no possible way to run an attack before this. So this solution poses no security issues.

Best regards,

Simon