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There's one major problem: software updater itself is vulnerable. So in theory the only way to get a clean system is to download a fresh, fixed installer via a channel that's not affected and reinstall. What a mess.

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This is a problem only if updates are not signed.

No, it's not. The updater needs to know the correct signatures and it doesn't have a trusted way of retrieving them. Also, any previous update may have been malicious and replaced the updater. The updater may very well have embedded keys, but we don't know, so the secure option would be not to trust it.

That's not how signing works. Also, if the signing is indeed implemented correctly, then the updater cannot have been compromised earlier.

The updater verifies the signature using the public counterpart to Apple's current private key. It can only be changed by updating the updater (or a separate keys-only package), with the correct signature.

Or at least that's how it's supposed to work.

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