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I want to completely wipe a MacBook Pro and install macOS High Sierra, but without trusting the self-install mechanism on the laptop (it's second-hand, so it may have been compromised by a rootkit or something). So I want to somehow download it and install it from an external boot disk, like in the old days.

How can I do this? I've googled but I can't seem to find any clear advice on this.

2 Answers 2

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If you don't trust the build-in Internet Recovery installer you can't.
Because that same installer handles installation from USB, DVD or external hard-drive.

Compromising that installer is possible, but it is beyond the capabilities of most people to do so.
Unless you are sure the original owner was a hacker with exceptional skills and a grudge against you (why did you buy the laptop from him then ?) I wouldn't worry about it.

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  • How can the installer be compromised? It comes directly from Apple live on demand & checks its own signature before it will run.
    – Tetsujin
    Jan 3, 2018 at 12:54
  • @Tetsujin This is still a mini-OS installed in flash-rom. This mini-OS makes the connection to the Apple servers and initiates the download/installation. As it is flashable from within the regular OS (it is upgraded by Apple sometimes) it can be compromised. But it would take a great deal of skill and knowledge to do it. Other attack would be to intercept the download from the Apple server and present the user with fake OSX image to install from.
    – Tonny
    Jan 3, 2018 at 13:04
  • I think the whole concept would require considerably more determination & forethought than anyone short of an international terrorist ought to be afraid of.
    – Tetsujin
    Jan 3, 2018 at 13:08
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    @Tetsujin I completely agree and that is why I said "I wouldn't worry about it" in my answer.
    – Tonny
    Jan 3, 2018 at 15:42
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The install cannot be compromised - because it is not yet on the Mac at all, nor is it ever on the Mac's internal drive until you know it's clean.

  • Boot to Internet Recovery - Cmd ⌘ Opt ⌥ R at the chimes.
    This will boot to a completely clean environment downloaded from Apple, which exists entirely in RAM.
  • Use Disk Utility to completely erase the internal drive
  • Setup the machine fresh using Reinstall macOS

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This procedure can change slightly depending on the age of the machine - add specific machine & last-known OS details to your question & I can adjust this answer to suit.

Complete info at Apple KB - How to reinstall macOS

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