[electroly]

> HTTPS says, importantly, "You're getting the content from whom you think you're getting it from."

You need certificate pinning to know this for sure, due to the existence of MITM HTTPS spoofing in things like corporate firewalls. HTTPS alone isn't enough; you have to confirm the certificate is the one you expected. (You can pin the CA cert rather than the leaf certificate if you want, if you trust the CA; that still prevents MITM spoofing.)

[brirec]

I’m not aware of any HTTPS MITM that can function properly without adding its own certificate to the trusted roots on your system (or dismissing a big red warning for every site), so I don’t think certificate pinning is necessary in such an environment (if the concern is MITM by a corporate firewall).

An attacker would still need to either have attacked the domain in question, or be able to forge arbitrary trusted certificates.

[yjftsjthsd-h]

If an attack requires compromising my operating system certificate store, I'm reasonably comfortable excluding it from most of my threat models.

[electroly]

Obviously you choose your own relevant threat models, but it's common to do in iOS apps--many apps are including it in their threat models. Pinning the CA cert is what Apple recommends to app developers. It's not an unreasonable thing to do.

https://developer.apple.com/news/?id=g9ejcf8y

[yjftsjthsd-h]

That link discusses how to do it but not why. The most likely thing that occurs to me is that iOS apps consider the user a potentially hostile actor in their threat model, which is... technically a valid model, but in the context of this thread I don't that counts as a real concern.