Segwit & Offline Signing
Answered on Twitter


  The hash of the provided prevtx must match the prevtx hash given in the input. The attacker is unable to construct a fake prevtx that hashes to a predefined value. The prevtx hash in the input cannot be changed either, otherwise the network will reject the tx.


Basically, you can't create fake previous transactions because that would require producing a sha256 collision, which we assume can't happen.