In the hypothetical scenario the attacker doesn't have the client's private key so it can't authenticate to the server. It can pass along the session key from the server but then it won't be able to read the data.
Re:Duplicate SSH Keys Everywhere
by jrochkind1 on Tuesday February 17, 2015 @11:08PM
(#9066935)
Ah, I get it now. It's not just the authentication step, it's the fact that the data is encrypted so only the original client can read it? That's right?
The attacker just has to authenticate with the correct signature to each, but if it's an attacker, why would it just pass along the victim's data anyway? The whole point of an MITM is that you can modify data before it hits its target.