Attacker in a key exchange
I think that this notation "$(A,N_X,T_B),K_{BS}$" isn't correct and must be transcripted as "$(A,N_X,T_B).K_{BS}$", which could be interpreted as the resulting cipherText of "$(A,N_X,T_B)$ under the Key $K_{BS}$".
Under these conditions, we could try to understand that the attacker X begin by impersonating A, by sending to B the plainText $(A,N_X)$


X: Impersonating A:    X(A) -> $(A,N_X)$
B: Ask the Server to generate a shared key for A and B, But the message is intecepted by the Attacker who doesn't know the Master Key $K_{BS}$
The attacker then return back the initial cipherText to B, who can easilly decrypt as he knows its shared Key $K_{BS}$ with the trusted third part.


But there is a problem, this protocol doesn't work, because there is no generation of shared Key by the Server for A and B, and then B after checking the existence of Nonce and TimeStamp, and the absence of shared Key, must logically abort the session, under the assumption that a master Key is never and NEVER send in a plainText.