How Does Man-in-the-middle Attack Exploit The Security Of Diffie Hellman Key Agreement?

How Does Man-in-the-middle Attack Exploit The Security Of Diffie Hellman Key Agreement? A person in the middle may establish two distinct Diffie–Hellman key exchanges, one with Alice and the other with Bob, effectively masquerading as Alice to Bob, and vice versa, allowing the attacker to decrypt (and read or store) then re-encrypt the messages passed