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The vulnerability associated with Diffie-Hellman key exchange primarily stems from its susceptibility to man-in-the-middle attacks. In this scenario, an attacker could intercept and alter the public keys exchanged between two parties. Since Diffie-Hellman does not inherently authenticate the parties involved, the attacker can present their own public key to each participant. As a result, both parties believe they are communicating securely with each other, while in reality, the attacker is able to decrypt, read, and manipulate the transmitted messages.

To enhance the security of the Diffie-Hellman exchange and safeguard against such threats, it is essential to implement additional authentication mechanisms. This can involve using digital signatures or integrating the key exchange with certificate authorities that confirm the identities of the communicating parties.

Other vulnerabilities listed, such as brute-force attacks, key collision, and replay attacks, pertain to different weaknesses or issues inherent to various cryptographic systems, but they do not specifically reflect the fundamental issue present with Diffie-Hellman itself. The absence of authentication is what distinctly makes Diffie-Hellman vulnerable to man-in-the-middle attacks.