A 'Collision attack' specifically targets the vulnerability of a hash function by finding two different inputs that produce the same hash value. This is a significant vulnerability because hash functions are expected to generate unique output for every unique input. Successful collision attacks undermine the integrity of cryptographic systems by allowing attackers to substitute a legitimate message or file with a fraudulent one without detection. The incorrect answers are related to other cryptography concepts: 'Birthday attack' exploits the mathematics behind probability and is focused on finding any two inputs that hash to the same output, rather than a specific one; 'Known plaintext attack' involves deducing the key used for encryption when the attacker has access to both the plaintext and its corresponding ciphertext; 'Man-in-the-middle attack' is a type of eavesdropping attack where the attacker secretly relays and possibly alters the communication between two parties who believe they are directly communicating with each other.