Birthday attack
A method of cracking cryptographic algorithms through matches in hash functions, they are made against hash algorithms that are utilized to confirm the integrity of a message, software, or digital signature. Cryptography is an indispensable tool for protecting information in computer systems.
The birthday attack is based off of the birthday paradox, according to which the probability of two people sharing a birthday is far higher than it seems, it states that in order for there to be a 50% chance that someone in a given room shares your birthday, you need 253 people in the room (or for a group of 23 people, for example, the probability is 50%).
A birthday attack doesn't break the code, it just causes collision. This can be dangerous in that an attacker can use that duplicate hash to authenticate a harmful, dangerous file and pass it on as legitimate.